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  • Assessment of the stability, sorption, and exchangeability of marine dissolved and colloidal metals
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-16
    L.T. Jensen; N.J. Wyatt; W.M. Landing; J.N. Fitzsimmons

    The size partitioning of dissolved trace metals is an important factor for determining reactivity and bioavailability of metals in marine environments. This, alongside the advent of more routine shipboard ultrafiltration procedures, has led to increased attention in determining the colloidal phase of metals such as Fe in seawater. While clean and efficient filtration, prompt acidification, and proper storage have long been tenets of trace metal biogeochemistry, few studies aim to quantify the kinetics of colloidal exchange and metal adsorption to bottle walls during storage and acidification. This study evaluates the effect of storage conditions on colloidal size partitioning, the kinetics of colloid exchange over time, and the timescale of bottle wall adsorption and desorption for dissolved Fe, Cu, Ni, Zn, Cd, Pb, Mn and Co. We report that preservation of dissolved size partitioning is possible only for Fe and only under frozen conditions. All metals except Mn and Cd show regeneration of the colloidal phase following its removal in as short as 14 h, validating the importance of prompt ultrafiltration. Adsorption of metals to bottle walls is a well-known sampling artifact often cited for Fe and assumed to be potentially significant for other metals as well. However, only Fe and Co showed significant proclivity to adsorption onto low density polyethylene bottle walls, sorbing a maximum of 91 and 72% over 40 months, respectively. After 20 weeks of acidification neither Fe nor Co desorbed to their original concentrations, leading to an acidified storage recommendation of 30 weeks prior to analyses following storage of unacidified samples for long periods of time. This study provides empirical recommendations for colloidal and dissolved trace metal methodology while also paving the way for much-needed future methods testing.

  • Determining seawater mercury methylation and demethylation rates by the seawater incubation approach: A critique
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-15
    Kang Wang; Kathleen M. Munson; Debbie Armstrong; Robie W. Macdonald; Feiyue Wang

    The discovery of a pervasive subsurface methylmercury maximum in the world's oceans highlights the importance of understanding the processes that produce and destroy this potent neurotoxin in seawater. Two approaches have been published in the literature, one based on the spatial distribution of mercury species in seawater and, more recently, a second approach based on seawater incubation studies using additions of isotopically enriched mercury species. The reaction rates determined by these two approaches differ by several orders of magnitude. Using Arctic Ocean seawater and the incubation approach to determine mercury methylation and demethylation rates, we observed unexplainable methylation and demethylation at time zero and poor fitting of the data to first-order kinetics. A critical analysis of previous incubation studies reveals similar deficiencies, especially when the mercury was added at low concentrations to simulate ambient seawater conditions. These findings cast doubt on the validity of mercury methylation and demethylation rates determined by the seawater incubation approach, and call for the development of alternative methods to determine these crucially important rate constants in ambient seawater.

  • Elemental mercury in the marine boundary layer of North America: Temporal and spatial patterns
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-15
    Kathleen M. Munson; Jeffrey Latonas; Wen Xu; Ashley Elliot; Debbie A. Armstrong; Gary A. Stern; Feiyue Wang

    Gaseous elemental mercury (Hg0) comprises the majority of atmospheric mercury (Hg) and results in long-range transport of Hg. Secondary emissions from the marine surface mixed layer result in spatial and temporal Hg0 variability, which subsequently determine marine pools of divalent mercury (HgII) available for methylation. We measured atmospheric Hg0 concentrations in the marine boundary layer (MBL) around North America in regions of the Atlantic, Pacific, and Arctic Oceans (6.7°-77.3° N, 52.3°-150.0° W). Observed spatial Hg0 distributions across these regions match patterns revealed from current published biogeochemical models. These models have supported marine processes as the major factors controlling MBL Hg0. We observe differences in variability between ocean basins. Asian emissions appear to be well-mixed in the Eastern Pacific MBL, with stable measurements across a 3200 km transect in the North Pacific. Hg0 concentrations decrease but remain relatively stable in tropical regions of the Pacific and Atlantic. In contrast, we measured several 100–200 km regions of elevated Hg0 above North Atlantic surface waters. Repeated offshore sampling of Arctic Hg0 reveals strong seasonal trends that are similar to those collected at coastal long-term Arctic monitoring stations. These similarities suggest marine processes control seasonal Arctic trends, which are linked to sea ice formation and melt. Together our observations are consistent with marine controls on MBL Hg0 concentrations although further study is needed to fully distinguish between marine and atmospheric controls.

  • Investigating the cycling of chromium in the oxygen deficient waters of the Eastern Tropical North Pacific Ocean and the Santa Barbara Basin using stable isotopes
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-15
    Simone B. Moos; Edward A. Boyle; Mark A. Altabet; Annie Bourbonnais

    Cr isotope geochemistry is being explored in the context of a variety of geological problems as well as the environmental remediation of pollutant Cr(VI). There is a strong Cr isotope fractionation during reduction of oxidized Cr(VI) to reduced Cr(III). We present chromium concentration and Cr isotope data for samples from highly reducing environments ([O2] < 2 μmol/kg) in the Eastern Tropical North Pacific (ETNP) Oxygen Deficient Zone (ODZ) off of Mexico and the deep Santa Barbara Basin off of California. Total dissolvable Cr in the upper ETNP ODZ is slightly depleted (by up to 0.8 nmol/kg) and δ53Cr is up to 0.1–0.2‰ heavier compared to oxic waters of the same density seen at the SAFe station (30°N, 140°W), presumably both a result of reduction of Cr(VI) and removal of light Cr(III) by sinking particles. The Cr depletion and Cr isotope fractionation peak at the same depth as the highest δ15N of NO3− and decrease within the equally oxygen-deficient waters below, implying that microbial reduction dependent on the sinking organic matter flux may be the mechanism of Cr reduction. These data are consistent with a fractionation mechanism with a net isotope fractionation factor of ε ≈ −0.44‰. In the deepest anoxic waters of the Santa Barbara Basin in July 2014, dissolved (<0.2 μm) Cr is depleted by up to 1.8 nmol/kg and δ53Cr is up to 0.5‰ heavier compared to SAFe station waters of the same density. This is consistent with a net isotope fractionation factor of ε ≈ −0.65‰. At the Santa Barbara Basin site, it is possible that abiotic Fe(II) reduction (from Fe(II) diffusing out of reducing continental shelf sediments) also contributes to Cr reduction in addition to the microbial reduction mechanism.

  • Variability in 210Pb and 210Po partition coefficients (Kd) along the US GEOTRACES Arctic transect
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-10
    Wokil Bam; Kanchan Maiti; Mark Baskaran; Katherine Krupp; Phoebe J. Lam; Yang Xiang

    The naturally occurring 210Po (t1/2 = 138.3d) and 210Pb (t1/2 = 22.3 y) radionuclide pair has been extensively used to study particle scavenging, cycling and transport in the ocean. However, the role of particle composition in the scavenging, sorption and fractionation of 210Po210Pb needs to be better understood to successfully utilize these radionuclides as oceanographic tracers. Here, we measure dissolved and particulate 210Pb and 210Po activities for both small (1-51 μm) and large particles (> 51 μm) with abundance of major phases of suspended particulate matter (SPM) from the US GEOTRACES Arctic transect to examine the role of the concentration and composition of SPM in scavenging and fractionation of these radionuclides along the coastal, Canada Basin and interior stations. 210Po210Pb disequilibria were observed throughout the water column for all stations. The Pacific end member had the highest activity of 210Po and 210Pb in dissolved samples. The calculated partition coefficients (Kd) of 210Po and 210Pb varied between 3.2 × 105 to 9.9 × 108 L kg−1 and 7.9 × 105 to 6.9 × 108 L kg−1 respectively and followed a log-linear trend with SPM for particle size fractions. Principal Component Analysis (PCA) and correlation analysis for six major particle phases indicated a significant positive correlation between 210Pb and the lithogenic phase. Both 210Po and 210Pb had significant negative correlations with opal. Our results suggest that particle composition alone cannot explain the observed variability in Kd across the different oceanic regime along the transect. Estimated Kd values from an end member mixing model were found to be within the range of observed Kd values and suggest that higher particulate Mn concentration in the Arctic may be partly responsible for the higher Kd observed in the basin compared to Atlantic and Pacific.

  • Isotopic and optical heterogeneity of solid phase extracted marine dissolved organic carbon
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-10
    Christian B. Lewis; Brett D. Walker; Ellen R.M. Druffel

    Marine dissolved organic carbon (DOC) is the ocean's largest exchangeable reservoir of organic carbon. The biogeochemical cycling of DOC plays an important role in ocean carbon storage on various timescales. Solid-phase extraction (SPE) is a process used to isolate DOC from seawater for biogeochemical analysis. This study examines how DOC isotopic (Δ14C, δ13C) and optical (absorbance) properties of SPE-DOM change as a function of eluent volume (and hydrophobicity). These properties were measured in 28 SPE-DOC fractions incrementally eluted from Bond Elut PPL (styrene-divinylbenzene polymer) cartridges, totaling 32 mL of methanol. We show that the early eluted SPE-DOC has distinctly different ∆14C and δ13C values than those eluted later. This study reveals isotopic heterogeneity as a function of SPE-DOC elution volume. These results show a partitioning of two distinct sources of SPE-DOC during elution, indicating a gradual transition from “marine-like” DOC to “terrestrial-like” DOC along a hydrophobicity continuum.

  • Variations in sediment production of dissolved iron across a continental margin not dominated by major upwelling or riverine inputs
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-09
    Eryn M. Eitel; Shannon M. Owings; Keaton M. Belli; Jordon S. Beckler; Anna Williams; Benjamin P. Fields; Malory Brown; Joel Craig; Olivia Bailey Studebaker; Donald B. Nuzzio; Martial Taillefert

    Despite the undeniable effect of iron on shaping patterns of ocean productivity, the relative importance of the different sources of this limiting nutrient to the ocean is still under debate. Although global estimates indicate that the benthic input of iron to the oceans is significant, most studies have investigated continental margins exposed to either upwelling or large riverine inputs, environments that are not representative of the majority of the oceans. Additionally, the number of studies that report dissolved iron concentrations in continental slope sediments is limited, despite the fact that these regions between the shelf edge and the continental rise make up >5% of the sedimentary surface area of the global ocean. The sedimentary flux of iron has traditionally been considered negligible due to the rapid oxidation of Fe2+ in oxic waters and poor solubility of the Fe(III) product. The recent realization that ferric iron may be stabilized in solution by organic ligands during oxidation near the sediment-water interface suggests that a significant fraction of the dissolved iron pool may be present under the form organic-Fe(III) complexes that could eventually reach the overlying waters. In this study, the speciation and biogeochemical importance of iron was determined in intact sediment cores along a transect across the entire continental margin near Cape Lookout, North Carolina, a region not dominated by upwelling or riverine inputs that is representative of most passive continental margins. Rates of diffusive oxygen uptake (DOU) and maximum diffusive fluxes of both dissolved Fe2+and organic-Fe(III) complexes decreased from the coastal zone to the continental shelf, remained low on the shelf and the upper continental slope, but rebounded to reach a maximum in mid-slope sediments where concentrations of Fe(III) oxides were the highest along the transect. In turn, DOU decreased and dissolved iron was below detection in lower-slope sediments, indicating that mid-slope sediments represent depocenters where Fe(III) oxides and organic matter may accumulate. Pore water sulfate and sulfide concentrations as well as separate sediment incubations confirmed that sulfate reduction does not greatly influence the cycling of iron in these sediments. The production of dissolved organic-Fe(III) in these continental margin sediments is likely regulated by a combination of aerobic oxidation in the presence of natural organic ligands near the sediment-water interface, dissimilatory iron reduction, or chemical oxidation of Fe(II) complexed to natural organic ligands. Fluxes of Fe2+ and organic-Fe(III) complexes across the sediment-water interface were not observed. However, diffusive fluxes of Fe2+ and organic-Fe(III) complexes into the oxic zone of these sediments (<1 cm from the sediment-water interface) and production of dissolved Fe(III) in sediment slurry incubations suggest that complexation of Fe(III) in these sediments may contribute to the stabilization and potential transport of dissolved iron into oxygenated deep ocean waters. Extrapolation to the global ocean suggests that mid-slope depocenters contribute considerably to the iron inventory of the ocean, thus warranting the need for measurement of benthic iron fluxes and dissolved iron speciation in these environments.

  • Sources of particulate Ni and Cu in the water column of the northern South China Sea: Evidence from elemental and isotope ratios in aerosols and sinking particles
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-08
    Shotaro Takano; Wen-Hsuan Liao; Hung-An Tian; Kuo-Fang Huang; Tung-Yuan Ho; Yoshiki Sohrin

    We present isotope ratios of Ni (δ60Ni) and Cu (δ65Cu) in sinking particles, aerosols, and seawater collected from the northern South China Sea to identify sources and transformation processes of the two metals. In aerosols, δ60Ni values are in the range +0.05‰ to +0.56‰, and δ65Cu values are in the range −0.33‰ to +0.83‰. The isotope ratios are different from those of lithogenic materials, indicating that the aerosols are anthropogenic in origin. In sinking particles collected at depths of 2000 and 3500 m, δ60Ni values are in the range +0.01‰ to +0.54‰ at 2000 m and −0.18‰ to +0.54‰ at 3500 m, and the values exhibit similar temporal variation pattern between 2000 and 3500 m. Based on the significant correlation between δ60Ni and the ratio of P/Ni or organic-C/Ni, we hypothesize that the main sources of Ni in the sinking particles originate from both resuspended marine sediments off southwest Taiwan, and biogenic organic particles. The δ60Ni in biogenic particles is estimated to be +0.6‰ to +1.0‰, which is 0.3–0.7‰ lighter than that of dissolved Ni in seawater. The isotope ratios of Cu in sinking particles are fairly constant (+0.13‰ to +0.36‰), and the range is between those of marine sediments and labile fractions of marine particles. Thus, Cu in sinking particles is likely to be from marine sediments and biogenic organic particles. Compared with Ni, the correlation between Cu and P or organic-C is weaker, suggesting that the Cu/P and Cu/organic-C ratios are not constant in organic matter or there are additional sources of particulate Cu, such as Fe-Mn oxides and anthropogenic aerosols.

  • Dissolved iron in the Bermuda region of the subtropical North Atlantic Ocean: Seasonal dynamics, mesoscale variability, and physicochemical speciation
    Mar. Chem. (IF 2.713) Pub Date : 2020-01-03
    P.N. Sedwick; A.R. Bowie; T.M. Church; J.T. Cullen; R.J. Johnson; M.C. Lohan; C.M. Marsay; D.J. McGillicuddy; B.M. Sohst; A. Tagliabue; S.J. Ussher

    Water-column data from seven cruises in 2007–2008 reveal pronounced temporal and spatial variations in the distribution of dissolved iron (DFe, <0.4 μm) over the upper 1000 m of the Sargasso Sea near Bermuda, in the western subtropical North Atlantic Ocean. In near-surface waters, DFe exhibits a clear seasonal cycle, increasing from ~0.1–0.3 nM in spring to ~0.4–1.0 nM in summer-early fall. The observed seasonal ranges appear to reflect the extent of winter convective mixing and of summer dust deposition, both of which are closely tied to atmospheric circulation processes. Surface DFe concentrations also show significant (~two-fold) submesoscale lateral variations during summer, perhaps as a result of lateral inhomogeneities in wet deposition and wind-driven mixing. The summer vertical profiles reveal pronounced DFe minima and sometimes deeper maxima in the lower euphotic zone, which likely reflect biological uptake and shallow remineralization, and eddy-driven lateral gradients in these processes. Significant variability is also seen in the mesopelagic zone, with a DFe concentration range of ~0.4–0.7 nM at 1000 m depth, which may reflect mesoscale isopycnal displacements and/or lateral advection of iron-rich waters in the lower thermocline. Physicochemical iron speciation measurements indicate that the major fraction of DFe that accumulates in surface waters of the Sargasso Sea during summer is colloidal-sized Fe(III), which appears to be complexed by strong, iron-binding organic ligands. Concentrations of soluble iron (sFe, <0.02 μm) were considerably lower than DFe in the upper euphotic zone during summer, except over the subsurface DFe minima, where sFe accounts for ~50–100% of the DFe pool. Labile Fe(II), on average, accounted for around 20% of DFe, with maximum concentrations of around 0.1 nM in near-surface waters and in the lower thermocline. The seasonal-scale DFe changes that we have documented near Bermuda are of the same magnitude as basin-scale lateral gradients across the North Atlantic, underscoring the importance of time-series observations in understanding the behavior of trace elements in the upper ocean.

  • Partitioning of iron and plutonium to exopolymeric substances and intracellular biopolymers: A comparison study between the coccolithophore Emiliania huxleyi and the diatom Skeletonema costatum
    Mar. Chem. (IF 2.713) Pub Date : 2019-12-13
    Peng Lin; Chen Xu; Wei Xing; Luni Sun; Kathleen A. Schwehr; Antonietta Quigg; Peter H. Santschi

    Iron (Fe), a micronutrient for algal growth, and plutonium (Pu), an anthropogenic radionuclide, share some common features. This includes similar oceanic distributions when different input modes are taken into account, as well as their chemical behavior, such as a high affinity to natural organic matter (NOM). The NOM produced by various phytoplankton communities can potentially influence Fe cycling in the ocean, and likely also influence the transport behavior of Pu. We conducted laboratory incubation experiments using the coccolithophore Emiliania huxleyi and the diatom Skeletonema costatum, in the presence of 59Fe and 238Pu as radiotracers, in order to differentiate Fe and Pu uptake by extracellular exopolymeric substances (EPS) and intracellular biopolymers. The Fe and Pu distributions in select organic compound classes produced by these two types of phytoplankton, including proteins, total carbohydrates (TCHO) and uronic acids (URA), were compared. Our results indicated that most of the Fe and Pu (>95%) were found concentrated in E. huxleyi-derived non-attached EPS, while much less (<2%) was present in the intracellular fraction of E. huxleyi. In contrast, in the diatom S. costatum, Fe and Pu were both distributed with EPS > intracellular biopolymers > outer cell covering (i.e., frustule). In fact, over 50% of the Fe was concentrated in S. costatum-derived attached EPS and intracellular biopolymers. The diatom derived Fe-EPS complexes were more hydrophobic, with stronger tendency to aggregate in seawater. Fe binding to biopolymers in both E. huxleyi and S. costatum cultures was related to URA concentrations, but the overall distribution of URA between these two phytoplankton species was different (e.g., high intracellular abundance of URA in S. costatum but low intracellular URA abundance in E. huxleyi). Our findings suggest that the presence of URA on cellular surfaces of S. costatum (i.e., attached EPS) and its high intracellular fraction could be an indicator for Fe transport from the surrounding seawater to the diatom cells. However, for the coccolithophore E. huxleyi, Fe was not efficiently taken up during its growth. Instead, the more hydrophilic non-attached EPS (i.e., low protein/TCHO ratio) produced by E. huxleyi could have stabilized Fe in the colloidal form as Fe-EPS complexes. Similar partitioning behavior of Fe and Pu suggests that Pu isotopes can potentially serve as a tracer for Fe biogeochemistry in the ocean.

  • Can the protein/carbohydrate (P/C) ratio of exopolymeric substances (EPS) be used as a proxy for their ‘stickiness’ and aggregation propensity?
    Mar. Chem. (IF 2.713) Pub Date : 2019-12-09
    Peter H. Santschi, Chen Xu, Kathleen A. Schwehr, Peng Lin, Luni Sun, Wei-Chun Chin, Manoj Kamalanathan, Hernando P. Bacosa, Antonietta Quigg

    Microbially secreted exopolymeric substances (EPS), rich in polysaccharides and proteins, make up an important part of natural organic matter in the ocean, especially marine snow. While the attention in the oceanographic literature is focused more on the role of polysaccharides and less of proteins in EPS functions, the role of proteins, especially the role of the protein/carbohydrate (P/C) ratio of particles and colloids (macromolecular fraction) remains to be explored. EPS associated with particles forms a biofilm, where proteins are not only involved in cell surface attachment, but also in the stabilization of the biofilm matrix, and the development of a three-dimensional biofilm architecture. Here, we provide a perspective based on the most recent literature on EPS, marine oil spills and waste water treatment to describe the relationship between the P/C ratio of EPS and a number of biophysical properties related to biopolymer aggregation propensity, e.g., relative hydrophobicity, surface activity and surface tension, attachment efficiency, light-induced chemical crosslinking, and sedimentation efficiency of marine snow in marine environments.

  • Character and sedimentation of “lingering” Macondo oil to the deep-sea after the Deepwater Horizon oil spill
    Mar. Chem. (IF 2.713) Pub Date : 2019-11-29
    Uta Passow, Scott A. Stout

    During the active 87-day Deepwater Horizon spill in the northern Gulf of Mexico, a significant fraction of the spilled Macondo oil was transported to the seafloor via the sedimentation of marine snow. Here we present a detailed characterization of oil that arrived together with marine snow at a 1400 m deep sediment trap six weeks to 13 months after the spill had ended. These data give insight into the nature and evolution of the sedimentation of the marine snow and oil, the latter of which remained as droplets in the water column after the spill ended. Four pulses of oil flux were recognized; three of which were associated with peak sedimentation rates of diatoms. Detailed chemical analysis (TPH, alkylated PAH, and petroleum biomarker fingerprints) reveal the sinking oil's lack of evaporation and photo-oxidation, which indicated it was not derived from the sea surface but had “lingered” within the water column after the spill. Measurable amounts of the increasingly weathered (biodegraded and water-washed) Macondo oil was collected in the trap for ~1 year after the active spill ended, over which time the oil flux decreased overall. The results indicate that sinking diatom aggregates and other marine snow scavenged measurable amounts of weathered Macondo oil droplets remaining in the water, and carried them to the deep-seafloor for approximately 1-year after the spill ended.

  • Hydrothermal signatures in sediments from eastern Southwest Indian ridge 63°E to 68°E
    Mar. Chem. (IF 2.713) Pub Date : 2019-11-27
    Deepak Kumar Agarwal, Parijat Roy, L. Surya Prakash, Palayil John Kurian

    Hydrothermal vent incidence is proportional to the spreading rate of the mid-oceanic ridges (MORs) (Baker et al., 1996). However, the ultra-slow spreading Southwest Indian Ridge (SWIR) is unique and has a relatively higher incidence of hydrothermal vent fields compared with other ultra-slow spreading ridges (German et al., 1998; Baker and German, 2004). In the present study, sediment samples from the less studied easternmost section of the ultra-slow spreading SWIR are investigated to decipher the signatures of hydrothermal activity. The sediments are mainly composed of calcium carbonate (55–85%), Fe-Mn-(oxyhydr)oxide (13–27%), and residual aluminosilicate phases (2–17%). The metalliferous nature, low Mn concentration (393–1772 ppm), negative Ce, and positive Y anomalies in the shale-normalized rare earth element (REE) pattern of the bulk sediments provide evidence for the presence of hydrothermal inputs in most of the sediments. The fractionation among lighter REEs over heavier REEs in bulk sediments indicates the high-temperature hydrothermal plume fall-out to the sediments. The leached fraction also shows similar negative Ce and positive Y anomalies, and indicates that the Fe-Mn-(oxyhydr)oxide precipitates in these sediments may be dominantly of hydrothermal origin. In addition to hydrothermal Fe-Mn-(oxyhydr)oxide phases, the residual fraction of the sediments also consists of minor quantities of hydrothermally sourced sulfate phases. Otherwise, the residual fraction is significantly dominated by mid-oceanic ridge basalt- (MORB) derived components in addition to serpentinized peridotite and terrigenous material. Among the studied sediments, sample E-3-16 from Fuji Dome (influenced by hydrothermal activity) has high zinc concentrations, which indicates the possibility of a new vent field in the vicinity. The present study also introduces a new geochemical tool (Zn/Fe vs Co/Zn proxy) for assessing the contribution of hydrothermal components in the sediments. The Zn/Fe vs Co/Zn proxy will be more useful in MOR settings hosting sulfides dominated by sphalerite. The influence of hydrothermal activity in most of the studied sediments from the eastern SWIR indicates that the ultra-slow spreading eastern SWIR has the potential to possess a larger number of hydrothermal vent fields.

  • Tracing water mass fractions in the deep western Indian Ocean using fluorescent dissolved organic matter
    Mar. Chem. (IF 2.713) Pub Date : 2019-11-06
    Jeonghyun Kim, Yeseul Kim, Hyoun-Woo Kang, Suk Hyun Kim, TaeKeun Rho, Dong-Jin Kang

    The meridional distributions of fluorescent dissolved organic matter (FDOM) and various hydrologic properties were investigated along 67°E in the western Indian Ocean. Our results showed that the highest fluorescence of the humic FDOM (FDOMH) was discovered in the Indian Deep Water (IDW), and relatively lower values were observed in the intruding water masses from the upper layer (e.g., Circumpolar Deep Water (CDW), Antarctic Intermediate Water (AAIW), and South Indian Central Water (SICW)). The deep FDOMH was robustly correlated with apparent oxygen utilisation (AOU), as suggested by previous studies. In particular, the slopes of the regression line AOU on FDOMH varied for different water masses and the two humic components. In this study, to identify the factor inducing the variations of the slope, we estimated the relative water mass fraction of different water masses using a three-end-member mixing model with a salinity-FDOMH diagram. The distribution of water mass fractions was in good agreement with water mass distribution from the conventional method from temperature and salinity distribution and previous studies. The FDOMH components were positively correlated with the aged water mass fraction (i.e., IDW; r = 0.93) and negatively correlated with fresher ones originating from the upper water (r = −0.93, −0.51, and − 0.95 for CDW, AAIW, and SICW, respectively). The fluorescence ratio between the two FDOMH components was also observed to be linked to the water mass fractions. The results indicate that the distribution of FDOMH is attributed to the mixing of various deep-water masses during the global ocean circulation.

  • Electrodialysis as a sample processing tool for bulk organic matter and target pollutant analysis of seawater
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-31
    M.A. Wirth, M. Sievers, F. Habedank, U. Kragl, D.E. Schulz-Bull, M. Kanwischer
  • Organic geochemistry of sediments from the continental margin off southern New England, U.S.A.--Part II. Lipids.
    Mar. Chem. (IF 2.713) Pub Date : 1987-01-01
    M I Venkatesan,E Ruth,S Steinberg,I R Kaplan

    Organic geochemical measurements of the lipid fraction, comparing saturated and aromatic hydrocarbons, fatty acids, alcohols and sterols, have been carried out on six sediments cores collected from the Atlantic shelf, slope and the rise areas to evaluate the cross-shelf transport of the organic carbon. The concentration of most of the organic compound classes studied is correlated with the total organic carbon, which decreases from the shelf through slope to the rise. Terrigenous carbon is recognizable even in the slope and rise sediments, but terrestrial influx decreases relative to marine generated lipids in the slope and rise organic matter. We estimate that approximately 50% of the shelf organic matter is exported to the slope. Data of sediment trap material collected at 1200 m from 1250 m water depth are discussed and compared with that of surface sediment from 1280 m water depth (slope). Fluxes for specific organic compound classes have been computed. The fluxes are of the same magnitude as for equatorial North Atlantic trap particulates at comparable water depth, studied by other investigations.

  • Organic geochemistry of sediments from the continental margin off southern New England, U.S.A.--Part I. Amino acids, carbohydrates and lignin.
    Mar. Chem. (IF 2.713) Pub Date : 1987-01-01
    S M Steinberg,M I Venkatesan,I R Kaplan

    Total organic carbon (TOC), lignin, amino acids, sugars and amino sugars were measured in recent sediments for the continental margin off southern New England. The various organic carbon fractions decreased in concentration with increasing distance from shore. The fraction of the TOC that was accounted for by these major components also decreased with increasing distance from shore. The concentration of lignin indicated that only about 3-5% of the organic carbon in the nearshore sediment was of terrestrial origin. The various fractions were highly correlated, which was consistent with a simple linear mixing model of shelf organic matter with material form the slope and rise and indicated a significant transport of sediment from the continental shelf to the continental slope and rise.

  • Method for determining stable isotope ratios of dissolved organic carbon in interstitial and other natural marine waters.
    Mar. Chem. (IF 2.713) Pub Date : 1991-01-01
    J E Bauer,R I Haddad,D J Des Marais

    A procedure is described for the analysis of the stable carbon isotopic composition of dissolved organic carbon (DOC) in natural waters from marine and higher-salinity environments. Rapid (less than 5 min) and complete oxidation of DOC is achieved using a modification of previous photochemical oxidation techniques. The CO2 evolved from DOC oxidation can be collected in less than 10 min for isotopic analysis. The procedure is at present suitable for oxidation and collection of 1-5 micromoles of carbon and has an associated blank of 0.1-0.2 micromole of carbon. Complete photochemical oxidation of DOC standards was demonstrated by quantitative recovery of CO2 as measured manometrically. Isotopic analyses of standards by photochemical and high-temperature sealed-tube combustion methods agreed to within 0.3%. Photochemical oxidation of DOC in a representative sediment pore-water sample was also quantitative, as shown by the excellent agreement between the photochemical and sealed-tube methods. The delta 13C values obtained for pore-water DOC using the two methods of oxidation were identical, suggesting that the modified photochemical method is adequate for the isotopically non-fractionated oxidation of pore-water DOC. The procedure was evaluated through an analysis of DOC in pond and pore waters from a hypersaline microbial mat environment. Concentrations of DOC in the water column over the mat displayed a diel pattern, but the isotopic composition of this DOC remained relatively constant (average delta 13C = -12.4%). Pore-water DOC exhibited a distinct concentration maximum in the mat surface layer, and delta 13C of pore-water DOC was nearly 8% lighter at 1.5-2.0-cm depth than in the mat surface layer (0-0.5-cm depth). These results demonstrate the effectiveness of the method in elucidating differences in DOC concentration and delta 13C over biogeochemically relevant spatial and temporal scales. Carbon isotopic analysis of DOC in natural waters, especially pore waters, should be a useful probe of biogeochemical processes in recent environments.

  • Factors controlling the photochemical degradation of methylmercury in coastal and oceanic waters.
    Mar. Chem. (IF 2.713) Pub Date : 2018-03-09
    Brian P DiMento,Robert P Mason

    Many studies have recognized abiotic photochemical degradation as an important sink of methylmercury (CH3Hg) in sunlit surface waters, but the rate-controlling factors remain poorly understood. The overall objective of this study was to improve our understanding of the relative importance of photochemical reactions in the degradation of CH3Hg in surface waters across a variety of marine ecosystems by extending the range of water types studied. Experiments were conducted using surface water collected from coastal sites in Delaware, New Jersey, Connecticut, and Maine, as well as offshore sites on the New England continental shelf break, the equatorial Pacific, and the Arctic Ocean. Filtered water amended with additional CH3Hg at environmentally relevant concentrations was allowed to equilibrate with natural ligands before being exposed to natural sunlight. Water quality parameters - salinity, dissolved organic carbon, and nitrate - were measured, and specific UV absorbance was calculated as a proxy for dissolved aromatic carbon content. Degradation rate constants (0.87-1.67 day-1) varied by a factor of two across all water types tested despite varying characteristics, and did not correlate with initial CH3Hg concentrations or other environmental parameters. The rate constants in terms of cumulative photon flux values were comparable to, but at the high end of, the range of values reported in other studies. Further experiments investigating the controlling parameters of the reaction observed little effect of nitrate and chloride, and potential for bromide involvement. The HydroLight radiative transfer model was used to compute solar irradiance with depth in three representative water bodies - coastal wetland, estuary, and open ocean - allowing for the determination of water column integrated rates. Methylmercury loss per year due to photodegradation was also modeled across a range of latitudes from the Arctic to the Equator in the three model water types, resulting in an estimated global demethylation rate of 25.3 Mmol yr-1. The loss of CH3Hg was greatest in the open ocean due to increased penetration of all wavelengths, especially the UV portion of the spectrum which has a greater ability to degrade CH3Hg. Overall, this study provides additional insights and information to better constrain the importance of photochemical degradation in the cycling of CH3Hg in marine surface waters and its transport from coastal waters to the open ocean.

  • Seasonal variations in C:N:Si:Ca:P:Mg:S:K:Fe relationships of seston from Norwegian coastal water: Impact of extreme offshore forcing during winter-spring 2010.
    Mar. Chem. (IF 2.713) Pub Date : 2017-11-24
    Svein Rune Erga,Stig Bjarte Haugen,Gunnar Bratbak,Jorun Karin Egge,Mikal Heldal,Kjell Arne Mork,Svein Norland

    The aim of this study was to reveal the relative content of C, N, Ca, Si, P, Mg, K, S and Fe in seston particles in Norwegian coastal water (NCW), and how it relates to biological and hydrographic processes during seasonal cycles from October 2009-March 2012. The following over all stoichiometric relationship for the time series was obtained: C66N11Si3.4Ca2.3P1Mg0.73S0.37K0.35Fe0.30, which is novel for marine waters. A record-breaking (187-year record) negative North Atlantic Oscillation (NAO) index caused extreme physical forcing on the Norwegian Coastal Current Water (NCCW) during the winter 2009-2010, and the inflow and upwelling of saline Atlantic water (AW) in the fjord was thus extraordinary during late spring-early summer in 2010. The element concentrations in fjord seston particles responded strongly to this convection, revealed by maximum values of all elements, except Fe, exceeding average values with 10.8 × for Ca, 9.3 for K, 5.3 for S, 5.1 for Mg, 4.6 for Si, 4.0 for P, 3.8 for C, and 3.3 for N and Fe. This indicates that the signature of the Atlantic inflow was roughly two times stronger for Ca and K than for the others, probably connected with peaks in coccolithophorids and diatoms. There is, however, 1.5 × more of Si than Ca contained in the seston, which could be due to a stronger dominance of diatoms than coccolithophorids, confirming their environmental fitness. In total our data do not indicate any severe nutrient limitation with respect to N, P and Fe, but accumulation of iron by Fe-sequestering bacteria might at times reduce the availability of the dissolved Fe-fraction. There is a high correlation between most of the measured elements, except for Ca, which together with Fe only weakly correlated with the other elements. It is to be expected that environmental alterations in NCW related to climate change will influence the seston elemental composition, but the full effect of this will be strongly dependent on the future dominance of the high pressure versus low pressure systems (i.e. NAO index), since they are key regulators for the direction of wind driven vertical convection (i.e. upwelling or downwelling). Changes in stratification, temperature, light, pH (ocean acidification), CaCO3 concentrations (carbon pump) and availability of nutrients in the euphotic zone (biogeochemical cycling) are essential for the future dominance of coccolithophorids versus diatoms.

  • Bioaccumulation of methylmercury in a marine diatom and the influence of dissolved organic matter.
    Mar. Chem. (IF 2.713) Pub Date : 2017-12-20
    Cheng-Shiuan Lee,Nicholas S Fisher

    The largest bioconcentration step of most metals, including methylmercury (MeHg), in aquatic biota is from water to phytoplankton, but the extent to which dissolved organic matter (DOM) affects this process for MeHg largely remains unexplored in marine systems. This study investigated the influence of specific sulfur-containing organic compounds and naturally occurring DOM on the accumulation of MeHg in a marine diatom Thalassiosira pseudonana. Initial uptake rate constants and volume concentration factors (VCFs) of MeHg were calculated to evaluate MeHg enrichment in algal cells in the presence of a range of organic compound concentrations. At environmentally realistic and higher concentrations, the addition of glycine and methionine had no effect on algal MeHg uptake, but thiol-containing compounds such as cysteine and thioglycolic acid reduced MeHg accumulation in algal cells at high added concentrations (> 100 times higher than naturally occurring concentrations). However, environmentally realistic concentrations of glutathione, another thiol-containing compound as low as 10 nM, resulted in a decline of ~ 30% in VCFs, suggesting its possible importance in natural waters. Humic acid additions of 0.1 and 0.5 mg C/L also reduced MeHg VCFs by ~ 15% and ~ 25%, respectively. The bioaccumulation of MeHg for T. pseudonana in coastal waters with varying levels of dissolved organic carbon (DOC) was inversely correlated with bulk DOC concentrations. Generally, naturally occurring DOM, particularly certain thiol-containing compounds, can reduce MeHg uptake by phytoplankton.

  • Conservation of dissolved organic matter molecular composition during mixing of the deep water masses of the northeast Atlantic Ocean.
    Mar. Chem. (IF 2.713) Pub Date : 2015-12-20
    Roberta L Hansman,Thorsten Dittmar,Gerhard J Herndl

    Characterizing the composition of marine dissolved organic matter (DOM) is important for gaining insight into its role in oceanic biogeochemical cycles. Using Fourier transform ion cyclotron resonance mass spectrometry, we analyzed the molecular composition of solid phase extracted (SPE) DOM from the northeast Atlantic to investigate the specificity of the DOM pool of the individual major water masses of the North Atlantic. All 272 measured samples from depths ranging from 87 to 5609 m and latitudes from 24°N to 68°N shared 96% similarity (on a Bray-Curtis scale) in their DOM composition. Small variations between subsurface and deep samples and among latitudinal groupings were identified, but overall, water mass specific SPE-DOM composition was not apparent. A strong correlation between a calculated degradation index and water mass age indicates variability in portions of the DOM pool, and ocean-scale differences were observed between the North Atlantic and deep North Pacific. However, within the deep northeast Atlantic, conservative mixing primarily drives the molecular composition of SPE-DOM.

  • The effect of sediment mixing on mercury dynamics in two intertidal mudflats at Great Bay Estuary, New Hampshire, USA.
    Mar. Chem. (IF 2.713) Pub Date : 2016-03-01
    Lauren E Brown,Celia Y Chen,Mary A Voytek,Aria Amirbahman

    Estuarine sediments store particulate contaminants including mercury (Hg). We studied Hg sediment dynamics in two intertidal mudflats at Great Bay estuary, NH, over multiple years. Sediments at both mudflats were physically mixed down to ~10 cm, as determined by 7Be measurements, albeit via different mechanisms. Portsmouth mudflat (PT) sediments were subject to bioturbation by infaunal organisms and Squamscott mudflat (SQ) sediments were subject to erosion and redeposition. The presence of higher concentrations of fresh Fe(III) hydroxide at PT suggested bioirrigation by the polychaetes (Nereis virens). At depths where infaunal bioirrigation was observed, pore-water inorganic Hg (Hgi) and methylmercury (MeHg) were lower potentially due to their interaction with Fe(III) hydroxide. Methylmercury concentrations increased immediately below this zone in some samples, suggesting that the observed increase in material flux in bioirrigated sediments may initiate from lower depths. Pore water in sediment at PT also had higher fractions of more protein-like and labile DOC than those at SQ that can lead to increased MeHg production in PT, especially at depths where Hgi is not removed from solution by Fe(III) hydroxide. Where sediment erosion and redeposition were observed at SQ, Hg species distribution was extended deeper into the sediment column. Moreover, methyl coenzyme M reductase (MCR) and mercury reductase (mer-A) genes were higher at SQ than PT suggesting differences in conditions for Hg cycling. Results showed that the near-surface region of high MeHg concentrations commonly observed in unmixed sediments does not exist in physically mixed sediments that are common in many estuarine environments.

  • Sources of water column methylmercury across multiple estuaries in the Northeast U.S.
    Mar. Chem. (IF 2.713) Pub Date : 2016-01-26
    Prentiss H Balcom,Amina T Schartup,Robert P Mason,Celia Y Chen

    Estuarine water column methylmercury (MeHg) is an important driver of mercury (Hg) bioaccumulation in pelagic organisms and thus it is necessary to understand the sources and processes affecting environmental levels of MeHg. Increases in water column MeHg concentrations can ultimately be transferred to fish consumed by humans, but despite this, the sources of MeHg to the estuarine water column are still poorly understood. Here we evaluate MeHg sources across 4 estuaries and 10 sampling sites and examine the distributions and partitioning of sediment and water column MeHg across a geographic range (Maine to New Jersey). Our study sites present a gradient in the concentrations of sediment, pore water and water column Hg species. Suspended particle MeHg ranged from below detection to 187 pmol g-1, dissolved MeHg from 0.01 to 0.68 pM, and sediment MeHg from 0.01 to 109 pmol g-1. Across multiple estuaries, dissolved MeHg correlated with Hg species in the water column, and sediment MeHg correlated with sediment total Hg (HgT). Water column MeHg did not correlate well with sediment Hg across estuaries, indicating that sediment concentrations were not a good predictor of water MeHg concentrations. This is an unexpected finding since it has been shown that MeHg production from inorganic Hg2+ within sediment is the primary source of MeHg to coastal waters. Additional sources of MeHg regulate water column MeHg levels in some of the shallow estuaries included in this study.

  • An examination of the factors influencing mercury and methylmercury particulate distributions, methylation and demethylation rates in laboratory-generated marine snow.
    Mar. Chem. (IF 2.713) Pub Date : 2015-12-09
    Veronica L Ortiz,Robert P Mason,J Evan Ward

    In the marine environment, settling particulates have been widely studied for their role as effective vertical transporters of nutrients and metals scavenged from the euphotic zone to the benthos. These particulates are composed of transparent exopolymers, plankton and bacterial cells, detritus and organic matter, and form various size fractions from colloids (<0.2μm) to aggregates, and finally marine snow (>300 μm). As marine snow forms in the water column, anoxic layers form around and within the aggregation potentially creating a prime environment for the methylation of mercury (Hg), which occurs primarily in low oxygen environments. To examine this process, marine aggregates were produced from sieved estuarine seawater (100 μm) in 1-L glass bottles spiked with stable isotope enriched methylmercury (CH3199Hg) and inorganic mercury (200Hg(II)) at 18° C using a roller-table. After the rolling period, different particle-size fractions were collected and analyzed, including: visible marine snow (>300μm), particulates 8 to 300 μm, and particulates 0.2 to 8μm. Particulate analysis indicated higher incorporation of both forms of Hg into marine snow compared to unrolled treatments, with greater incorporation of 200Hg(II) than CH3199Hg. In addition, inorganic Hg was methylated and CH3Hg was demethylated in the larger particulate fractions (>8μm). Methylation and demethylation rates were assessed based on changes in isotopic composition of Hg(II) and CH3Hg, and found to be comparable to methylation rates found in sediments. These results indicate that net Hg methylation can occur in marine snow and smaller aggregates in oxic coastal waters, and that this net formation of CH3Hg may be an important source of CH3Hg in both coastal and open ocean surface environments.

  • River-derived humic substances as iron chelators in seawater.
    Mar. Chem. (IF 2.713) Pub Date : 2015-09-29
    Regina Krachler,Rudolf F Krachler,Gabriele Wallner,Stephan Hann,Monika Laux,Maria F Cervantes Recalde,Franz Jirsa,Elisabeth Neubauer,Frank von der Kammer,Thilo Hofmann,Bernhard K Keppler

    The speciation of iron(III) in oxic seawater is dominated by its hydrolysis and sedimentation of insoluble iron(III)-oxyhydroxide. As a consequence, many oceanic areas have very low iron levels in surface seawater which leads to iron deficiency since phytoplankton require iron as a micronutrient in order to grow. Fortunately, iron solubility is not truly as low as Fe(III) solubility measurements in inorganic seawater would suggest, since oceanic waters contain organic molecules which tend to bind the iron and keep it in solution. Various iron-binding organic ligands which combine to stabilize dissolved iron have been detected and thoroughly investigated in recent years. However, the role of iron-binding ligands from terrestrial sources remains poorly constrained. Blackwater rivers supply large amounts of natural organic material (NOM) to the ocean. This NOM (which consists mainly of vascular plant-derived humic substances) is able to greatly enhance iron bioavailability in estuaries and coastal regions, however, breakdown processes lead to a rapid decrease of river-derived NOM concentrations with increasing distance from land. It has therefore been argued that the influence of river-derived NOM on iron biogeochemistry in offshore seawater does not seem to be significant. Here we used a standard method based on 59Fe as a radiotracer to study the solubility of Fe(III)-oxyhydroxide in seawater in the presence of riverine NOM. We aimed to address the question how effective is freshwater NOM as an iron chelator under open ocean conditions where the concentration of land-derived organic material is about 3 orders of magnitude smaller than in coastal regions, and does this iron chelating ability vary between NOM from different sources and between different size fractions of the river-borne NOM. Our results show that the investigated NOM fractions were able to substantially enhance Fe(III)-oxyhydroxide solubility in seawater at concentrations of the NOM ≥ 5 μg L- 1. Terrigenous NOM concentrations ≥ 5 μg L- 1 are in no way unusual in open ocean surface waters especially of the Arctic and the North Atlantic Oceans. River-derived humic substances could therefore play a greater role as iron carriers in the ocean than previously thought.

  • Temporally-variable productivity quotients on a coral atoll: Implications for estimates of reef metabolism
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-25
    Isaiah W. Bolden, Julian P. Sachs, Alexander C. Gagnon

    Measurements of net ecosystem productivity (NEP) and calcification (NEC) from contemporary coral reefs provide a baseline for monitoring the impacts of future stressors like ocean acidification and sea-surface warming. However, separating secular trends from natural variability requires NEP and NEC records across a wide range of spatial and temporal scales. One promising way to make these measurements is with autonomous pH and O2 sensors. Crucially, the accuracy of this approach relies on knowledge of the in situ ecosystem productivity quotient (Q), which indicates the moles of O2 consumed per mole CO2 produced. Using co-located measurements of total dissolved inorganic carbon (DIC), total alkalinity, and dissolved O2, we empirically determined Q during a three-year field campaign on Tetiaroa Atoll, French Polynesia. Empirical values of daily net Q (−1.02 ≤ Qnet ≤ −0.47) frequently differed from both the canonical value of −1.45 for the ocean and the value of −1 often assumed for tropical reef ecosystems. Furthermore, Q changed on hourly timescales, and integrated daily values differed between days. We hypothesize that captive bubbles on the surfaces of coral, macroalgae and other substrates can explain these variations in Q, with other influential mechanisms being mixing between parcels of water, sedimentary denitrification, and ammonium-fueled primary productivity. Our findings, which are robust to changes in the model that is used to correct for advection and gas exchange, as well as changes in model parameters, suggest that future investigations of metabolism on reefs should be based on measurements of carbonate chemistry variability, rather than O2 evolution. Otherwise, large biases in NEP and NEC could result (>40%).

  • Mangrove removal: Effects on trace metal concentrations in temperate estuarine sediments
    Mar. Chem. (IF 2.713) Pub Date : 2019-07-26
    Ujwal Bastakoti, John Robertson, Cyril Marchand, Andrea C. Alfaro

    Trace metal accumulation in mangrove sediments is of increasing global concern, as contaminants can easily remobilise to surrounding environments through a range of sediment geochemistry processes. Such processes are driven by various physical or biological disturbances, which tend to remobilise metals profligately. The present study aimed to investigate the effects of mangrove removal activities on trace metal concentrations in temperate mangrove sediments and adjacent mudflats. To understand patterns of trace metal mobilization due to the removal of mangroves, we measured levels of Fe, Cu, Zn, Pb, and Cd along with other sediment parameters (organic matter, pH, bulk density) during pre- and post-mangrove removal activities. Sediment cores from three different depths down to 30 cm were collected from the mangrove removal location and adjacent mudflats at Mangawhai Harbour Estuary, New Zealand, during pre/post-removal sampling events over a four-year period. Results indicate that levels of OM and trace metals were higher in mangrove sediments than adjacent mudflats, which is consistent with the inherent capacity of mangrove sediments to sequester trace metals. However, after the removal of mangroves, the levels OM and trace metals decreased in removal sites. During this period, we observed a decrease in OM and lower sediment pH levels, indicating an increase in sediment acidity which is associated with a lower sediment sequestering capacity. We also observed increases in trace metal concentrations in adjacent mudflats during the post-removal period, thus highlighting the far-reaching effects of mangrove removal activities. These findings provide important information to assist with future management strategies of temperate mangrove ecosystems.

  • Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)
    Mar. Chem. (IF 2.713) Pub Date : 2019-07-05
    Alison M. Agather, Katlin L. Bowman, Carl H. Lamborg, Chad R. Hammerschmidt

    Mercury (Hg) in the Arctic Ocean is a concern due to unusually high concentrations of monomethylmercury (MMHg) in fish and marine animals. Increased human exposure from consumption of these animals is a significant health concern that is related to Hg contamination in nature. Most Arctic marine Hg research has investigated the amounts, distributions, and cycling in animals, snow, and ice, while few studies have examined the aqueous behavior and fate of Hg in the polar ocean. Here we present the most comprehensive dataset published to date detailing Hg speciation and distribution of elemental Hg (Hg0), dimethylmercury (DMHg), and filtered and particulate total Hg and MMHg in the western Arctic Ocean. This data was obtained as part of the U.S. Arctic GEOTRACES cruise (GN01) in 2015. Many water masses sampled appeared to be enriched with anthropogenic Hg. The Transpolar Drift supplied HgT and Hg0 to the central Arctic Ocean, but not methylated Hg. Gaseous Hg0, but not DMHg, was elevated in surface waters under the ice cover. Monomethylmercury levels, which averaged 0.054 ± 0.050 pM, are lower than other major ocean basins, suggesting ambient MMHg levels in western Arctic Ocean seawater do not by themselves explain anomalously high Hg in Arctic animals.

  • Sources and transformations of iron in the sediments of the Gulf of Aqaba (Red Sea)
    Mar. Chem. (IF 2.713) Pub Date : 2019-08-02
    Valeria Boyko, Barak Blonder, Alexey Kamyshny
  • Diurnal to interannual variability of sea surface pCO2 and its controls in a turbid tidal-driven nearshore system in the vicinity of the East China Sea based on buoy observations
    Mar. Chem. (IF 2.713) Pub Date : 2019-07-29
    Qian Liu, Xu Dong, Jinshun Chen, Xianghui Guo, Zhirong Zhang, Yi Xu, Ningli Huang, Minhan Dai

    We examined the diurnal to seasonal dynamics of the sea surface partial pressure of carbon dioxide (pCO2) in a subtropical nearshore estuarine system, Hangzhou Bay, adjacent to the Changjiang Estuary in the vicinity of the East China Sea, based on data collected between July 30, 2010 to September 20, 2011 by a surface buoy equipped with an autonomous pCO2 system along with hydrological and other chemical sensors. The study site (122.37° E, 30.55° N) is influenced by the river plumes of both the Changjiang and Qiantang River and is characterized by strong tidal circulation and highly turbid waters. The amplitude of pCO2 changes increased from winter to summer over both diurnal and spring-neap tidal cycle timescales. The average surface water pCO2 was slightly undersaturated with respect to the atmosphere in winter (382 ± 18 μatm), but supersaturated in spring (500 ± 56 μatm) and summer (687 ± 110 μatm). Overall the study site was a source of atmospheric CO2 with an average sea to air flux of 14 ± 9 mmol C m−2 d−1 from January to October 2011. We revealed factors controlling the pCO2 dynamics at different timescales. Over seasonal timescales, temperature and estuarine mixing dominated the seawater pCO2 variability. Over spring-neap tidal timescales in winter and spring, the major drivers were similarly water mass mixing and temperature. However, in summer, biological activity and air-sea exchange became the two principal factors controlling the variations in surface seawater pCO2. Our mass balance models further suggested that biological processes impacted surface pCO2 differently during different tidal phases. Respiration was revealed to promote the increase in pCO2 during spring tide in August, but in neap tides of the same month biological production was evident and resulted in the drawdown of pCO2. This is because photosynthesis was generally limited by light in summer at the study site due to high turbidity, except during neap tides when turbidity was dramatically drawn down, triggering high biological productivity. At the diurnal timescale, sea surface pCO2 was primarily controlled by tidal mixing, except during neap tides in summer when sea surface pCO2 was greatly influenced by biological metabolism. This study also revealed significant inter-summer differences between 2010 and 2011, showing lower sea surface pCO2 in August 2010 as compared to August 2011, which was likely due to the enhanced biological uptake as a result of the relatively low turbidity caused by weak tidal currents and enhanced river flow in August 2010. Our study highlights a highly dynamic system primarily driven by tidal mixing, which not only modulates water mass mixing but also affects turbidity, which subsequently controls biological production. These processes led to a synergy of CO2 dynamics in a tidally driven and highly turbid nearshore system, where high frequency time-series observations are essential to reveal the complex controls of CO2 dynamics.

  • Characterization of the extracellular polymeric substances (EPS) of Virgibacillus strains capable of mediating the formation of high Mg-calcite and protodolomite
    Mar. Chem. (IF 2.713) Pub Date : 2019-08-14
    Zulfa Ali Al Disi, Nabil Zouari, Maria Dittrich, Samir Jaoua, Hamad Al Saad Al-Kuwari, Tomaso R.R. Bontognali

    The origin of dolomite –a common mineral in the geological record– is the subject of an ongoing debate. Among different hypotheses, it has been proposed that extracellular polymeric substances (EPS) excreted by microbes include organic molecules that catalyze the incorporation of Mg in the carbonate mineral. However, limited information exists on the composition of the EPS produced by CaMg carbonate-forming microbes, which in turn hampers a precise understanding of their role in the mineralization mechanism. Here, we present the results of laboratory experiments in which we cultured different strains of microbes, characterized their EPS, and identified components associated with carbonate minerals with high mol% Mg. Two Virgibacillus strains known to mediate the formation of Mg-rich carbonates, as well as a strain of Bacillus licheniformis –a negative control that does not mediate mineral formation but produces EPS, were grown under different salinities and temperatures, which caused them to produce EPS with different compositions. The EPS were subsequently characterized by measuring total carbohydrate (TCHO) and total protein (TP) contents, as well as by Fourier-transform infrared spectroscopy (FTIR). At the tested conditions, we found that Mg-carbonates with a mol% Mg higher than 40% (i.e., potential dolomite precursor phases) formed exclusively in association with EPS rich in carbohydrates (TCHO > than 75% of the total mass). FTIR spectra of CaMg carbonate-forming strains were distinct from those of the non-mineral-forming strain in areas associated with the protein structures responsible for the formation of hydrogels, which contribute to hydration or dehydration of ionic clusters; further differences have been observed in the regions of phosphoryl functional groups. These results provide insight on which fraction of organic molecules and specific functional groups are, among the many constituents of EPS, important for mineral nucleation and incorporation of Mg into carbonate minerals, a crucial step for the formation of dolomite in natural environments.

  • Vanadium cycling in the Western Arctic Ocean is influenced by shelf-basin connectivity
    Mar. Chem. (IF 2.713) Pub Date : 2019-09-10
    Laura M. Whitmore, Peter L. Morton, Benjamin S. Twining, Alan M. Shiller

    Water in the western Arctic Ocean tends to show lower dissolved vanadium concentrations than profiles observed elsewhere in the open ocean. Dissolved V in Pacific-derived basin waters was depleted by approximately 15–30% from the effective Pacific Ocean endmember. The depletion originates on western Arctic shelves and is not a result of mixing with a water mass with low V. While biological uptake may account for some of the V removal from the water column, adsorption onto particulate Fe is likely the dominant factor in removing V from shelf waters to the sediments. Once in the sediments, reduction should result in sequestering the V while Fe (and Mn) can be remobilized. A similar Fe-shuttling mechanism for V was previously described for the Peru margin (Scholz et al. 2011). Off the shelves, particulate Mn concentrations often exceed particulate Fe concentrations and thus may exert greater control on the V distribution in basin waters. Nonetheless, particulate V concentrations are much lower in basin waters and dissolved V thus behaves largely conservatively away from the shelf environment. Dissolved V concentrations in Atlantic-derived and Arctic deep waters were as much as 5 nmol/kg lower than those observed in deep waters of other ocean basins. The uniformity in deep water dissolved V between the sampled basins suggests that slow removal of V from the deep basins is probably not a factor in the deep water depletion. Vanadium-depleted incoming Atlantic waters (i.e., the source of Arctic deep waters) and/or removal of vanadium from incoming waters that pass over the shelves probably accounts for the deep water dissolved V depletion. Overall, our results demonstrate the utility of the V distribution as an additional tool to help understand the Arctic marine system. Furthermore, our work is pertinent to questions related to the net effect of marginal basin shelves on oceanic vanadium cycling, its isotopic balance, and how climate-induced changes in shelf biogeochemical cycling will impact vanadium cycling.

  • Annual variability of dissolved manganese in Northeast Pacific along Line-P: 2010–2013
    Mar. Chem. (IF 2.713) Pub Date : 2019-09-11
    Nari Sim, Kristin J. Orians

    The distribution of dissolved manganese (dMn) in the northeast Pacific across the Line-P transect was evaluated to investigate the mechanisms responsible for the spatial and temporal variability of dMn. A total of 299 filtered seawater samples (< 0.4 μm) collected in August at the five major stations each year from 2010 to 2013 were analyzed. Vertical profiles of dMn showed a clear distinction between onshore and offshore stations. Within the Summer Mixing layer (SML), we observed high dMn concentrations in all years driven by external sources, such as river water, coastal sediments or eolian dust, as well as photo-reduction. At the onshore stations, the absolute concentration of dMn at the surface was annually variable, depending on the strength of the Ekman transport. Within the subsurface layer, dMn decreased rapidly with depth down to 150 m due to particle scavenging. Within the Oxygen Minimum Zone (OMZ), near the continental margin, we observed elevated and annually variable dMn. The high dMn concentration is likely due to mixing of remobilized Mn from the continental margin, which varies year-to-year depending on the intensity of sedimentary Mn reduction process. At the offshore stations, dMn showed subsurface maxima, within the Winter Mixing Layer (WML) rather than surface maxima. We attribute this to the combined effect of biological drawdown of dMn in the surface during the spring and summer, where iron (Fe) is depleted in this High Nutrient and Low Chlorophyll (HNLC) region, and remnant dMn deeper in the WML, from earlier in the year. Using a simple one-dimensional advection and diffusion model, we identified that dMn found in North Pacific Intermediate Water (NPIW) at the western end of the transect can be advected as far as the outermost of the onshore stations in 2011 and 2013, while dMn in the central area of the transect is likely diluted by low dMn water from the south in 2010 and 2012. Within the OMZ, dMn showed elevated and annually variable concentrations, yet the enhancement of dMn of this study area was found to be less intense than in other regions. Lastly, an eddy found in 2010 enhanced dMn concentrations within the surface mixing layers.

  • Perspective on identifying and characterizing the processes controlling iron speciation and residence time at the atmosphere-ocean interface
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-24
    Nicholas Meskhidze, Christoph Völker, Hind A. Al-Abadleh, Katherine Barbeau, Matthieu Bressac, Clifton Buck, Randelle M. Bundy, Peter Croot, Yan Feng, Akinori Ito, Anne M. Johansen, William M. Landing, Jingqiu Mao, Stelios Myriokefalitakis, Daniel Ohnemus, Benoît Pasquier, Ying Ye
  • Biodegradability of hydrothermally altered deep-sea dissolved organic matter
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-22
    Christian T. Hansen, Jutta Niggemann, Helge-Ansgar Giebel, Meinhard Simon, Wolfgang Bach, Thorsten Dittmar

    Deep-sea dissolved organic matter (DOM) constitutes a huge carbon reservoir in the worlds' oceans that – despite its abundance – is virtually unused as a substrate by marine heterotrophs. Heating within hydrothermal systems induces major molecular modifications of deep-sea DOM. Here, we tested the hypothesis that hydrothermal heating of deep-sea DOM enhances bioavailability. Aliquots of DOM extracted from the deep North Pacific (North Equatorial Pacific Intermediate Water; NEqPIW) were re-dissolved in artificial seawater and subjected to temperatures of 100 and 200 °C (40 MPa) using Dickson-type reactors. In agreement with earlier findings we observed a temperature-related drop in dissolved organic carbon (DOC) concentration (−6.1% at 100 °C, −21.0% at 200 °C) that predominantly affected the solid-phase extractable (SPE-DOC) fraction (−18.2% at 100 °C, −51.4% at 200 °C). Fourier-transform ion cyclotron resonance mass spectrometric (FT-ICR-MS) analysis confirmed a temperature-related reduction of average molecular mass, O/C ratios, double bond equivalents (DBE) and a relative increase in aromaticity (AImod). This thermally altered DOM was added (25 μmol L−1 DOC) to deep-water samples from the South West Pacific (Kermadec Arc, RV Sonne / SO253, 32° 37.706′ S | 179° 38.728′ W) and incubated with the prevailing natural microbial community. After 16 days at 4 °C in the dark, prokaryotic cell counts in incubations containing the full spectrum of thermally-degraded DOM (extractable and non-extractable compounds) had increased considerably (on average 21× for DOM100°C and 27× for DOM200°C). In contrast, prokaryotic growth in incubations to which only solid-phase extractable thermally-altered DOM was added was not enhanced compared to control incubations. The experiments demonstrate that temperature-driven degradation of deep-sea recalcitrant DOM within hydrothermal systems turns fractions of it accessible to microbes. The thermally-produced DOM compounds that stimulate microbial growth are not retained on reversed-phase resins (SPE-DOM) and are likely low-molecular mass organic acids. Despite the comprehensive compositional modifications of SPE-DOM (SPE-DOM) through heating, it remains inaccessible to microbes at the investigated concentration levels. The microbial incubation resulted in only minor and mostly insignificant overall changes in SPE-DOM molecular composition and concentration.

  • Seasonal and spatial changes in carbon and nitrogen fluxes estimated using 234Th:238U disequilibria in the North Pacific tropical and subtropical gyre
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-16
    Blaire P. Umhau, Claudia R. Benitez-Nelson, Hilary G. Close, Cecelia C.S. Hannides, Laura Motta, Brian N. Popp, Joel D. Blum, Jeffrey C. Drazen

    Particle dynamics are an essential component of global ocean biogeochemistry as they transport essential nutrients, carbon, and other reactive elements and compounds from the surface ocean to depth in the water column. The North Pacific Ocean is characterized by spatial and temporal variations in particle export mediated by a diverse food web and variations in environmental conditions, such as oxygenation. Here we explored temporal variability in the downward flux of particulate carbon (PC) and nitrogen (PN) using 238U234Th disequilibria, sediment traps and in situ pumps in winter, spring and summer at the time-series site Station ALOHA, a region characterized by a well-defined summer export pulse that influences the composition and structure of the biological community within the mesopelagic zone. We further explored spatial variability in PC and PN fluxes along a latitudinal gradient (17.5°-5°N x 150°W) that transitions from a low to high productivity region influenced by equatorial upwelling, with several stations further characterized by a shallow (~ 130 m) oxygen minimum zone. Winter PC and PN fluxes at 150 m at Station ALOHA were low, while summer and spring fluxes were significantly higher, coinciding with a seasonal export pulse associated with diazotrophs. PC and PN fluxes along the 155°W transect were also low at 150 m and similar to those measured at Station ALOHA in winter. At Station ALOHA zooplankton impart a greater influence over both small and large PC (and PN) fluxes in February relative to September or May, when heterotrophic bacteria play a proportionally larger role in particle remineralization and cycling. Along the transect stations, PC fluxes were too low to discern any clear trends with latitude, likely due to El Niño conditions at the time of sampling. Where vertical water column profiles of PC and PN fluxes were available, PC (and PN) fluxes were found to peak in the subsurface at 8°N (50 m) and 5°N (75 m) with zooplankton grazing and microbial remineralization following patterns similar to those found in February at Station ALOHA. Combined, these results support the hypothesis that small and large particles contribute to mesopelagic carbon demand depending on season, with smaller particles having greater contributions to mesopelagic food webs when surface derived particle export is low.

  • Biological degradation of ethanol in Southern California coastal seawater
    Mar. Chem. (IF 2.713) Pub Date : 2019-10-05
    Warren J. de Bruyn, Catherine D. Clark, Mary Senstad, Natalie Toms, Aaron W. Harrison

    The increased use of ethanol as an additive in gasoline is expected to increase atmospheric ethanol levels. Understanding future impacts requires an understanding of the current atmospheric ethanol budget. Air-sea exchange is one of the largest sources of uncertainty in budget estimates. Understanding processes that produce or destroy ethanol in seawater will help constrain air-sea exchange estimates. The rate of biological consumption of ethanol in seawater was measured in Southern California surface coastal water sampled from a tidally flushed river mouth over a six-month period. First-order rate constants (k) for biological degradation of approximately ambient ethanol levels of 89 nM ranged from zero to 1.9 ± 0.1 × 10−2 min−1 with an average of 3.8 ± 5.9 × 10−3 min−1 (n = 20). This corresponds to an average biological turnover time (1/k) of approximately 4.4 h or an average biological half-life (0.693/k) of approximately 3 h. There were several rain events over this time period resulting in flow from upstream in the river; these were associated with increased bacterial levels, higher absorption coefficients and reduced salinity. Rate constants, in general, increased with the number of bacterial colony forming units in the water sample. Excluding the two rate constant maxima events the background biological degradation rate constant in the absence of rain/bacteria input from upstream, is 1.3 ± 1.0 × 10−3 min−1. This corresponds to an average biological turnover time of approximately 13 h and an average biological half-life of approximately 9 h. Autoclaved samples (to remove bacteria) showed no measurable ethanol degradation, suggesting that chemical loss processes are not significant.

  • The 210Po/210Pb method to calculate particle export: Lessons learned from the results of three GEOTRACES transects
    Mar. Chem. (IF 2.713) Pub Date : 2019-08-12
    Yi Tang, Gillian Stewart

    The deviation from secular equilibrium between the natural radionuclide 210Po (half-life: 138.4 d) and its radioactive grandparent 210Pb (half-life: 22.3 y) has been used to examine particle export from the surface ocean. Here we combine 210Po and 210Pb activity results from three GEOTRACES transects: two transects of the North Atlantic Ocean (GA03: 15–40°N, and GA01: 40–60°N) and one transect of the South Pacific Ocean (GP16: 10–15°S), and estimate 210Po export fluxes at the base of the primary production zone (PPZ) by assuming steady state (SS) without advection or diffusion of the isotopes. The SS 210Po flux was sometimes lower at basin margins than at the open-ocean stations along the transects. High SS 210Po flux estimations derived in the North Atlantic subtropical gyre may be associated with the atmospheric deposition of 210Pb to the surface ocean. In this paper we also question the validity of the SS assumption and discuss the influence of vertical advection and diffusion on the overall 210Po activity balance. The SS model may have underestimated the export flux of 210Po at margin stations in the GA03 and GP16 transects and along the GA01 cruise track. We found that upwelling in the Peruvian coastal region and near the Greenland shelf had a dramatic impact on the estimated 210Po flux balance. Vertical diffusion had limited influence on the 210Po export fluxes along GA03 and GA01 in the North Atlantic whereas it added 210Po export fluxes by as much as 190% in GP16 in the Pacific, especially at the shelf stations 1 and 4. Further, analysis of the partitioning coefficient suggested the importance of small particles in the scavenging of radionuclides. This suggests it is wise to sample small particles along with large particles to determine the ratio of the concentration of particulate organic carbon (POC) to 210Po activity (POC/210Po) for the lower limit of POC export flux estimations. Finally, the observation of the deficit of 210Po relative to 210Pb activity (210Po/210Pb < 1) in seawater concurrent with a deficit of 210Po in particles contradicts our understanding of the conceptual 210Po flux model, which assumes that 210Po activity is more effectively removed from the surface ocean via particles than 210Pb activity. While this observation deserves more attention, we propose two possible solutions: (1) the deficit of total 210Po relative to 210Pb activity in the surface ocean may be due to an input of 210Pb activity instead of/concurrent with a relative removal of 210Po activity via particle export; or (2) the particles collected may not be identical to the ones that have originally created the observed deficit in total 210Po activity.

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上海纽约大学William Glover