Determination and geochemical implication of multiple series of long-chain oxygen-bearing compounds trapped in kerogen in the Lucaogou Formation, Santanghu Basin, NW China Org. Geochem. (IF 3.081) Pub Date : 2018-04-11 Bin Cheng, Jianbing Xu, Yungan Liang, Qian Deng, Yankuan Tian, Zewen Liao
Studies of occluded oxygen-bearing compounds inside kerogen are scarce, although such studies are important in understanding early-stage transformation of organic matter. A relatively low-maturity kerogen from the late Permian mudstone of the Lucaogou Formation of the Santanghu Basin, northwestern China, was successively extracted with n-hexane, acetone and dichloromethane to release the adsorbed components, and the extracted kerogen was treated with a mild oxidation reagent H2O2 to release the occluded components. Gas chromatography–mass spectrometry analysis of the adsorbed and occluded components revealed a suite of oxygen-bearing compounds, including n-alkan-2-ones, n-aldehydes and fatty acid methyl esters. The n-alkan-2-ones were mainly detected in the adsorbed fraction and n-aldehydes in the occluded component. Comparison of the long-chain oxygen-bearing compound distributions and their individual stable carbon isotope characteristics both suggest that the oxygen-bearing compounds released by H2O2 treatment were occluded in the kerogen. The shorter-chain (C15 to C19) occluded n-aldehydes reflected similar isotopic distribution trends to the adsorbed n-alkan-2-ones, and the δ13C values of the mid-length (C21 to C25) occluded n-aldehydes were much closer to the adsorbed n-alkanes. Bacterially mediated methylation and decarbonylation of the n-aldehydes during kerogen formation may be important source(s) of n-alkan-2-ones and n-alkanes, respectively. The mid-chain ketones mainly originated from hydrothermal alteration of n-alkanes after kerogen formation. Uplift of the Lucaogou Formation in the Yuejingou section allowed surface-enhanced aerobic reactions.
Origin of polar organic sulfur compounds in immature crude oils revealed by ESI FT–ICR MS Org. Geochem. (IF 3.081) Pub Date : 2018-04-05 Weimin Liu, Yuhong Liao, Quan Shi, Chang Samuel Hsu, Bin Jiang, Ping'an Peng
Organic sulfur compounds (OSCs) are abundant in immature crude oils, including “polar” OSCs that are defined here as compounds containing one or more heteroatom(s) in addition to sulfur atoms having sufficient polarity to be analyzable by electrospray ionization (ESI) without derivatization. An understanding of the origins of polar OSCs in crude oils has been hampered by limitations in their analytical characterization. In this paper, we employed a high field (9.4 T) Fourier transform ion cyclotron resonance mass spectrometry (FT–ICR MS) coupled with ESI to study the sulfur-rich immature crude oils from the Jianghan Basin in China. The results show that the polar OSCs usually have non-sulfur counterparts, i.e., heteroatom-containing compounds without sulfur, with similar carbon number ranges, but slightly lower DBE (double bond equivalence) distributions. The similar or even identical carbon number distributions of the polar OSCs and their non-sulfur counterparts indicate their inheritance from the same precursors. The sulfur rings in polar OSCs are formed by intramolecular sulfurization that leads to the increases in DBEs compared to their non-sulfur counterparts. The results also indicate that the number of sulfur rings that can be formed in polar OSCs is largely controlled by the number of available reactive functional groups in their precursors. This paper extends our knowledge of intramolecular sulfurization during early diagenesis, especially for polar heteroatom-containing compounds.
Sediment records of long chain alkyl diols in an upwelling area of the coastal northern South China Sea Org. Geochem. (IF 3.081) Pub Date : 2018-04-03 Xiaowei Zhu, Guodong Jia, Shengyi Mao, Wen Yan
Here, Long chain alkyl diols in sediment cores were studied in the Yuedong Upwelling (YDU) area in the coastal northern South China Sea to investigate whether sea surface temperature (SST) and upwelling intensity over the last few decades can be traced by the diol-derived indices, i.e., the LDI (long chain diol index) as proxy for SST and the DI-1 and DI-2 (diol index 1 and 2) as proxies for upwelling intensity. The influence of riverine long chain alkyl diols (e.g., 1,13- and 1,15-diols), which may affect LDI-reconstructed SSTs, was largely ruled out based on the comparison between diol distributions in suspended particulate materials in the Pear River estuary and the core sediments. LDI-derived SSTs in downcore sediments matched well with local annual mean SSTs. The records of DI-2 changed in parallel with the local wind stress and inversely with SST at times of upwelling, suggesting that DI-2 as an effective proxy for upwelling intensity of YDU. The DI-1 showed a generally inverse variation pattern with DI-2, implying a major influence by SST, and it is thus not an applicable upwelling indicator for YDU. A negative relationship of DI-1 and a positive relationship of DI-2 with LDI-derived SST were observed. This occurrence might be attributed to El Niño–Southern Oscillation (ENSO), because the SST and upwelling intensity were largely modulated by ENSO, showing a warm annual mean SST during the years of enhanced El Niño and intensified summer upwelling.
Modelling leaf wax n-alkane inputs to soils along a latitudinal transect across Australia Org. Geochem. (IF 3.081) Pub Date : 2018-03-29 S. Howard, F.A. McInerney, S. Caddy-Retalic, P.A. Hall, J.W. Andrae
Leaf wax n-alkanes provide a valuable palaeoecological proxy, but their interpretation requires an understanding of the scale of temporal and spatial integration in soils. Leaf wax n-alkanes are continually deposited into soils directly from local plants as well as from more distant plants via wind or water transport. In addition, n-alkanes can persist in soils for thousands of years, and tend to decrease in age with shallower depth. To explore whether the uppermost soils reflect recent leaf fall inputs we compared surface soils and modern vegetation from 20 sites along a transect across Australia. At each site, the three most dominant plant species and a soil sample from the top 3 cm were analysed for n-alkane concentration, average chain length (ACL), proportional abundance of C33 and C29 (Norm33) and carbon preference index (CPI). Chain length distributions differ between trees and grasses, with a higher proportion of C29 in trees and C33 in grasses. Norm33 in soils correlates with proportional grass to tree cover across the transect. To model n-alkane inputs for each site, we calculated a predicted ACL, Norm33 and CPI using the dominant plants at that site, weighted by proportional species cover and n-alkane concentration. Predicted ACL, Norm33 and CPI inputs were generally higher than the soils, demonstrating that recent and local inputs do not dominate soil n-alkanes at our study sites. Thus, n-alkane distributions in surface soils do not correlate with local, current vegetation, but do correlate with proportional grass and tree cover, suggesting they provide a faithful record of large scale ecosystems structure.
Archaeal and bacterial glycerol dialkyl glycerol tetraether (GDGT) lipids in environmental samples by high temperature-gas chromatography with flame ionisation and time-of-flight mass spectrometry detection Org. Geochem. (IF 3.081) Pub Date : 2018-03-28 Sabine K. Lengger, Paul A. Sutton, Steven J. Rowland, Sarah J. Hurley, Ann Pearson, B. David A. Naafs, Xinyue Dang, Gordon N. Inglis, Richard D. Pancost
Archaeal isoprenoidal glycerol dibiphytanyl glycerol tetraether lipids (iGDGTs) and their non-isoprenoidal branched bacterial analogues (brGDGTs) have widespread applications in biogeochemistry and paleothermometry. Analysis of GDGTs usually involves separation using high performance liquid chromatography, typically coupled via atmospheric pressure chemical ionisation to mass spectrometric detection in selected ion-monitoring mode (HPLC–APCI-MS). However, reliable determination of ratios and, in particular, quantification by this technique, can be challenging due to differences in ionisation efficiencies of the various compounds. Quantification of GDGTs also relies on external calibration of the relative response to an internal standard with authenticated GDGTs, which are often not readily accessible. Here, we tested the suitability of high temperature gas chromatography with flame ionisation detection (HTGC-FID) for the determination of concentrations and tetraether lipid-based ratios in marine and terrestrial samples. For this, we identified GDGTs in environmental samples using HTGC coupled to time-of-flight mass spectrometry (HTGC–MS). Using a purified GDGT standard, we show we can quantify GDGT-0 in environmental samples by GC-FID. Some GDGT-based ratios measured by HTGC-FID exhibited a linear correlation (1:1) with ratios derived from HPLC–MS and weight-based ratios of mixtures of purified standards. However, ratios relying on minor isomers, such as TEX86 and MBT/CBT have many unresolved challenges for determination by HTGC. Detection limits were higher than for HPLC–MS. However, the advantages of employing HTGC-based methods include: (1) the independence from MS tuning-related differences in ionisation energies; (2) the potential for direct comparison with other, non-GDGT based biomarkers; and (3) a more complete insight into biomarker distributions in environmental samples by the extension of the temperature range. Quantitative elution of GDGTs from a HTGC column as demonstrated herein, will also enable their analysis by compound-specific isotope ratio mass spectrometry.
On the changes of polycyclic aromatic compounds in waterflooded oil and their implications for geochemical interpretation Org. Geochem. (IF 3.081) Pub Date : 2018-03-27 Xiangchun Chang, Yue Wang, Yaohui Xu, Jing Cui, Tao Wang
Qudi oilfield had been producing for over twenty years and became waterflooded ten years ago. Samples from nine producing wells drilled recently in the Qudi oilfield were collected during the three recovery stages spanning an eight month interval of waterflooding and were analyzed for changes in their polycyclic aromatic compounds (PACs). The results indicate that most of waterflooded oils are characterized by increased tricyclic, tetracyclic, pentacyclic and triaromatic steroids concentrations. The irregular changes in parameters based on two or fewer alkyl substituted naphthalenes (MNR, ENR, DBR, DNR-1) implies a different depletion order for the PACs because the numerators or denominators are controlled by the predominance of water washing and biodegradation over a certain time. The increase in DBTs and DBFs with water washing reflects their strong resistance to biodegradation. Lower aqueous solubility and strong resistance to biodegradation are possibly responsible for the mainly unchanged values of the three or more alkylnaphthalene-related parameters (TNR-1, TDE-1, TDE-3, TBR and TeBR), tricyclic aromatics (MPI-1 and PP-1), tetracyclic aromatics (MCI1, MCI2, 2-MC/1-MC, and BaA/(BaA + Chry)), triaromatic steroids parameters (C26-TAS/C28-TAS, C27-TAS/C28-TAS, C28-TAS 20S/(20R + 20S), and TA(I)/TA(I + II)), and heterocyclic aromatic parameters (4-MDBT/DBT, 4-MDBT/1-MDBT, and 4,6-DMDBT/(1,4 + 1,6)-DMDBT), confirming their validity for geochemical interpretation even after extensive water washing. However, pentacyclic aromatic parameters (BeP/Pe, (BeP + BaP)/Pe, BF/Pe, and BF/BeP), which are theoretically expected to be unaffected, change significantly with the advance of the waterflooding process, implying a complex alteration mechanism that needs further investigation.
Sources and distribution of tetraether lipids in sediments from the Zhejiang–Fujian coastal mud area, China, over the past 160 years: Implications for paleoclimate change Org. Geochem. (IF 3.081) Pub Date : 2018-03-23 Lilei Chen, Jian Liu, Jiasheng Wang, Gang Xu, Feng Li, Xingliang He, Yuanyuan Zhang, Li Li
Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids in surface sediments have been successfully used to reconstruct sea surface temperature (SST), mean annual air temperature (MAAT), and soil pH in the East China Sea and its adjacent ocean margins. In this study, two cores from the Zhejiang–Fujian coastal mud area (ZFCMA) were studied in detail to investigate temporal changes in the source and distribution of tetraether lipids in the area, and the implications of such data for paleoclimate change. The results indicate that isoprenoid GDGTs (isoGDGTs) in the ZFCMA were derived mainly from Thaumarchaeota, with a minor contribution from in situ and allochthonous methanogenic archaea in upwelling areas, whereas branched GDGTs (brGDGTs) originated mainly from terrigenous bacteria and in situ organisms. However, the minor methanogen-derived and allochthonous isoGDGTs were considered to have had no effect on inferred SSTs in this study. The presence of brGDGTs produced in situ might corrupt the relationship between the methylation index of branched tetraethers or the cyclization index of branched tetraethers (MBT/CBT) and MAAT or pH, particularly in hypoxic areas. However, #ringtetra data, used here as an index to evaluate the sources of the brGDGTs, indicated that the brGDGTs buried in the sediments are predominantly derived from soil erosion. Furthermore, temporal changes in the GDGT-based proxy records indicate that climate changes and events (e.g., relating to the East Asian Monsoon, Kuroshio Current (KC)/Pacific Decadal Oscillation (PDO) intensity, and flood events), as well as anthropogenic influences (e.g., dam construction, soil conservation, fertilizer usage, and discharge of industrial waste water and domestic sewage) affect the signal in the core sediments. SSTs fluctuated over the past 160 years in the ZFCMA, with higher values in the 20th century than in the 19th century. In addition, the humidity, as inferred from the relationship between precipitation/soil water content and pH, and continental temperature in the mid–lower Yangtze River basin switched between dry–hot/dry–cool and wet–hot/wet–cool over the last 160 years. The results also indicate that PDO/KC intensity plays a key role in temperature changes and has a larger impact on terrestrial ecosystems than on ocean ecosystems in the ZFCMA and the adjacent continent.
Variations in abundance and distribution of methyltrimethyltridecylchromans (MTTCs) in sediments from saline lacustrine settings in Cenozoic lacustrine basins, China Org. Geochem. (IF 3.081) Pub Date : 2018-03-21 Kaixi Jiang, Chunming Lin, Xia Zhang, Chunfang Cai, Fei Xiao, Wenxiang He, Li Peng
This study examines the abundance and distribution of MTTCs and biomarker compositions in samples of MTTC-rich sediments obtained from Paleogene strata in the Bohai Bay, Jianghan and Western Qaidam Basins in China. Biomarker signatures, including high C35/C34 hopane ratios, high gammacerane index values, low pristane/phytane (Pr/Ph) ratios and the presence of aryl isoprenoids, indicate saline lacustrine settings and photic zone euxinia (PZE). The distributions of MTTCs indicate significant palaeosalinity differences in the depositional environments of the three sets of samples obtained from the Bohai Bay, Jianghan and Western Qaidam Basins. These sets of samples were deposited under normal to mesosaline, meso- to hypersaline and hypersaline environments, respectively. The δ-MTTC% values are positively correlated with the values of the gammacerane index, but are negatively correlated with the Pr/Ph ratios. This result demonstrates that MTTCs do not form in the upper or lower parts of stratified water columns, but may form in the euxinic portions of photic zones. Therefore, MTTC parameters can only be used to reconstruct palaeosalinity in the euxinic portions of photic zones and are not applicable to the upper or lower parts of stratified water columns. This paper infers that variations in palaeosalinity in the euxinic portions of photic zones likely play a key role in controlling the distribution and abundance of MTTCs.
Temperature effect on abundance and distribution of leaf wax n-alkanes across a temperature gradient along the 400 mm isohyet in China Org. Geochem. (IF 3.081) Pub Date : 2018-03-21 Jia Wang, Ergu Axia, Yunping Xu, Guoan Wang, Liping Zhou, Yufu Jia, Zixun Chen, Jiazhu Li
Long chain n-alkanes are plant wax biomarkers that are widely used in paleoenvironmental reconstructions. Temperature is an important influence on the composition of leaf wax n-alkanes. However, to date, understanding of changes in the abundance and distribution of leaf wax n-alkanes with temperature remains unclear. The main reason is that the interference of precipitation on leaf wax n-alkanes has not been effectively removed in most investigations conducted at natural ecosystems. We sampled 357 plants across a temperature gradient along an isohyet in China to minimize the influence of precipitation and evaluate the effect of temperature on the abundance and distribution of leaf wax n-alkanes. Most plant functional groups showed similar distributions in total n-alkane concentration (∑alk), carbon preference index (CPI) and average chain length (ACL). CPI values were not sensitive to temperature change. ∑alk values were negatively correlated with summer mean temperature (TJJA, June to August) for most plant functional groups. ACL values were always positively related to temperature, especially TJJA. However, large variability in ∑alk and ACL occurred within plants from the same site. Therefore, this study suggests that ∑alk and ACL can indicate relative environmental temperature, but they should not be used as quantitative proxies of paleo-temperature.
Influence of vegetation type on n-alkane composition and hydrogen isotope values from a high latitude ombrotrophic bog Org. Geochem. (IF 3.081) Pub Date : 2018-03-21 Nicholas L. Balascio, William J. D'Andrea, R. Scott Anderson, Stephen Wickler
The composition and hydrogen isotope values of leaf wax components can be powerful tools in reconstructing past climate and environments. However, interpretation of past environmental conditions from such components in sediments is complicated by species-specific influences and there is a need to better understand how vegetation type affects leaf wax composition and isotope ratios in modern environments. In this study we analyzed leaf wax (n-alkane) distributions and hydrogen isotope values of plants from a high latitude ombrotrophic bog in northern Norway. The isotopic analysis of surface water was also conducted on samples from 15 lakes along a ca. 150 km transect to contextualize the bog water isotopic composition and constrain fractionation factors among n-alkane homologues. We identified 14 different plant types growing on the bog surface, including mosses, graminoids and other herbs, sub-shrubs and a tree. n-Alkanes from the leaves of the modern plants had average chain length from 25 to 30.5, with a variety of distributions, and with the dominant compound of longer chain length (C27, C29, or C31). δD values of n-C25 to n-C33 for the vegetation samples ranged from −197‰ to −116‰, with an average of −162‰. The data also revealed that the δD values for the homologues for half of the vegetation types had ranges that were ≥20‰. Using the average isotopic value of bog water samples, −60‰, we calculated apparent fractionation factors that ranged from −66‰ to −134‰ (avg. −108 ± 22‰), similar to other sites across Europe and to a global data compilation. Our results demonstrate the range of species-specific influences on leaf wax composition and isotopic values at this site and presumably other ombrotrophic bog environments, and provide a dataset to help evaluate the influence of vegetation type on regional sedimentary leaf wax records.
Patterns of change in permanganate oxidizable soil organic matter from semiarid drylands reflected by absorbance spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry Org. Geochem. (IF 3.081) Pub Date : 2018-03-14 Carlos M. Romero, Richard E. Engel, Juliana D'Andrilli, Chengci Chen, Catherine Zabinski, Perry R. Miller, Roseann Wallander
Organic matter (OM) oxidized by slightly alkaline KMnO4, termed permanganate-oxidizable carbon (POXC), has recently emerged as a standardized indicator of active, labile carbon within soil quality frameworks. Yet, qualitative information on POXC, particularly in semiarid drylands, is very scarce. The aim of this study was to characterize POXC within three long-term field experiments in Montana, USA: (i) across a wide range of edaphic (e.g.,% clay) and management conditions (e.g., cropping intensity) (n = 148); and (ii) to identify the molecular composition of soil OM before and after KMnO4 treatment using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The content of POXC was significantly greater under perennial (382–685 mg/kg) or annual cropping (404–607 mg/kg) than fallow-wheat (359–543 mg/kg) systems. Soil OM changes, however, were equally or better expressed when considering soil organic carbon (SOC) concentration. The occurrence of POXC paralleled SOC (R = 0.87; P <0.001) and total nitrogen (TN) (R = 0.82; P <0.001) concentration, regardless of soil textural differences. The ESI FT-ICR MS analyses of aqueous soil extracts indicated that the KMnO4 reaction oxidized dissolved OM of diverse molecular character. OM molecular composition after KMnO4 treatment was enriched by strongly reduced chemical constituents (O/C <0.4) at greater condensed aromaticity (AI >0.67) and hydrogen saturation (aliphatic composition; H/C >1.5) across all heterogeneous groups (CcHhNnOoSs). Although POXC is a rapid assay widely used for characterizing soil OM dynamics, it may not provide a clear advantage over SOC concentration in semiarid drylands. The view of POXC as a merely labile, simple biodegradable OM fraction needs to be reconsidered.
Glacial-interglacial vegetation changes in northeast China inferred from isotopic composition of pyrogenic carbon from Lake Xingkai sediments Org. Geochem. (IF 3.081) Pub Date : 2018-03-10 Weiwei Sun, Enlou Zhang, Enfeng Liu, Jie Chang, Rong Chen, Ji Shen
Understanding the changes in monsoon intensity and ecosystem response at different timescales is crucial for the well-being of humans, yet the paleoclimatic interpretation of stable carbon isotope (δ13C) values from northeast China records is debatable. In this study, reported δ13C data from 76 surface soils in northeast China are compiled, and a δ13C record of pyrogenic carbon (δ13CPyC) from Lake Xingkai in northeast China since the last interglacial period is presented. The aim was to investigate the orbital timescale environmental implication of geological δ13CPyC data for northeast China. The results showed a distinct increase in δ13C values of surface soils, which correlated with increasing temperature of the warmest month. Higher temperature favored the expansion of C4 plants, while precipitation had only a weak correlation with δ13C values of surface soils in the region. On an orbital timescale, the δ13CPyC record from Lake Xingkai generally reflected paleovegetation change, suggesting that the abundance of C4 plants was relatively high during the warm periods, changing to almost purely C3 plants during the cold periods. Both modern and geological analyses suggest that the climatic factor determining the δ13C in northeast China was temperature of the warmest month. This is similar to the situation for mid-latitudes such as the Chinese Loess Plateau, in contrast to low latitudes such as southern China.
Amberene and 1-methylamberene, isolated and identified from Kuji amber (Japan) Org. Geochem. (IF 3.081) Pub Date : 2018-03-09 Tetsuaki Kawamura, Hiroyuki Koshino, Takemichi Nakamura, Yoshiyuki Nagasawa, Hidetaka Nanao, Masayuki Shirai, Shota Uesugi, Misa Ohno, Ken-ichi Kimura
Abundant labdatriene derivatives, named amberene [15,19,20-trinor-5,7,9-labdatriene (or 1,6-dimethyl-5-isopentyltetralin) (1)] and 1-methylamberene [15,20-dinor-5,7,9-labdatriene (or 1,1,6-trimethyl-5-isopentyltetralin) (2)] were isolated from the methanol extract of Kuji amber by HPLC. The structures were elucidated by spectral analyses including EIMS, 1D and 2D NMR and some previous assignments corrected.
n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa Org. Geochem. (IF 3.081) Pub Date : 2018-03-08 Andrea Baker, Joyanto Routh, Alakendra N. Roychoudhury
The sub-tropical Mfabeni fen is the only continuous coastal peat deposit that documents glacial and interglacial palaeoenvironmental conditions since the late Pleistocene (ca. 47 cal kyr BP) in southern Africa. Published bulk geochemical, biomarker and leaf wax δ13C data, along with palynology and stratigraphic studies of the Mfabeni peat sequence, renders it an ideal record for testing new palaeoreconstruction proxies. In this study, we aimed to establish the proxy potential of n-alkan-2-one (n-ket) compounds by tracing their source/origin and post-depositional diagenetic change, and if they preserve or not a robust palaeoenvironment signal that complements our understanding of palaeoclimatic variations. In the Mfabeni archive the most likely source for n-kets is via microbial decarboxylation of n+1-alkanoic acids (n-FAs) and, to a lesser degree, oxidation of same chain length n-alkanes (n-alks). The n-ket average chain length (ACLket) and n-C23 and C25ket/precursor ratios displayed a statistical significant negative relationship with the n-alk aquatic plant proxy (Paq), suggesting the source of n-kets to be submerged aquatic plants during waterlogged conditions that suppressed microbial activity during the ensuing anoxic conditions. Both the mid-chain and long chain n-ket/precursor ratios displayed predominant water level fluctuation controls, with temperature as a secondary regulator. By comparing the n-ket data with published environmental and climate reconstructions from the same core, and with geomorphology and palynological studies of the Mfabeni basin, we conclude that the n-kets show promise as a palaeoclimate proxy and can be used in conjunction with other biomarker proxies to reconstruct ancient hydrological changes in sub-tropical peatlands.
Reconstructing the palaeoecosystem and palaeodepositional environment within the Upper Devonian–Lower Mississippian Bakken Formation: a biomarker approach Org. Geochem. (IF 3.081) Pub Date : 2018-03-07 Titilade Aderoju, Stephen Bend
Biomarkers have been used extensively to correlate oil families, oil–source rock relationships and source inputs, and to help identify the palaeodepositional environment for a given geological sedimentary unit. Despite their application in characterising the Bakken Formation shales, a number of biomarkers still appear unable to provide us a consistent understanding regarding heterogeneity in source organic matter and palaeoenvironment. Here, we infer the palaeodepositional environment within the Bakken Formation shales using a biomarker suite and biomarker depth profiles. We also use the occurrence and abundance of biomarkers to define organic facies and to make inferences regarding biotic interactions drawn from modern analogues. The variations and co-occurrence of biomarkers is explained by the existence of a proposed complex algal–bacterium–bacteriovore palaeoecosystem. This approach not only helps explain the depth variation in occurrence and concentration of biomarkers and the molecular composition of associated organic matter, but also advances the use of biomarker analysis and biomarker associations to determine a complex environment-sensitive palaeoecosystem.
Sedimentary records of human activity and natural environmental evolution in sensitive ecosystems: A case study of a coral nature reserve in Dongshan Bay and a mangrove forest nature reserve in Zhangjiang River estuary, Southeast China Org. Geochem. (IF 3.081) Pub Date : 2018-03-02 Dongyi Li, Yonghang Xu, Yunhai Li, Jianjun Wang, Xijie Yin, Xiang Ye, Aijun Wang, Lian Wang
Total organic carbon (TOC), total nitrogen (TN), δ13Corg, δ15N, and grain size were measured in two sediment cores from reserves adjacent to Dongshan Bay, China. The aims were to identify organic matter (OM) sources and to evaluate the combined impact of human activity and natural environmental processes over the last ∼100 yr. In a coral reserve far from the estuary of Zhangjiang River, the average sediment accumulation rate (SAR) was 0.51 cm/yr from ca. 1839-2015, and the values of δ13Corg and C/N (atomic ratio of TOC to total N) ranged from -21.9‰ to -23.7‰ and from 5.7 to 8.1, respectively. In a mangrove forest reserve near the estuary, SAR averaged 0.49 cm/yr from ca. 1891-1985 and 2.41 cm/yr from ca. 1985-2015, and the δ13Corg and C/N values ranged from -22.7‰ to -26.3‰ and from 9.3 to 21.7, respectively. By combining a comparison plot of C/N and δ13Corg and a binary mixing model (based on δ13Corg and Norg/Corg, respectively), the major OM sources were assigned and their sedimentary contributions estimated. The sedimentary OM in the coral reserve had mainly an offshore marine authigenic signature with an average terrigenous contribution of <27%. In the mangrove forest reserve, the sedimentary OM was a mixture of terrestrial and marine sources, with an average terrigenous contribution of >56%. While input from the Zhangjiang River declined from the 1980s, urban development, deforestation and land reclamation contributed to an overall increase in the total mass accumulation rate (MAR). The coral was also severely damaged by the advent of poaching around the same time. These two factors collectively resulted in a rapid decrease in OM content and OC accumulation rate (CARorg) of the sediments until the establishment of the coral reserve in 1997. Changes in terrain and climate and the construction of water conservancy facilities were traditionally the primary factors responsible for changes in the sedimentary environment of the mangrove forest reserve. Since the 1980s, however, the expansion of the mangrove forest along with human activity has led to rapid increases in MAR and CARorg; whilst, protective measures and a favourable climate have also promoted the growth of the mangrove forest.
Effect of simulated diagenesis on the compositions, chemical stability and sorption properties of natural and engineered organic matter with different mineral contents Org. Geochem. (IF 3.081) Pub Date : 2018-03-02 Lanfang Han, Ziying Wang, Marco Keiluweit, Yan Yang, Ke Sun, Zheyun Zhang, Baoshan Xing
Assessing the diversity of lipids formed via Fischer-Tropsch-type reactions Org. Geochem. (IF 3.081) Pub Date : 2018-03-02 Helge Mißbach, Burkhard C. Schmidt, Jan-Peter Duda, Nils Keno Lünsdorf, Walter Goetz, Volker Thiel
Lipid biomarkers are commonly used for tracking life through Earth’s history and are also gaining in importance in the search for extraterrestrial life. However, some lipids may also be formed in-situ via abiotic Fischer-Tropsch-type (FTT) reactions. These processes have been considered as a source of prebiotic organic matter. Here we report on a FTT synthesis experiment performed under hydrothermal conditions, focusing on more complex, previously undescribed FTT products that may potentially mimic biological signals. The experiment was carried out in stainless steel reactors by heating aqueous solutions of oxalic acid mixed with montmorillonite to 175 °C for 3 days. Organic extracts of the products and extraction residues were analyzed by gas chromatography-mass spectrometry (GC-MS) and combustion-infrared detection, attenuated total reflectance Fourier transform infrared spectroscopy and pyrolysis GC-MS, respectively. FTT reactions led to a number of biomarker-like lipids such as linear and methyl-branched alkanes and alkanols as well as n-alkanoic acids. However, FTT reactions a priori produce unimodal chain length distributions and isomeric mixtures of methyl-branched compounds, as opposed to biolipids which typically show preferences of individual homologs and/or isomers. Experimental maturation of an immature kerogen demonstrated how these biological signatures may pass into unimodal distributions similar to FTT products by thermal overprint. Therefore, discrimination of biologically derived compounds from FTT organics may become increasingly problematic with ongoing thermal maturation. FTT extraction residues contained organic material, possibly in form of an organic polymer. However, it remains to be tested whether these residual organics can evolve into more complex macromolecular networks.
Fatty acids and other biomarkers in two Early Jurassic concretions and their immediate host rocks (Lias δ, Buttenheim clay pit, Bavaria, Germany) Org. Geochem. (IF 3.081) Pub Date : 2018-03-02 Volker Thiel, Michael Hoppert
Two carbonate- and phosphate-rich concretions from Early Jurassic strata at Buttenheim (Bavaria, Germany) and their immediate claystone host rocks were analysed to study the mechanisms of biomarker preservation in concretions. Superimposed on a common sedimentary background signal, distinctive biomarkers reflect microbial processes involved in concretion formation. When normalized to the HCl-insoluble insoluble rock portion, concretions show ∼1.5 to 3 times higher TOC values as their host rocks, but identical absolute quantities of most extractable hydrocarbons. In contrast, n-fatty acids and α,ω-dicarboxylic acids are by an order more abundant as compared to the host rocks, suggesting an enhanced preservation or selective accumulation mechanism for carboxylic acids in the concretions. Further, linear and iso-/anteiso-fatty acids with unusually short carbon chains (as low as C8) are exclusively observed in the concretions and are interpreted as biodegradation products of lipids derived from in-situ bacteria. Likewise, strongly enhanced neohop-13(18)-enes and hopanoic acids with more ‘immature’ isomer distributions indicate an in-situ contribution of microbial biomass during formation of the concretions. Pyrolysis released ample additional saturated short-chain n-fatty acids from the kerogens of the concretions, but not from the host rocks. We hypothesize that these compounds are remnants of locally accumulated OM rich in fatty acyl moieties (e.g. plant polymers or triglyceride storage lipids) whose hydrolysis products were initially precipitated as fatty acid salts (‘adipocere’). During shallow burial, these substances may have delivered a steadily flowing energy source for sedimentary microorganisms, with their anaerobic decomposition supplying both, carbonate and metal ions. In concert with other factors, this may have promoted carbonate precipitation and thus, the formation of the Buttenheim concretions.
Different temperature dependence of the bacterial brGDGT isomers in 35 Chinese lake sediments compared to that in soils Org. Geochem. (IF 3.081) Pub Date : 2018-02-20 Xinyue Dang, Weihua Ding, Huan Yang, Richard D. Pancost, B. David A. Naafs, Jiantao Xue, Xiao Lin, Jiayi Lu, Shucheng Xie
During the last decade, the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in lacustrine sediments has been widely used to reconstruct past variations in lake temperature. A prerequisite for the application of brGDGTs to lacustrine paleoclimate reconstructions is to understand the sources of brGDGTs in lake systems and the processes that influence their distribution. In this study, we investigated the distribution of brGDGTs in core-top sediments from 35 lakes across China, with a broad mean annual air temperature (MAAT) range, but a constrained pH range, to explore the effect of temperature. The results reveal a contrasting response of MBT'5ME and MBT'6ME to temperature in lake environments compared to that in soils. The sedimentary distributions of 5- and 6-methyl brGDGTs exhibit different relationships with temperature, with most of the latter being correlated to MAAT while the former responding to temperature by only hexamethylated compounds. In both global and Chinese soils, most 6-methyl brGDGTs have no relationship with MAAT but the distribution of 5-methyl brGDGTs is correlated with MAAT. The different behaviors suggest that communities producing 5- or 6-methyl brGDGTs might be different in lakes and soils. In addition, in lakes from cold regions (MAAT < 5 °C), the brGDGT distributions correlate only with warm season temperatures (April to October) but exhibit no correlation with cold seasons, suggesting a seasonal bias in brGDGT production in these lakes. This bias towards the warm season is not found in lakes from warmer regions (MAAT > 5 °C). Based on these results we propose new temperature calibrations for paleotemperature reconstructions in Chinese alkaline lakes.
Distribution of long chain alkyl diols along a south-north transect of the northwestern Pacific region: Insights into a paleo sea surface nutrient proxy Org. Geochem. (IF 3.081) Pub Date : 2018-02-15 Jong-Ku Gal, Jung-Hyun Kim, Kyung-Hoon Shin
Long chain alkyl diols (LCDs) have previously been used to develop several diol indices as a proxy for upwelling or stratification conditions and sea surface temperatures. However, there is still a lack of data on the distribution of LCDs in the Pacific Ocean, which calls into question to what extent these diol indices can serve as a proxy in the Pacific Ocean. Hence, we investigated the distribution of LCDs in suspended particulate matter (SPM) samples collected in July 2015 along a south-north transect ranging from the East Sea of Korea to the Bering Sea in the northwestern Pacific region. Our results showed that both saturated and unsaturated C28 and C30 1,14-diols were dominant in most SPM samples investigated. The principal component analysis (PCA) results of the SPM data revealed that the C28:1 and C30:1 1,14-diols were positively associated with nutrient concentrations. However, only the C28:1 1,14-diol in addition to the C28 1,14-diol was positively associated with nutrient concentrations in the global surface sediment data set previously published, while the C30:1 1,14-diol was related to sea surface temperature (SST). Based on these observations, we defined a new diol index, i.e., the nutrient diol index (NDI), which included the C28 1,14-diol as well as the C28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C32 1,15-diol, in the denominator. The NDI was positively associated with the ocean atlas values of surface water phosphate and nitrate concentrations for the SPM and surface sediment data sets. Accordingly, the results of our study showed that the NDI has potential as a quantitative nutrient proxy for estimating sea surface phosphate (or nitrate) changes. Further studies should be undertaken in order to assess the applicability of the NDI as a paleo-nutrient proxy for confidently reconstructing surface water nutrient variations in the past, by analyzing more SPM, surface sediments, and sediment cores collected from various oceanic settings.
Biodegradation in the Nkh 3-4 reservoir at Vankor Field (West Siberia Basin, Russia) is strongly controlled by rock properties Org. Geochem. (IF 3.081) Pub Date : 2018-02-12 Nikolay V. Oblasov, Ivan V. Goncharov, Vadim V. Samoilenko, Ger W. van Graas
The Vankor Field, located in the West Siberia Basin, Russia, contains oils at various levels of biodegradation. A study of reservoir oils and rock extracts from the Lower Cretaceous sandstones in the Nkh 3-4 interval reveals that increasing biodegradation when moving from the gas-oil contact (GOC) towards the oil-water contact (OWC) is strongly controlled by rock properties. Biodegradation is most advanced where the OWC is located in the high permeability reservoir interval. High permeability rocks also allow for more extensive upward mixing of biodegraded oil. Changes in the oil and gas fractions due to biodegradation also affect the gas-oil ratio as dry gas is less soluble in heavy oil. This explains the presence of reservoir oil saturated with gas near the OWC. The latter also can be partly due to the admixture of methane through methanogenic processes at the OWC. Upward movement of microbial gas bubbles enhances the mixing of biodegraded oil in the high permeability reservoir intervals.
Determination of position-specific carbon isotope ratios of propane from natural gas Org. Geochem. (IF 3.081) Pub Date : 2018-02-12 Yun Li, Lin Zhang, Yongqiang Xiong, Shutao Gao, Zhiqiang Yu, Ping'an Peng
Experimental investigation of the role of rock fabric in gas generation and expulsion during thermal maturation: anhydrous closed-system pyrolysis of a bitumen-rich Eagle Ford Shale Org. Geochem. (IF 3.081) Pub Date : 2018-02-12 Deyong Shao, Geoffrey S. Ellis, Yanfang Li, Tongwei Zhang
Gold-tube pyrolysis experiments were conducted on miniature core plugs and powdered rock from a bitumen-rich sample of Eagle Ford Shale to investigate the role of rock fabric in gas generation and expulsion during thermal maturation. The samples were isothermally heated at 130, 300, 310, 333, 367, 400, and 425 °C for 72 h under a confining pressure of 68.0 MPa, corresponding to six levels of induced thermal maturity: pre-oil generation (130 °C/72 h), incipient oil/bitumen generation (300 and 310 °C/72 h), early oil generation (333 °C/72 h), peak oil generation (367 °C/72 h), early oil cracking (400 °C/72 h), and late oil cracking (425 °C/72 h). Experimental results show that gas retention coupled with compositional fractionation occurs in the core plug experiments and varies as a function of thermal maturity. During the incipient oil/bitumen generation stage, yields of methane through pentane (C1–C5) from core plugs are significantly lower than those from rock powder, and gases from core plugs are enriched in methane. However, the differences in C1–C5 gas yield and composition decrease throughout the oil generation stage, and by the oil cracking stage no obvious compositional difference in C1–C5 gases exists. The decrease in the effect of rock fabric on gas yield and composition with increasing maturity is the result of an increase in gas expulsion efficiency. Pyrolysis of rock powder yields 4–16 times more CO2 compared to miniature core plugs, with δ13CCO2 values ranging from –2.9‰ to –0.6‰, likely due to carbonate decomposition accelerated by reactions with organic acids. Furthermore, lower yields of gaseous alkenes and H2 from core plug experiments suggest that the rock fabric plays a role in promoting hydrogenation reactions of alkenes.
Carbon isotope ratios of coccolith–associated polysaccharides of Emiliania huxleyi as a function of growth rate and CO2 concentration Org. Geochem. (IF 3.081) Pub Date : 2018-02-11 Elise B. Wilkes, Renee B.Y. Lee, Harry L.O. McClelland, Rosalind E.M. Rickaby, Ann Pearson
The calcite plates, or coccoliths, of haptophyte algae including Emiliania huxleyi are formed in intracellular vesicles in association with water–soluble acidic polysaccharides. These coccolith–associated polysaccharides (CAPs) are involved in regulating coccolith formation and have been recovered from sediment samples dating back to ∼180 Ma. Paired measurements of the carbon isotopic compositions of CAPs and coccolith calcite have been proposed as a novel paleo–pCO2 barometer, but additional proxy validation and development are still required. Here we present culture results quantifying carbon isotopic offsets between CAPs and other cellular components: bulk organic biomass, alkenones, and calcite. E. huxleyi was grown in nitrate–limited chemostat experiments at growth rates (µ) of 0.20–0.62/d and carbon dioxide concentrations of 10.7–17.6 µmol/kg. We find that CAPs are isotopically enriched by 4.5‰ to 10.1‰ relative to bulk organic carbon, exhibiting smaller isotopic offsets at faster growth rates and lower CO2 concentrations. This variability suggests that CAPs record a complementary signature of past growth conditions with different sensitivity than alkenones or coccolith calcite. By measuring the isotopic compositions of all three molecules and minerals of self-consistent origin, the ratio µ/[CO2(aq)] may be reconstructed with fewer assumptions than current approaches.
Modern sediment records of stanol to sterol ratios in Lake Suigetsu, Japan: An indicator of variable lacustrine redox conditions Org. Geochem. (IF 3.081) Pub Date : 2018-02-10 Masatoshi Nakakuni, Junichi Kitano, Hitoshi Uemura, Shuichi Yamamoto
We tested the usefulness of the 5 α (H)-stanol/ Δ 5-sterol ratios obtained by the tetramethylammonium hydroxide (TMAH) thermochemolysis gas chromatography–mass spectrometry (GC–MS) method used to reconstruct redox events in marine sediments as a redox tracer in lake sediments with cores from Lake Suigetsu, Japan. The lake was previously a freshwater lake and is now brackish owing to excavation works conducted during the 17th century. Other redox tracer analyses (e.g., based on farnesol) have shown that C26(nor-24) Δ22/C26(nor-24) Δ5,22 (24-nordehydrocholestanol/24-nordehydrocholesterol), C28(24Me) Δ22/C28(24Me) Δ5,22 (diatomstanol/diatomsterol), and C27 Δ22/C27 Δ5,22 (22-dehydrocholestanol/22-dehydrocholesterol) ratios have risen (∼ 0.53) following strong anoxic conditions that formed in Lake Suigetsu (A.D. 1848 and 1935). This study is the first to show that 5 α (H)-stanol/ Δ5-sterol ratios determined using the TMAH method could be useful as a redox tracer in lacustrine environments. In particular, the ratio of diatomsterol, which is detected in both brackish and fresh water in relatively high abundance, has the potential to be a very useful tracer of redox events. On the other hand, the other 5 α(H)-stanol/Δ5-sterol ratios (C27Δ0/C27Δ5 [cholestanol/cholesterol], C28(24Me)Δ0/C28(24Me)Δ5 [campestanol/campesterol], and C29(24Et) Δ0/C29(24Et)Δ5 [sitostanol/sitosterol] ratios) showed no increasing trends, suggesting the influence of terrestrial inputs from degraded products.
Dissolved Organic Matter Compositions in 0.6–3.4 km Deep Fracture Waters, Kaapvaal Craton, South Africa Org. Geochem. (IF 3.081) Pub Date : 2018-02-09 Thomas L. Kieft, Clifford C. Walters, Meytal B. Higgins, Anthony S. Mennito, Catherine F.M. Clewett, Verena Heuer, Michael J. Pullin, Sarah Hendrickson, Esta van Heerden, Barbara Sherwood Lollar, Maggie C.Y. Lau, T.C. Onstott
The composition of dissolved organic matter (DOM) in extremely deep groundwaters has rarely been investigated. Focusing on deep fracture waters in billion year old rocks in South Africa, we hypothesized the DOM would be predominantly derived from in situ microbial sources, consistent with H2-driven lithoautotrophic ecosystems previously documented in these fluids. We collected groundwater from 0.6–3.4 km depths via boreholes in mines, and characterized the DOM by a variety of analytical methods. Dissolved organic carbon concentrations ranged from 0.25 to 4.9 mg C/L. Low molecular weight organic acids (formate, acetate, lactate, propanoate) and amino acids (after hydrolysis) accounted for varying fractions of the total dissolved organic carbon. UV spectrophotometry, scanning fluorimetry, and nuclear magnetic resonance (NMR) spectra indicated lower concentrations of aromatic compounds than are commonly found in surface waters and shallow groundwaters. Molecular weights of aromatic compounds measured by size-exclusion chromatography were dominantly 500-1000 Da. Scanning fluorimetry revealed humic substances in two shallow (578 and 1300 m) samples; deeper samples showed peaks indicating aromatic amino acids. Intensive study of one sample from a previously sampled borehole in Beatrix mine resulted in large NMR peaks for aliphatics and carboxyls with lesser aromatics peaks; negative electrospray ion Fourier transform ion cyclotron resonance-mass spectrometry detected protein-, lipid-, and lignin-like compounds. These results are consistent with DOM in the form of microbial metabolites, and modified cell components (membrane lipids and proteins). These findings support previous geomicrobiological reports of chemolithoautotrophic microbial ecosystems functioning in isolation from photosynthetic primary production.
Phenolic compounds in oil-bearing fluid inclusions: implications for water-washing and oil migration Org. Geochem. (IF 3.081) Pub Date : 2018-02-06 Carl A. Peters, Christian Hallmann, Simon C. George
In comparison to genetically related crude oils, oil bearing fluid inclusions are often relatively enriched in polar compounds. Phenolic, carbazolic and benzocarbazolic compounds, which are part of the polar fraction, are characterised by elevated solubilities in water, rendering them potentially useful indicators for oil migration and entrapment, as well as water-washing of oil-bearing fluid inclusions. These compounds were investigated in a set of crude oils and fluid inclusions, and methods were modified to allow for the direct and simultaneous analysis of phenolic and carbazolic compounds. By avoiding chromatographic column fractionation, this new analytical approach allows for faster analyses and higher throughput. Phenolic and carbazolic compounds were found to be present in all investigated crude oil samples, yet they represent only minor constituents of oil-bearing fluid inclusions. A newly described phenol–cresol index (PCI) is systematically elevated (> 0.7) in oil-bearing fluid inclusions, while crude oils are characterised by significantly lower values (< 0.5). Supported by observations of increasing PCI during progressive water-washing experiments, our data suggest oil-water interaction as a possible reason for the elevated PCI in oil-bearing fluid inclusions. Moreover, some of the phenolic compounds detected in oil-bearing fluid inclusion samples may derive from co-occurring aqueous fluid inclusions, whose simultaneous co-extraction cannot be avoided on this spatial scale. Our study highlights the importance of oil-water interaction in the subsurface and offers new techniques to aid understanding of this commercially relevant phenomenon.
Seasonal variations of leaf wax n-alkane molecular composition and δD values in two subtropical deciduous tree species: Results from a three-year monitoring program in central China Org. Geochem. (IF 3.081) Pub Date : 2018-02-03 Xianyu Huang, Bingyan Zhao, Kui Wang, Yu Hu, Philip A. Meyers
We investigated the seasonal variations of the molecular and hydrogen isotopic compositions of leaf cuticular n-alkanes (δDalk) in two subtropical deciduous tree species (Quercus chenii, Liquidambar formosana) over a three-year period. Average chain length values of long chain n-alkanes in both species are lower in young leaves than in mature ones. The δDalk values in both tree species show seasonal fluctuations that can be as large as 78‰ in a single year. The seasonal changes in molecular and isotopic compositions suggest that leaf wax n-alkanes in these subtropical deciduous tree species integrate environmental information over a large portion of the annual growth cycle in response to fluctuations in environmental stresses (e.g. storms, droughts). In addition, the changes reveal that seasonal patterns of δDalk can differ between different deciduous species at the same location and between different years. Spring leaves collected in 2014 had more negative δDalk values than those collected in 2015 and 2016, possibly responding to differences in annual precipitation D/H ratios and associated atmospheric circulation. These preliminary results highlight that extended seasonal monitoring of leaf wax δDalk values can improve their application to reconstructing paleohydrological histories.
Waxy bitumen stranding in southern Australia: A geochemical study of multiple oil families and their likely origins Org. Geochem. (IF 3.081) Pub Date : 2018-02-02 Dianne S. Edwards, David M. McKirdy, Steven J. Rowland, David J. Heath, Patricia S. Gray
Reports of bitumen stranding on the ocean beaches of southern Australia date back to the early days of European settlement. Previous investigations have shown that this ‘coastal bitumen’ comprises three categories of stranded petroleum: waxy bitumen, asphaltite and oil slicks. All three varieties are physically and chemically distinct from each other, and bear no geochemical resemblance to any indigenous Australian crude oil. This study focuses on the most common variety, waxy bitumen, which accounted for 90% of the strandings on six South Australian beaches repeatedly surveyed during 1991–1992. Geochemical analysis of 96 individual specimens collected from these survey sites and other beaches in South Australia and western Victoria has shown them to be variously weathered high-wax crude oils of paraffinic to aromatic-intermediate bulk composition. Elemental, isotopic and biomarker differences allow their assignment to at least five oil families with inferred source facies that range from deep freshwater lacustrine through paludal and deltaic to euxinic marine, possibly deposited within different sedimentary basins. Family 1, 2 and 3 waxy bitumens all contain biomarkers derived from the freshwater alga Botryococcus sp. and tropical angiosperms (notably dipterocarps). Similar biomarker assemblages are unknown in Australian sedimentary basins but are common in Cenozoic crude oils and source rocks throughout western Indonesia. Family 4 waxy bitumens lack these biomarkers, but do contain dinosterane and 24-n-propylcholestane, indicative of a marine source affinity, while the carbon isotopic signatures and high pristane/phytane (Pr/Ph) ratios of Family 5 waxy bitumens are consistent with their origin from coal-rich source rocks deposited in fluvial to deltaic sedimentary successions. The majority of these waxy bitumens represent an oceanic influx of non-indigenous, Southeast Asian crude oils carried into the waters of southern Australia by the Leeuwin Current. Although they are likely to originate from natural seepage within the Indonesian Archipelago, it is unknown whether the parent oils emanate from submarine seeps or from inland seepages which are then carried to the sea by rivers. The common practice of tanker cleaning operations in the Java and Banda seas may augment the supply of natural bitumen to the beaches of Australia.
Biotic and abiotic degradation of the sea ice diatom biomarker IP25 and selected algal sterols in near-surface Arctic sediments Org. Geochem. (IF 3.081) Pub Date : 2018-02-02 Jean-François Rontani, Simon T. Belt, Rémi Amiraux
The organic geochemical IP25 (Ice Proxy with 25 carbon atoms) has been used as a proxy for Arctic sea ice in recent years. To date, however, the role of degradation of IP25 in Arctic marine sediments and the impact that this may have on palaeo sea ice reconstruction based on this biomarker have not been investigated in any detail. Here, we show that IP25 may be susceptible to autoxidation in near-surface oxic sediments. To arrive at these conclusions, we first subjected a purified sample of IP25 to autoxidation in the laboratory and characterised the oxidation products using high resolution gas chromatography–mass spectrometric methods. Most of these IP25 oxidation products were also detected in near-surface sediments collected from Barrow Strait in the Canadian Arctic, although their proposed secondary oxidation and the relatively lower abundances of IP25 in other sediments probably explain why we were not able to detect them in material from other parts of the region. A rapid decrease in IP25 concentration in some near-surface Arctic marine sediments, including examples presented here, may potentially be attributed to at least partial degradation, especially for sediment cores containing relatively thick oxic layers representing decades or centuries of deposition. An increase in the ratio of two common phytoplanktonic sterols – epi-brassicasterol and 24-methylenecholesterol – provides further evidence for such autoxidation reactions given the known enhanced reactivity of the latter to such processes reported previously. In addition, we provide some evidence that anaerobic biodegradation processes also act on IP25 in Arctic sediments. The oxidation products identified in the present study will need to be quantified more precisely in downcore records in the future before the effects of degradation processes on IP25-based palaeo sea ice reconstruction can be fully understood. In the meantime, a brief overview of some previous investigations of IP25 in relatively shallow Arctic marine sediments suggests that overlying climate conditions were likely dominant over degradation processes, as evidenced from often increasing IP25 concentration downcore, together with positive relationships to known sea ice conditions.
Main factors influencing the formation of thermogenic solid bitumen Org. Geochem. (IF 3.081) Pub Date : 2018-02-02 Rui Lei, Yongqiang Xiong, Yun Li, Li Zhang
Highly mature solid bitumen, a residue of oil cracking, is widespread in the lower Paleozoic paleo-oil reservoirs of southern China. Solid bitumen is not a simple, pure component, but rather a compositionally and structurally variable mixture of materials. This study investigated the formation of thermogenic solid bitumen and the effects of oil composition and reservoir environment. Seven series of gold-tube pyrolysis experiments were conducted: three used the main fraction groups (i.e., saturated, aromatic, and resin+asphaltene fractions) of crude oil to evaluate the effect of oil composition on the formation of solid bitumen during cracking; the other four tested the effects of water and pressure in reservoirs by simulating the cracking of crude oil under different reservoir conditions. Quantitative analyses of pyrolytic products (including methane, C2–C5 gaseous hydrocarbons, C6–C13 light hydrocarbons, C13+ heavy hydrocarbons, and solid bitumen) indicated that thermogenic solid bitumen formed at different stages of oil cracking, and its formation was clearly affected by oil composition. In contrast, water and pressure in the reservoir had little effect on the formation of solid bitumen.
Sources, distributions, and burial efficiency of terrigenous organic matter in surface sediments from the Yellow River mouth, northeast China Org. Geochem. (IF 3.081) Pub Date : 2018-01-31 Dayang Sun, Jianhui Tang, Yuxin He, Weisen Liao, Yongge Sun
Organic geochemical studies on surface sediments from Laizhou Bay, the Yellow River mouth, northeast China, were conducted to reveal the sources, distributions, and burial efficiency of terrigenous OC exported by the Yellow River. Bulk OC contents and δ13Corg values from 204 samples show that terrigenous OC mainly accumulates in two muddy patches, basically controlled by hydrodynamic conditions. Distributions and stable carbon isotopic compositions of individual n-alkanes from 45 samples suggest that short-chain n-alkanes, with even carbon-number predominance, are most likely microbial products from degradation of riverine fossil OC, whereas long-chain odd-numbered n-alkanes (n-C27, n-C29 and n-C31) and long-chain even-numbered n-alkanes (n-C26,n-C28 and n-C30) are representative of riverine soil OC and fossil OC, respectively. Integrated with previous reports of ∼100% burial efficiency of soil OC in the Yellow River mouth, concentration ratios of n-C27+29+31 to OC and n-C26+28+30 to n-C27+29+31 from the riverine particles to surface sediments in Laizhou Bay were constructed to indicate relative terrigenous OC contributions and fossil OC burial efficiency, respectively. The results suggest that the terrigenous OC contribution ranges from 14% to 57% in Laizhou Bay, with the highest values in the two muddy patches as well as in areas near the river mouth. The fossil OC burial efficiency ranges from 36% to 76%, with an TOC-weighted mean value of 42%. The 58% fossil OC lost during the transport and burial processes in the Yellow River mouth may provide a significant CO2 source.
Organic matter in sediments of a tropical and upwelling influenced region of the Brazilian continental margin (Campos Basin, Rio de Janeiro) Org. Geochem. (IF 3.081) Pub Date : 2018-01-31 Lívia G.M.S. Cordeiro, Angela L.R. Wagener, Renato S. Carreira
Lipid biomarkers [fatty acids (FAs), sterols and alcohols] and carbon stable isotopes (δ13C values) in surface sediments from 9 cross shelf transects (25 to 3000 m water depth) from the Campos Basin, SE Brazilian continental margin were analyzed. The aim was to investigate the link between the prevailing regional specific oceanographic conditions (upwelling events, intrusion of cold and nutrient-rich water, low river input) and the nature and distribution of organic matter (OM) in the basin. A general predominance of OM from autochthonous processes, but with a relevant spatial gradient in the quality and quantity of the sedimentary OM, was observed. On the shelf (< 150 m), concentrations of lipids were usually low, except in areas influenced by upwelling, but the presence of labile compounds suggest the occurrence of fresh OM in the sediment. The export of continental OM was observed only in shelf sediments near the Paraíba do Sul River. The upper and middle slope (400 to 1300 m) exhibited the highest concentrations of total organic carbon (TOC) and lipids, but lipid biomarkers suggest the presence of OM with a high degree of bacterial degradation. This may result from the export of material from shallow areas, possibly due to the action of eddies and meandering of the Brazil Current and bottom currents in the region. On the lower slope (1900 to 3000 m), only the more recalcitrant compounds were above detection limit. The presence of labile lipids in high amount in the shelf and slope suggests the presence of OM with a high potential for supplying the food requirements of heterotrophic organisms in the sediment, which may in turn have a major influence on the ecology of benthic communities.
The importance of mass accuracy in selected ion monitoring analysis of branched and isoprenoid tetraethers Org. Geochem. (IF 3.081) Pub Date : 2018-01-31 Nina Davtian, Edouard Bard, Guillemette Ménot, Yoann Fagault
Among the new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs), the BIT index (Branched and Isoprenoid Tetraether index) is one of the most difficult to determine accurately, as shown by two round-robin GDGT studies. Sensitivity to mass spectrometer settings and tuning, and a diversity of mass spectrometry techniques may explain the relatively large observed interlaboratory scatter. However, the mass defect difference between crenarchaeol and branched GDGTs (brGDGTs) has never been specifically scrutinized. In this study, we analyzed five sediment samples with contrasting BIT values using about 60 m/z values to assess the shape of GDGT peaks using selected ion monitoring. We then assessed the biases on relative GDGT signals and mass spectrometry-derived BIT values under two scenarios which ignore the systematic mass defect difference between crenarchaeol and brGDGTs. Our results show that approximate mass selection for GDGT analysis using selected ion monitoring generates losses of relative GDGT signals of up to 36%. The observed effects on BIT values are maximal for intermediate BIT values, with shifts of BIT values of ±0.1 unit. The shifts of BIT values due to approximate mass selection are thus not negligible compared to the interlaboratory scatter evidenced by the latest round-robin GDGT study.
Occurrence of extended tetracyclic polyprenoid series in crude oils Org. Geochem. (IF 3.081) Pub Date : 2018-01-31 Bruno Q. Araújo, Francisco R. Aquino Neto, Débora A. Azevedo
A homologous series of tetracyclic polyprenanes from C24 to C41 was identified in crude oils from Espírito Santo Basin (Brazil) by comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry (GC×GC–TOFMS) and gas chromatography–tandem mass spectrometry (GC–MS-MS) analyses. These products were separately characterized by elution order in first- and second-dimension retention times (1tR and 2tR, respectively), time-of-flight mass spectra (TOFMS), MRM chromatograms, product ion spectra including diagnostic ions at m/z 191, m/z 259 and m/z 149 + 14n (n = 0 to 17), the latter a key mass fragment for the evaluation of side chain extension at the C-18 position in tetracyclic polyprenoids. The tetracyclic compounds were thus identified as scalarane-like structures related to abundant C30 tetracyclic polyprenoids, an important indicator of input from fresh to brackish water paleoenvironments.
Sterol identification in floating Arctic sea ice algal aggregates and the Antarctic sea ice diatom Berkeleya adeliensis Org. Geochem. (IF 3.081) Pub Date : 2018-01-31 Simon T. Belt, Thomas A. Brown, Lukas Smik, Philipp Assmy, C.J. Mundy
A number of common sterols were identified in sea ice diatoms from the Arctic and the Antarctic. The main sterols in floating sea ice algal aggregates collected from Resolute Passage (Canadian Arctic) and Nansen Basin (North Svalbard) in 2012 were 22E-dehydrocholesterol, cholesterol, epi-brassicasterol, 24-methylenecholesterol and 24-ethylcholesterol, although the distribution varied between the two locations, likely reflecting compositional differences in diatom taxa. The three major sterols in cells of Berkeleya adeliensis picked from a melted sea ice core collected from Ryder Bay in the Antarctic Peninsula in 2014, were 24-ethylcholesterol, cholesterol and 22E-dehydrocholesterol. We suggest that certain sea ice diatoms may thus contribute to the sedimentary budget of common sterols in seasonally sea ice-covered locations following ice melt.
The last forests on Antarctica: Reconstructing flora and temperature from the Neogene Sirius Group, Transantarctic Mountains Org. Geochem. (IF 3.081) Pub Date : 2018-01-12 Rhian L. Rees-Owen, Fiona L. Gill, Robert J. Newton, Ruza F. Ivanović, Jane E. Francis, James B. Riding, Christopher H. Vane, Raquel A. Lopes dos Santos
Fossil-bearing deposits in the Transantarctic Mountains, Antarctica indicate that, despite the cold nature of the continent’s climate, a tundra ecosystem grew during periods of ice sheet retreat in the mid to late Neogene (17–2.5 Ma), 480 km from the South Pole. To date, palaeotemperature reconstruction has been based only on biological ranges, thereby calling for a geochemical approach to understanding continental climate and environment. There is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm-conifer vegetation, or whether by this point conifers had disappeared from the continent. In order to address these questions, we have analysed, for the first time in sediments of this age, plant and bacterial biomarkers in terrestrial sediments from the Transantarctic Mountains to reconstruct past temperature and vegetation during a period of East Antarctic Ice Sheet retreat. From tetraether lipids (MBT’/CBT palaeothermometer), we conclude that the mean continental summer temperature was ca. 5 °C, in agreement with previous reconstructions. This was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter. Biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants, moss and algal mats growing in microenvironments in a glacial outwash system. Abietane-type compounds were abundant in some samples, indicating that conifers, most likely Podocarpaceae, grew on the Antarctic continent well into the Neogene. This is supported by the palynological record, but not the macrofossil record for the continent, and has implications for the evolution of vegetation on Antarctica.
Choosing a suitable εw-p by distinguishing the dominant plant sources in sediment records using a new Pta index and estimating the paleo-δDp spatial distribution in China Org. Geochem. (IF 3.081) Pub Date : 2018-01-11 Zheng Wang, Hu Liu, Yunning Cao
The hydrogen isotopic composition of plant wax (δDwax) inherits the isotopic values of the source water and isotopic fractionations that take place in lipid biosynthesis. The δDwax values of n-alkanes or n-acids in sediment records are considered to be effective paleo-δD for precipitation proxies for different timescales. However, the vegetation effect also contributes substantially to the apparent fractionation values (εw-p) between the precipitation and plant. Therefore, a method for evaluating the dominant plant source in the sediment records is key for reconstructing a reliable paleo-δDp record. In this study, based on field data from the extremely arid Qaidam Basin and the semi-arid central Chinese Loess Plateau (CLP), a plant type index (Pta), based on the traditional Paq and n-alkane C31/C29 index, was used to evaluate the dominant plant type that contributed to the sediments. In the arid and semi-arid region, the Pta values of the aquatic plants were < 90, while the shrub Pta ranged from 90 to 180 and the grass Pta was > 180. A test on contemporary soils and surface lake sediments also showed that the Pta index could estimate the regional vegetation distribution in modern environments. We also evaluated the dominant n-alkane source for four sediment records in China during the last millennium and reconstructed a regional paleo-δDp by choosing a different εw-p. The estimated paleo-δDp values showed a good correlation with the modern regional summer δDp. We suggest that a large number of field studies are needed to improve the Pta index for quantitatively calculating the precise ratios for various plant sources. Such data would play a key role in improving hydrogen isotopic hydroclimatic research.
Potential recycling of thaumarchaeotal lipids by DPANN Archaea in seasonally hypoxic surface marine sediments Org. Geochem. (IF 3.081) Pub Date : 2017-12-26 Yvonne A. Lipsewers, Ellen C. Hopmans, Jaap S. Sinninghe Damsté, Laura Villanueva
Steroid dissipation and formation in the course of farmyard manure composting Org. Geochem. (IF 3.081) Pub Date : 2017-12-23 Katharina Prost, Pia Loretta Bradel, Eva Lehndorff, Wulf Amelung
Steroids are used as biomarkers for tracing faecal material in the environment, but their dissipation behaviour in the course of composting has remained unclear. To assess their stability, we tracked the fate of Δ5-sterols, 5α-stanols, 5β-stanols, epi-5β-stanols, stanones and bile acids after 0, 7, 14, 28, 56, 112 and 168 days and after 0 and 168 days of composting cattle and horse farmyard manure, respectively. After composting, extractable steroid content decreased by 93.8-99.9% for cattle manure and by 54.0-100% for horse manure relative to the initial amount. The loss was, particularly pronounced for bile acids in the cattle manure compost. Our findings challenge the assumption that bile acids are generally more resistant towards degradation than other steroids. Contrary to the bile acids, the Δ5-sterol content did not decrease constantly, but showed a temporary increase, pointing to a delayed release from straw (β-sitosterol, stigmasterol and cholesterol) or to temporary production by fungi (cholesterol). Similarly, there was no continued loss of cholesterol transformation products (5α-cholestanol, cholestanone and epicoprostanol); they either increased temporarily or showed delayed degradation. These changes in steroid patterns complicate the identification of compost as faecal matter by way of commonly used steroid ratios and stress the importance of additional bile acid analysis in geo-archaeological research. Nevertheless, slower dissipation and smaller relative loss of epi-5β-stanols vs. 5β-stanols increased the value of r = epi-5β-stigmastanol/5β-stigmastanol + epicoprostanol/coprostanol during composting and allowed differentiation between fresh farmyard manure, and immature and mature compost.
Taphonomy and chemotaxonomy of Eocene amber from southeastern Australia Org. Geochem. (IF 3.081) Pub Date : 2017-12-21 Andrew J. Coward, Chris Mays, Antonio F. Patti, Jeffrey D. Stilwell, Luke A. O'dell, Pedro Viegas
Amber is a complex, organic polymer that offers unparalleled utility as a preservation medium, providing insights into past organisms and environments. However, under specific circumstances, this information can be compromised through alteration of the amber structure. Understanding the degradation of amber in the geosphere could improve prospecting techniques and maximise the quality and validity of chemical information from altered samples. This study analysed 114 amber samples retrieved from two new Eocene Australian deposits at Strahan, Tasmania and Anglesea, Victoria using a combination of attenuated total reflectance Fourier transform infrared and solid-state 13C cross-polarised magic angle spinning nuclear magnetic resonance spectroscopy. Results identified both Class Ib polylabdanoid and Class II cadinene-based amber. The presence of Class II amber in Australia suggests one of two possibilities: (1) a local Dipterocarpaceae source, the primary producer of Class II resins, despite the absence of this family from the Australian Eocene fossil record; or (2) a local, unidentified botanical source of cadinene-based amber. A third alternative, that Class II amber was transported to Australia from southeast Asia via ocean currents, is rejected. Taphonomic analysis revealed four mechanisms of alteration prevalent in amber across the two study regions, with evidence of oxidation and metal carboxylate formation. Both the nature and extent of these alterations were found to vary significantly between classes I and II, suggesting that amber class may play a defining role in determining the chemical pathways by which amber degrades. Of note was the high proportion of amber that exhibited no significant chemical changes despite extensive visible alteration features, suggesting the integrity of palaeobiological and palaeoenvironmental information in these samples may be preserved.
Assessing environmental controls on the distribution of long-chain alkenones in the Canadian Prairies Org. Geochem. (IF 3.081) Pub Date : 2017-12-20 Julien Plancq, Bianca Cavazzin, Steve Juggins, Heather A. Haig, Peter R. Leavitt, Jaime L. Toney
Long-chain alkenones (LCAs) have been used for decades to reconstruct quantitative sea-surface temperature records, but they also have a great potential for paleotemperature reconstructions in lacustrine settings. Here, we investigated how the presence and abundance of LCAs in surface sediments from 106 lakes varied with environmental conditions in lakes of the Northern Great Plains (Canadian Prairies) in southern Saskatchewan, Canada. Consistent with preliminary research, we found LCAs in 55% of surveyed lakes, with mean concentrations of 143 μg/g dry sediment, but very high concentrations (up to 2.3 mg/g dry sediment) in 7% of lakes. Statistical analyses indicate that salinity and stratification play key roles in determining LCA presence and abundance supporting previous research in Spain and the northern Great Plains, USA. Overall, the alkenone unsaturation index ( U 37 K ) was not correlated significantly with average summer water temperature, probably because the timing of maximum LCA production occurs during the spring season. We conclude that improved seasonal sampling is required within the study lakes to better identify the timing and habitat of haptophyte production, and allow development of environmental temperature reconstruction tools.
Supraglacial microbes use young carbon and not aged cryoconite carbon Org. Geochem. (IF 3.081) Pub Date : 2017-12-15 Drake O. McCrimmon, Michael Bizimis, Alexandra Holland, Lori A. Ziolkowski
Cryoconite holes are depressions in glacial ice surfaces filled with dark debris that reduce albedo. The relative contributions of combustion and microbial carbon to cryoconite carbon are currently not known. To constrain cryoconite organic carbon composition and carbon sources to microorganisms living on glacier surfaces, measurements of bulk organic carbon and microbial phospholipid fatty acids (PLFAs) from supraglacial cryoconite sediment within the ablation zones of Spencer and Matanuska glaciers in southern Alaska were coupled with radiocarbon (14C) analyses. The 14C content of bulk cryoconite organic carbon on both glaciers was depleted relative to the modern atmosphere, while the PLFAs contained carbon that was recently in equilibrium with the atmosphere. Because the bulk cryoconite material is isotopically distinct from the PLFAs, these results indicate that cryoconite organic carbon is not bioavailable to the microbes. Instead, modern carbon in the microbes suggests that carbon is being quickly cycled by them within the cryoconite. Biomarker and stable isotope analyses of cryoconite organic carbon points to fresh carbon inputs to cryoconite and indicates that combusted fossil carbon is a not a major component of cryoconite organic carbon. Trace element analyses of the bulk cryoconite show crustal inputs and no evidence for excess metals associated with recent combustions sources, indicating that the aged bulk cryoconite organic carbon is likely of rock origin (e.g., shale). This study highlights that microbes in cryoconite holes on these glaciers are fixing atmospheric carbon and not using the aged carbon surrounding them.
Distributions of 5- and 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) in East African lake sediment: Effects of temperature, pH, and new lacustrine paleotemperature calibrations Org. Geochem. (IF 3.081) Pub Date : 2017-12-15 James M. Russell, Ellen C. Hopmans, Shannon E. Loomis, Jie Liang, Jaap S. Sinninghe Damsté
The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, peats, and lake sediments has been shown to correlate with mean annual air temperature (MAAT) and has provided valuable new climate reconstructions. Here we use an improved chromatographic method to quantify the fractional abundances of 5- and 6-methyl isomers in surface sediments from 65 East African lakes spanning temperatures 1.6–26.8 °C, and investigate the relationships between these fractional abundances and temperature, lake pH, and other environmental variables. We find that temperature exerts a strong control on brGDGT distributions, including the relative abundances of 5- and 6-methyl isomers, whereas other environmental variables, including lake pH, are weakly correlated to the fractional abundances of the brGDGTs. The distributions of brGDGTs in our lake sediments differ from those of soils and peats, leading to temperature offsets if soil- and peat-based brGDGT temperature calibrations are applied. We develop new calibrations for MAAT for use in lake sediment based upon the MBT́5Me and Index 1 ratios, as well as a multivariate regression of brGDGT fractional abundances on temperature using stepwise forward selection. We obtain root mean square errors (RMSE) between ∼2.1 and 2.5 °C for these calibrations, highlighting the potential for brGDGTs to provide precise temperature reconstructions using lake sediment cores. Calibrations for lake pH perform more poorly, likely due to weak correlations between pH and brGDGT distributions in East African lakes. These results indicate that quantification of 5- and 6-methyl isomers separately in lake sediment can improve paleoclimatic reconstructions.
The application of compound-specific sulfur isotopes to the oil–source rock correlation of Kurdistan petroleum Org. Geochem. (IF 3.081) Pub Date : 2017-12-08 Paul F. Greenwood, Luma Mohammed, Kliti Grice, Malcolm McCulloch, Lorenz Schwark
The concentrations and δ34S values of thioaromatic compounds of a suite of oils from several major oil fields in Kurdistan and their corresponding regional Type II-S source rocks have been measured to investigate their source relationship. The oils of three fields (Khabbaz, Jambur, Ajeel) and the bitumen extracted from specific rock formations (Alan, Sargelu, Naokelekan, Chia Gara) showed particularly high abundances of thioaromatics consistent with a carbonate source deposited in a restricted sulfate-rich marine platform setting. The δ34S [V-CDT] values of the major organosulfur compounds (OSCs) in these petroleum samples were measured with a gas chromatograph coupled to a multi-collector inductively coupled plasma mass spectrometer. δ34S values of dibenzothiophenes and methyldibenzothiophenes were consistently in the range –4‰ to –12‰ and –9‰ to –18‰ for the oils and rocks, respectively. Separate groupings of oils and rocks were distinguishable by > 2‰ difference, given an analytical reproducibility of < 0.8‰. OSCs from rocks were consistently ∼2–4‰ depleted than in oils, reflecting a similar trend to previous bulk δ34S studies from which an initial evolution of 34S depleted H2S during diagenesis and thermal maturation had been proposed. Distinctive δ34SOSC data of the oils and rocks with particularly high thioaromatic abundances did suggest several oils–source rock relationships: the Ajeel and Jambur oils and sediments from the Chia Gara formation yielded relatively enriched δ34SOSC values, whereas consistently depleted δ34SOSC values were observed for the Khabbaz oil and Naokelekan source rocks. Results suggest that compound-specific S isotope analysis can help establish oil–source rock relationships of S-rich petroleum.
Early diagenetic alterations of sterol biomarkers during particle settling and burial in polluted and pristine areas of the Rio de la Plata Basin Org. Geochem. (IF 3.081) Pub Date : 2017-12-07 Eric Demian Speranza, Manuel Colombo, Carlos Norberto Skorupka, Juan Carlos Colombo
Reconstruction of prehistoric pottery use from fatty acid carbon isotope signatures using Bayesian inference Org. Geochem. (IF 3.081) Pub Date : 2017-12-07 Ricardo Fernandes, Yvette Eley, Marek Brabec, Alexandre Lucquin, Andrew Millard, Oliver E. Craig
Carbon isotope measurements of individual fatty acids (C16:0 and C18:0) recovered from archaeological pottery vessels are widely used in archaeology to investigate past culinary and economic practices. Typically, such isotope measurements are matched with reference to food sources for straightforward source identification, or simple linear models are used to investigate mixing of contents. However, in cases where multiple food sources were processed in the same vessel, these approaches result in equivocal solutions. To address this issue, we tested the use of a Bayesian mixing model to determine the proportional contribution of different food sources to a series of different mixed food compositions, using data generated both by simulation and by experiment. The model was then applied to previously published fatty acid isotope datasets from pottery from two prehistoric sites: Durrington Walls, near Stonehenge in southern Britain and Neustadt in northern Germany. We show that the Bayesian approach to the reconstruction of pottery use offers a reliable probabilistic interpretation of source contributions although the analysis also highlights the relatively low precision achievable in quantifying pottery contents from datasets of this nature. We suggest that, with some refinement, the approach outlined should become standard practice in organic residue analysis, and also has potential application to a wide range of geological and geochemical investigations.
Branched GDGT production at elevated temperatures in anaerobic soil microcosm incubations Org. Geochem. (IF 3.081) Pub Date : 2017-12-07 Yufei Chen, Fengfeng Zheng, Songze Chen, Haodong Liu, Tommy J. Phelps, Chuanlun Zhang
Branched glycerol dialkyl glycerol tetraethers (bGDGTs) are unique ether-linked bacterial membrane lipids, which occur in diverse environments. Acidobacteria are considered to produce them in acidic soils and peat bogs. However, the biological source(s) of bGDGTs other than acidobacteria in alkaline soils and their response to environmental changes are obscure. We designed a 1 year incubation experiment under elevated temperature conditions (30 °C, 45 °C and 85 °C) in order to accelerate the growth of mesophilic and thermophilic bacteria that may potentially produce bGDGTs and understand the mechanisms underlying the changes in bGDGT composition. The soil we selected was alkaline (pH > 8) with a small population of acidobacteria being detected using high throughput sequencing. The medium contained nutrients and organic carbon and samples were incubated under anaerobic conditions with pH of 7.5, in order to stimulate the growth of heterotrophic and anaerobic bacteria from the soil. Quantitative polymerase chain reaction (qPCR) was employed to monitor the growth status of bacteria at the beginning and the end of the experiment. The results showed multiple lines of evidence for bGDGT biosynthesis, including: (1) increased total abundance of both PL-bGDGTs (polar lipid-derived bGDGTs, “living”) and CL-bGDGTs (core lipid bGDGTs, “dead”) at 30 °C and 45 °C, but not 85 °C; (2) changes in bGDGT distribution at 30 °C and 45 °C; and (3) increased temperature inferred by the bGDGT-based proxy at an incubation temperature at 45 °C. Estimated turnover time of both PL- and CL-bGDGTs was 1–8 months at 30 °C or 45 °C under these laboratory conditions, which agrees with previous studies. Our study suggests that raising temperature may stimulate growth of bGDGT-producing bacteria that likely includes some anaerobic heterotrophs other than acidobacteria.
Drimane-type compounds in source rocks and fluids from fluvial-deltaic depositional settings in the North-West Shelf of Australia Org. Geochem. (IF 3.081) Pub Date : 2017-12-07 Jaime Cesar, Kliti Grice
Fluvial-deltaic petroleum systems with gas-prone source rocks are difficult to assess because the released fluids generally lack biomarkers and this limits the implementation of traditional organic geochemistry screening. However, there are a number of compound classes yet to be investigated in order to establish novel parameters for correlation studies in fluvial-deltaic geological settings. In this study we have focused on drimane-skeleton molecules (C15-decalins), particularly the compounds 1,2,2,5,5-pentamethyl-trans-decalin and 1,1,2,5,5-pentamethyl-trans-decalin (termed herein as rearranged drimanes RD1 and RD2, respectively) and drimane (D), in source rock extracts and fluids from fluvial-deltaic petroleum systems in the Carnarvon Basin, North-West Shelf of Australia (NWS). We have calculated the ratio (RD1+RD2)/D as proposed by Ji et al. (Ji, L., He, C., Zhang, M., Wu, Y., Li, X. . Bicyclic alkanes in source rocks of the Triassic Yanchang Formation in the Ordos Basin and their inconsistency in oil-source correlation. Marine and Petroleum Geology 72, 359–373) and compared their results with samples from the NWS. Our compilation of rearranged drimane ratios for a variety of lithologies suggests clay-catalysed rearrangement of C15-decalins in siliciclastic source rocks from fluvial-deltaic depositional environments. Therefore, the mineral content seems to be a key factor controlling the isomerisation of decalins. Samples with higher clay content, such as claystone from the Brigadier Formation, have higher (RD1+RD2)/D ratios (∼6) and higher C29 diasterane / C29 sterane (∼1.1) compared to silty claystone from the Legendre Formation (∼2 and ∼0.8 respectively). The effects exerted by thermal maturity of the source rock and redox conditions of the depositional environment require further investigation, although samples from the Yanchang Fomation and La Luna Formation, where deposition occurred under more reducing conditions, exhibit lower rearranged drimane ratios (<2).
Sources and degradation of sedimentary organic matter in the mud belt of the East China Sea: Implications from the enantiomers of amino acids Org. Geochem. (IF 3.081) Pub Date : 2017-11-28 Yan Chen, Gui-Peng Yang, Chong-Xiao Ji, Hong-Hai Zhang, Peng-Yan Zhang
Total organic carbon (TOC), total N (TN) and amino acids (AAs, including the bacterial biomarker D-AAs) were determined from the mud belt of the East China Sea (ECS) in sediment cores. The concentration of total hydrolyzable AAs (THAAs) ranged from 3.35 to 13.44 µmol/g dry wt (dw) of the sedimentary organic matter (SOM), exhibiting a decreasing trend downcore. Major constituents of THAAs were glycine (Gly), L-serine (L-Ser), L-alanine (L-Ala), L-glutamic acid (L-Glu) and L-aspartic acid (L-Asp), whereas the D-aspartic acid (D-Asp), D-glutamic acid (D-Glu), D-serine (D-Ser) and D-alanine (D-Ala), along with non-protein AAs (γ-aminobutyric acid and β-alanine) together accounted for ca. 7% of the THAA pool. Given the C/N ratio values, the OM in three cores was predominantly of marine origin. Based on D-Ala yield, bacterial OM represented on average 18% of TOC and ca. 30% of TN. C-normalized THAAss (THAA-C%) was a sensitive indicator of SOM diagenetic alteration in the upper 40 cm of sediment. However, the degradation index (DI) and reactivity index (RI) values in the cores did not exhibit any definite trend, which may indicate that these two indicators were not sensitive in the early stage of sediment degradation. Negative correlation between the D/L ratio of Ala and THAA-C% suggested a close coupling between the extent of degradation and the accumulation and/or selective preservation of bacterial material in the sediments. In addition, correlation between clay content and the THAA-C% suggested that fine grained particles played an important role in affecting the quality of SOM.
Distribution and export of isoprenoid tetraether lipids in suspended particulate matter from the water column of the Western Atlantic Ocean Org. Geochem. (IF 3.081) Pub Date : 2017-11-22 Sarah J. Hurley, Julius S. Lipp, Hilary G. Close, Kai-Uwe Hinrichs, Ann Pearson
The TEX86 paleotemperature proxy is based on the distribution of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids preserved in marine sediments, yet both the influence of different physiological factors on the structural distribution of GDGTs and the mechanism(s) by which GDGTs are exported to marine sediments remain unresolved. We investigated the abundance and structural distribution of GDGTs in the South-west and Equatorial Atlantic Ocean in four water column profiles spanning 48 degrees of latitude. The depth distribution of GDGTs was consistent with production by ammonia-oxidizing Thaumarchaeota; maximum GDGT concentration occurred at the base of the NO2- maximum, core GDGTs dominated the structural distribution in surface waters above the NO2- maximum, and intact polar GDGTs – potentially indicating live cells – were more abundant below the NO2- maximum. Between 0 and1000 m, > 98% of the integrated GDGT inventory was present in waters at and below the NO2- maximum. Depth profiles of TEX86 temperature values displayed local minima at the NO2- maximum, while the ratio of GDGT-2:GDGT-3 increased with depth. A model based on the results predicts an average depth of origin for GDGTs exported to sediments between ca. 80-250 m. In the model, exported TEX86 values are remarkably insensitive to change in the average depth of origin of GDGTs. However, net TEX86 values exported from the water column appear to reflect euphotic zone productivity, possibly due to the correlative intensity of organic matter remineralization providing substrates for ammonium oxidation. Predicting the influence of these regional controls on sedimentary TEX86 records requires a better understanding of the interaction between GDGT production, particle dynamics, and the depth of origin for exported organic matter.
Fractionation of dibenzofurans during subsurface petroleum migration: based on molecular dynamics simulation and reservoir geochemistry Org. Geochem. (IF 3.081) Pub Date : 2017-11-20 Meijun Li, Xiaoqiang Liu, T.-G. Wang, Weidong Jiang, Ronghui Fang, Lu Yang, Youjun Tang
Dibenzofurans, oxygen-containing heterocyclic aromatics, are ubiquitous in crude oils. Their geochemical implications in petroleum geochemistry have been studied in numerous previous publications. Here, we present the distribution of dibenzofuran and its methylated homologues, and calculate the polarity and solubility of methyldibenzofuran isomers based on the Connolly surfaces and molecular dynamics simulation. The results show that the geo-chromatographic fractionation effect is evident due to absorption onto the water/silica matrix carrier bed during the oil migration process. Two potential molecular indicators relative to dibenzofurans, i.e. the total content of dibenzofurans and the relative abundances of 1-methyldibenzofuran to 4-methyldibenzofuran, are proposed for tracing the oil migration orientation and filling pathways. These two indicators were applied successfully in Paleozoic marine carbonate and Tertiary lacustrine sandstone reservoirs. The variations of source input, depositional environment, and maturity for oils from same source bed/kitchen appear to have no significant effect on these two indicators. It suggested that these two dibenzofuran indicators are potentially effective molecular tracers for orientation and filling pathways in oil migration.
Synthesis of 26-methyl cholestane and identification of cryostanes in mid-Neoproterozoic sediments Org. Geochem. (IF 3.081) Pub Date : 2017-11-16 Pierre Adam, Philippe Schaeffer, Jochen J. Brocks
The biomarker distributions of mid-Neoproterozoic sediments (800 and 717 million yr, Ma) from various locations are characterized by the occurrence, besides cholestane isomers, of a novel series of C28 steranes, whereas classical C-24 alkylated steranes are absent. These unusual C28 steranes, termed cryostanes, seem to be restricted to pre-Snowball Earth sediments. We report their conclusive identification as 26-methyl cholestanes based on comparison of gas chromatographic (GC) behavior and mass spectrum of the last eluting cryostane isomer with those of a 26-methyl 5α (H),14 α (H),17 α (H),20R-cholestane standard obtained by way of synthesis. Sterols methylated at C-26 seem to be restricted to some demosponges and, based on molecular clock estimates, demosponges may have emerged in the time interval 800 to 700 Ma. A sponge origin of cryostanes is thereby conceivable, making cryostanes the oldest molecular markers for animals. However, other biological sources need to be explored, including the wide variety of eukaryotic protists.
Evidence for major input of riverine organic matter into the ocean Org. Geochem. (IF 3.081) Pub Date : 2017-11-14 Xiaoyan Cao, George R. Aiken, Kenna D. Butler, Thomas G. Huntington, William M. Balch, Jingdong Mao, Klaus Schmidt-Rohr
The changes in the structure of XAD-8 isolated dissolved organic matter (DOM) samples along a river (Penobscot River) to estuary (Penobscot Bay) to ocean (across the Gulf of Maine) transect and from the Pacific Ocean were investigated using selective and two dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy coupled with elemental and carbon isotope analysis. The results provide important insights into the nature of relatively stable structures in the river-to-ocean continuum and the enigma of the fate of terrestrial DOM in the marine system. First, lignin and carboxyl-rich alicyclic molecules (CRAMs), which are indistinguishable from mass spectrometry, were clearly differentiated with NMR spectroscopy. NMR unambiguously showed that CRAMs persisted along the river-to-ocean transect and in the Pacific Ocean, while lignin residues dramatically decreased in abundance from the river to the coastal ocean and from the Pacific Ocean. The results challenge a previous conclusion that lignin-derived compounds are refractory and can accumulate in the coastal ocean. The loss of terrestrial plant-derived aromatic compounds such as lignin and tannin residues throughout the sequence of riverine, coastal, and open ocean DOM extracts could also partially explain the change in carbon stable isotope composition from riverine DOM (δ 13C -27.6‰) to ocean DOM (δ 13C -23.0‰) extracts. The observation, from advanced NMR, of similar CRAM molecules in XAD-8 isolated DOM samples from the Penobscot River to the Penobscot Bay and from the ocean refutes a previous conclusion that XAD-isolated DOM from seawater and river are distinctly different. The alicyclic structural features of CRAMs and their presence as the major structural units in DOM extracts from the Penobscot River to Gulf of Maine transect, together with the deduced old 14C age of CRAMs in the ocean, imply that terrestrial CRAMs may persist on timescales long enough to be transported into the ocean.
Rapid and economical quantification of black carbon in soils using a modified benzene polycarboxylic acids (BPCA) method Org. Geochem. (IF 3.081) Pub Date : 2017-11-14 Mireia Llorente, María-Belén Turrión, Bruno Glaser
Black carbon, the ubiquitous stable product of incomplete combustion, is a potential sink for atmospheric CO2 and, therefore, a contributor to the Earth’s radiative heat balance. Accurate information about the distribution of black carbon in soils is important for climate change projections, given that the black carbon content of soils varies widely across the globe. In response to this issue, an accurate, rapid and cost-effective method to assess BC is needed. The use of benzene polycarboxylic acids (BPCA) as molecular markers of black carbon has the advantage of specifically tracking the polyaromatic backbone of the black carbon continuum. However, this method has three major disadvantages: (a) it is expensive; (b) it is very time-consuming; and (c) some of the literature states that care has to be taken when applying the BPCA method to organic-rich matrices. In response to these disadvantages, the aims of the present study were: (a) to check the validity of a new shorter and economical method for black carbon quantification based on BPCA as molecular markers; and (b) to study the correlation between BPCA formation during the analysis and the soil sample’s OC content. The proposed method for black carbon quantification is based on the BPCA method developed by Glaser et al. (1998), but uses a simple direct elemental analysis (EA) of C content in the residue obtained after a strong digestion instead of the molecular determination by gas chromatography with flame ionisation detector (GC-FID). This simplification of the method represents a great cost reduction (in time and expense) providing a rapid and accurate method for large soil sample sets.
Geochemical composition of oils in the Dunga Field, western Kazakhstan: Evidence for a lacustrine source and a complex filling history Org. Geochem. (IF 3.081) Pub Date : 2017-11-13 H.I. Petersen, D. Cumming, E. Dujoncquoy
The onshore Mangyshlak Basin, located in western Kazakhstan along the eastern coastline of the Caspian Sea, is a prolific hydrocarbon province and home to numerous oil and gas fields. The primary reservoirs are Jurassic clastic sandstones, but the Dunga Field contains hydrocarbons in Lower Cretaceous (Valanginian, Barremian, Aptian, Albian) clastic and carbonate reservoirs. The general perception is that these petroleum accumulations were charged from marine source rocks of likely Middle Triassic age. A detailed organic geochemical investigation of oils from the Cretaceous reservoirs of Dunga Field suggests that all were generated from the same lacustrine source rock facies (organofacies C) in mature kitchens in the offshore Mangyshlak Basin, followed by a complex filling history. The oils are characterised by relatively high wax content, low sulfur content (mainly < 0.2 wt%), typical lacustrine tricyclic terpane ratios (high T26/T25, high T24/T23, low T22/21) and low H31R/H30 hopane ratios. Maturity of the source rock that generated the oils would tentatively correspond to a VR of ∼0.9–1.1%Ro. The oils are non-biodegraded, but some minor water washing might have occurred. Diamondoid data suggest that the Barremian reservoir contains a mixed oil pool composed of the non-cracked lacustrine oil and another charge of oil that is cracked from 79% to 83% and likely also to be slightly fractionated. This requires a complex filling history of the Dunga Field involving two hydrocarbon charges, likely from two different fetch areas. One charge of non-cracked oil migrated into all four reservoirs whereas another charge was ‘hoteled’ and cracked before remigration to the Barremian reservoir only and mixed with the non-cracked oil.
Degradation changes in plant root cell wall structural molecules during extended decomposition of important agricultural crop and forage species Org. Geochem. (IF 3.081) Pub Date : 2017-11-13 Kathryn E. White, Frank J. Coale, James B. Reeves
Little is known regarding changes in cell wall structural molecules (lignin, cellulose and hemicellulose) as plant roots decompose, despite their importance for soil organic matter (OM) formation. The objectives of this study were to quantify changes in root composition during 270 d incubations of ten important grain and forage crops utilizing forage fiber analysis and to characterize the changes in cell wall composition and structure using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Large, species-dependent variation was observed in the extent of root tissue decomposition over time, ranging from 82.5% of initial mass for alfalfa to 21.5% for switchgrass. Fiber analysis revealed that initial rapid decomposition increased lignin concentration and cellulose concentration while hemicellulose declined, while all three moieties degraded proportionally thereafter. Similar trends were found in the ratios between the DRIFTS diagnostic peaks for lignin, cellulose and the carbonyls of hemicellulose and wax components. Spectra illustrated changes during decomposition, particularly in more extensively decomposed roots. Features potentially indicative of suberin preservation were found in the region between 2800 cm-1 and 3000 cm-1. Examination of the region between 1000 cm-1 and 1300 cm-1 revealed possible change in hemicellulose structure. The results illustrate the effect of differences in cell wall composition and structure during root decomposition and expand understanding of the role of roots in soil OM dynamics. Variability in root degradation and change in cell wall composition among species demonstrate that characterization of a broad range of individual species is necessary to predict root contributions to soil C.
Severe drought-influenced composition and δ13C of plant and soil n-alkanes in model temperate grassland and heathland ecosystems Org. Geochem. (IF 3.081) Pub Date : 2017-11-11 Kavita Srivastava, Guido L.B. Wiesenberg
Drought events are predicted to increase under future climate change. In temperate ecosystems, plants are capable of resisting drought due to their hydrophobic wax layer, in which n-alkanes are important constituents. In soils, plant-derived n-alkanes are comparatively resistant to degradation. To improve understanding of the significance of n-alkanes in plant-soil systems during a severe drought period (104 days), we investigated bulk organic carbon (Corg) concentration, total lipid extract (TLE) concentration, n-alkane molecular ratios such as average chain length (ACL), carbon preference index (CPI) and chain length ratios of different n-alkane compounds, in addition to the compound-specific isotope composition (δ13C) of n-alkanes in model temperate grassland and heathland plant-soil systems. Although plant communities of two (heathland) and four (grassland) species were available, only one representative species per biome was accessible for the current study. Heathland plants and soil revealed significantly higher concentrations of Corg and TLE compared with grassland. TLE and alkane composition responded quickly during the first drought phase (0 – 40 days). This indicates that plants were actively utilizing C and produced more n-alkanes in order to withstand drought, which was confirmed by increased (2 – 3‰) δ13C values for n-alkanes in shoot biomass. However, during later drought phases all the parameters remained constant for plants and soils. This suggests limited change in biosynthesis and cycling of plant lipids such as n-alkanes during intense drought. Surprisingly, during the first drought phase, increased ACL and CPI ratios in soil demonstrated a rapid input of plant-derived long chain n-alkanes to soil, which was not expected due to the decadal residence time of alkanes in soil. The study enabled tracing of plant metabolic response in terms of alkane biosynthesis under different phases of drought and rapid cycling of alkanes in the plant-soil system.
Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude Org. Geochem. (IF 3.081) Pub Date : 2017-10-31 Marie-Aimée Galeron, Olivier Radakovitch, Bruno Charrière, Frédéric Vaultier, John K. Volkman, Thomas S. Bianchi, Nicolas D. Ward, Patricia M. Medeiros, Henrique O. Sawakuchi, Suzanne Tank, Philippe Kerhervé, Jean-François Rontani
There exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autoxidative damage.
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