Aromatic hydrocarbons provide new insight into carbonate concretion formation and the impact of eogenesis on organic matter
Introduction
Carbonate concretions are a common geological feature that is encountered in sedimentary rocks throughout the geological record. They sometimes enclose exceptionally-preserved fossils (Martill, 1988, Martill, 1989, Long and Trinajstic, 2010, Williams et al., 2015, Grice et al., 2019). Carbonate concretions can also preserve information on the palaeoenvironment of deposition of their host sediments (Plet et al., 2016, Grice et al., 2019). Concretions have been the focus of a large number of studies, as highlighted in a review by Dietrich (2001)1. Early on, Berner, 1968, Berner, 1967 performed laboratory experiments investigating the processes and timing involved in the formation of carbonate concretions. More recently, Yoshida et al., 2018, Yoshida et al., 2015) succeeded in growing carbonate concretions around tusk shells in a matter of weeks. In parallel to laboratory experiments, several studies have investigated the geochemistry and petrology of carbonate concretions, indicating a pivotal role of microorganisms; in particular, sulfate reducing bacteria and possibly archaea involved in the anaerobic oxidation of methane (Martill, 1989, Coleman and Raiswell, 1995, Marshall and Pirrie, 2013, Dale et al., 2014). The following microbially-mediated reactions are believed to play a crucial role in the formation of the carbonate, with the product of each of these reactions utilised by other microbial processes or reacting with ions present in the immediate environment (Coleman and Raiswell, 1980, Coleman and Raiswell, 1995, Coleman, 1993, Lash and Blood, 2004, Hendry et al., 2006):
Bacterial sulfate reduction (BSR)2CH2O + SO42− → 2HCO3− + H2S
MethanogenesisCH3COOH → CH4 + CO2
Anaerobic oxidation of methane (AOM)CH4 + SO42− → HCO3− + H2S + OH−
Bacterial iron reduction (FeR)4FeOOH + CH2O + 7H+ → 4Fe2+ + HCO3− + 6H2O.
In particular settings, such as at methane seeps in the Black Sea, Reitner et al. (2005) investigated the microbial mats surrounding carbonate concretions using molecular analyses (DNA). These analyses evidenced the abundant sulfate reducing bacteria and archaea, in particular ANME-1 believed to be active players in anaerobic oxidation of methane (AOM). Unfortunately, accessible presently-forming carbonate concretions are rare (Pye et al., 1990, Coleman, 1993, Boni et al., 1994, Duan et al., 1996), and most carbonate concretions investigated come from the sedimentary record, therefore complicating our understanding of microbial communities involved in concretion formation. Although the exact microbial communities involved in concretion formation remain uncertain, models agree that carbonate concretions form around a centre of decaying organic matter (OM) harbouring complex microbial communities within anoxic to euxinic sediments (e.g. Coleman, 1993, Coleman and Raiswell, 1995).
Due to the labile nature of DNA molecules, DNA investigation of ancient carbonate concretions is limited (Briggs and Summons, 2014). However, since lipids are more resilient to geological time and processes, molecular studies of carbonate concretions can provide important insights into the genetic processes leading to concretions (Briggs and Summons, 2014, Grice et al., 2019). Lipids were first studied in carbonate concretions by Wolff et al., 1992. This study highlighted the presence of distinct lipid compositions for the different mineralogical zones of the concretion. In the dolomitic zone, the presence of pentamethylicosane (PMI) and squalane, at trace levels, were attributed to the presence of methanogenic archaea. Since then, a range of molecular studies on lipids from concretions has been conducted, revealing the occurrence of several biomarkers attributed to archaea and sulfate reducing bacteria, but also, information on thermal maturity and palaeoenvironments, for instance. An overview of these findings is presented in Table 1.
The investigations of lipids have largely focused on aliphatic hydrocarbons and fatty acids, whereas aromatic fractions of carbonate concretions have received much less attention (Table 1). Yet, aromatic compounds can provide insightful information on the catagenetic kerogen maturation state via investigation of PAHs or aromatic steroids, but also about eogenetic and palaeoenvironmental conditions. Carotenoids, for instance isorenieratene and its degradation products (i.e. isorenieratane and aryl isoprenoids), are indicative of photic zone euxinic conditions (e.g. Grice et al., 1996). Few studies have investigated these compounds in carbonate concretions. Melendez et al. (2013b) reported a range of carotenoids and aryl isoprenoids from a Devonian concretion; the occurrence of isorenieratane and aryl isoprenoids was also reported from the concretions investigated here (Plet et al., 2016). In the present study, we aim to gain further insights into the processes and micro-ecosystems involved in concretion formation by comparing the aromatic hydrocarbon distributions from concretions and host sediments using traditional gas chromatography-mass spectrometry (GC–MS) and comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-ToF-MS). These data were integrated with the results of stable isotopic analyses of carbonate and sedimentary sulfur (δ13C of carbonate and δ34S of mainly sulfide-bound S).
Section snippets
Geological setting
The Lower Toarcian (Early Jurassic; ~183 Ma) is marked by a global perturbation of the carbon cycle, characterised by a negative carbon isotopic excursion related to release of 13C-depleted carbon to the ocean-atmosphere system and global warming (e.g. Gómez et al., 2016, Hesselbo et al., 2000, Kemp et al., 2005). These processes, which were potentially forced by changes in the Earth’s solar orbit (Kemp et al., 2005, Boulila and Hinnov, 2017, Ruebsam et al., 2019), were accompanied by marine
Sampling location and preparation
The concretions and the host sediment Posidonia Shale studied herein were collected from the Holcim Cement quarry of Dotternhausen (SW-Germany) shortly after blasting. Although the exact stratigraphic position remains unknown due to the blasting, Plet et al. (2016) established that these concretions derived from the lower H. falciferum ammonite zone, likely from above the Unterer Stein carbonate horizon, based on the comparison of microfacies, lithology and geochemical signatures from the
Evaluation of the resilience of aromatic hydrocarbon compounds
All aromatic fractions were highly similar in their overall molecular compositions (Fig. 2). However, an unresolved complex mixture (UCM) was relatively enhanced in the concretions compared to the host sediment, suggesting a slightly higher degree of biodegradation in the concretions (Trolio et al., 1999). In addition, Plet et al. (2016) reported that the host sediment presents the highest hydrogen index (HI) values (820 mgHC/gTOC), whereas the carbonate concretion samples revealed moderately
Distribution of catagenetic aromatic hydrocarbons and microbial eogenesis: significance for kerogen quality
The HI indicates a better preservation of the organic matter in the host sediment compared to the concretion. This may appear contradictory given that carbonate concretions are known for their exceptional preservation of organic matter and fossils. Here, because of the low maturity of the host sediment (high HI and R0 ~ 0.5), we attribute the low HI in concretion samples to the microbial eogenetic activity, rather than to burial diagenesis. We suggest that in a context of more mature settings,
Conclusions
The detailed examination of the aromatic fractions from carbonate concretions and their host sediment highlights microbial processes involved in carbonate concretion formation. In addition to bulk stable isotopes supporting enhanced and complex microbial activity, the present study demonstrates that:
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Stable isotopic values of calcite and sedimentary sulfur (mainly pyrite) (δ13C and δ34S, respectively) co-vary. The 13C-depleted carbonate cement from the concretion body results from microbial
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by a Discovery Grant from the Australian Research Council (ARC-DORA- Kliti Grice, DP130100577) and DFG grants Schw554/23 and Schw554/29. Peter Hopper is thanked for GC-MS technical support. Marieke Sieverding is thanked for δ34S technical support. CP thanks Curtin University and The Institute of Geoscience Research for a PhD and a Top-up scholarship. The authors wish to acknowledge Anais Pages for constructive criticisms that contributed to the improvement of an
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