Sulfidic Bottom Waters Research Articles

Eutrophication and Deoxygenation Drive High Methane Emissions from a Brackish Coastal System.

Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate-methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2-8.6 mmol m-2 d-1) at sites where the SMTZ is located close to the sediment-water interface (2-10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m-2 d-1 and from 0.1 to 1.7 mmol m-2 d-1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.

Revisiting the applicability and constraints of molybdenum- and uranium-based paleo redox proxies: comparing two contrasting sill fjords

Abstract. Sedimentary molybdenum (Mo) and uranium (U) enrichments are often used as redox proxies to reconstruct bottom water redox changes. However, these redox proxies may not be equally reliable across a range of coastal settings due to varying depositional environments. Fjords vary greatly in their depositional conditions, due to their unique bathymetry and hydrography, and are highly vulnerable to anthropogenic and climatic pressures. Currently, it is unknown to what extent Mo and U sequestration is affected by variable depositional conditions in fjords. Here, we use pore water and sequential extraction data to investigate Mo and U enrichment pathways in sediments of two sill fjords on the Swedish west coast with contrasting depositional environments and bottom water redox conditions. Our data suggest that sedimentary authigenic Mo and U pools differ between the two fjords. At the (ir)regularly dysoxic (oxygen = 0.2–2 mL L−1) Gullmar Fjord, authigenic Mo largely binds to manganese (Mn) oxides and to a lesser extent to iron (Fe) oxides; Mo sulfides do not play a major role due to low sulfate reduction rates, which limits the rate of Mo burial. Authigenic U largely resides in carbonates. At the (ir)regularly euxinic (oxygen = 0 mL L−1; total hydrogen sulfide ≥ 0 mL L−1) Koljö Fjord, authigenic Mo is significantly higher due to binding with more refractory organic matter complexes and Mo-Fe-sulfide phases. Uranium is moderately enriched and largely bound to organic matter. We found no direct evidence for temporal changes in bottom water redox conditions reflected in Mo and U enrichments at either Gullmar Fjord or Koljö Fjord. While sulfidic bottom waters favor Mo sequestration at Koljö Fjord, enrichment maxima reflect a combination of depositional conditions rather than short-term low-oxygen events. Our data demonstrate that secondary pre- and post-depositional factors control Mo and U sequestration in fjords to such an extent that bottom water redox conditions are either not being systematically recorded or overprinted. This explains the large variability in trace metal enrichments observed in fjords and has implications for applying Mo and U as proxies for environmental redox reconstructions in such systems.

Constraining the major pathways of vanadium incorporation into sediments underlying natural sulfidic waters

Vanadium (V) is a redox-sensitive trace metal. Interconversion between different vanadium species (e.g., VIII, VIV, VV) occurs rapidly in association with changes in redox conditions in natural environments. Different vanadium species exhibit variable geochemical behavior that impacts if and how they are sequestered into sediments, which makes vanadium a useful proxy for ancient ocean redox conditions. However, we still lack constraints on how different dissolved vanadium sink pathways impact distributions of vanadium in water column, pore waters, and sediments, especially in natural sulfidic settings. In the present study, we combined field and experimental data to constrain the major scavenging processes that influence the partitioning of vanadium between aqueous and solid phases to provide an improved picture of vanadium geochemical cycling under anoxic-sulfidic conditions. We find that in the sulfidic bottom waters and sediment pore waters of the Chesapeake Bay, vanadium concentrations are correlated with manganese (Mn) concentrations in both aqueous and solid phases. Our experimental results support our interpretation that elevated concentrations of vanadium in anoxic-sulfidic waters result from the release of vanadium due to desorption from and/or dissolution of Mn oxides in anoxic/sulfidic waters. While sorption of vanadium results in strongly adsorbed complexes on both Mn oxides and pyrite surfaces, vanadium preferentially sorbs to Mn oxides rather than to pyrite. Thus, the impact of Mn oxides on vanadium geochemistry is greater than that imposed by scavenging via sorption on pyrite, although both processes should be considered. Mn oxides also have a greater impact on vanadium geochemistry than reductive formation of solid V(OH)3 by H2S. Sorption of vanadium is sensitive to pH conditions (especially in anoxic waters), moderately sensitive to ionic strength, and weakly sensitive to phosphate concentrations. In addition, our results show that as waters become more sulfidic, dissolved V/Mn molar ratios increase, whereas solid phase V/Mn molar ratios decrease, indicating that the V/Mn molar ratio can be used to trace redox changes. Finally, we investigate the V/Mn record of black shales that span the Proterozoic and Phanerozoic to show a framework of utilizing this proxy with applicability to identify changes in redox conditions associated with both oxygenation and anoxic events in ancient oceans.

Mariniplasma anaerobium gen. nov., sp. nov., a novel anaerobic marine mollicute, and proposal of three novel genera to reclassify members of Acholeplasma clusters II-IV.

A novel strictly anaerobic chemoorganotrophic bacterium, designated Mahy22T, was isolated from sulfidic bottom water of a shallow brackish meromictic lake in Japan. Cells of the strain were Gram-stain-negative, non-motile and coccoid in shape with diameters of about 600-800 nm. The temperature range for growth was 15-37 °C, with optimum growth at 30-32 °C. The pH range for growth was pH 6.2-8.9, with optimum growth at pH 7.2-7.4. The strain grew with NaCl concentrations of 5% or below (optimum, 2-3%). Growth of the strain was enhanced by the addition of thiosulfate. The major cellular fatty acids were C16:0 and anteiso-C15:0. Respiratory quinones were not detected. The complete genome sequence of strain Mahy22T possessed a 1 885 846 bp circular chromosome and a 12 782 bp circular genetic element. The G+C content of the genome sequence was 30.1 mol%. Phylogenetic analysis based on the 16S rRNA gene revealed that the novel strain belonged to the family Acholeplasmataceae, class Mollicutes. The closest relative of strain Mahy22T with a validly published name was Acholeplasma palmae J233T with a 16S rRNA gene sequence similarity of 90.5%. Based on the results of polyphasic analysis, the name Mariniplasma anaerobium gen. nov., sp. nov. is proposed to accommodate strain Mahy22T, along with reclassification of some Acholeplasma species into Alteracholeplasma gen. nov., Haploplasma gen. nov. and Paracholeplasma gen. nov.

A sequential extraction procedure for particulate manganese and its application to coastal marine sediments

An existing sequential extraction scheme for particulate iron (Fe) is evaluated for manganese (Mn) using a range of Mn standards. The scheme consists of 5 steps and quantifies 5 operationally defined Mn pools (1) poorly ordered Mn oxides and Mn phosphates (ascorbic acid extractable); (2) Mn carbonates and Mn sulfides (1M HCl extractable), (3 and 4) crystalline Mn oxides (citrate buffered dithionite and ammonium oxalate extractable, respectively) and (5) Mn associated with pyrite (concentrated HNO3 extractable). Application of the extraction scheme to coastal sediments from six locations (Black Sea, Baltic Sea, Bothnian Sea, Gulf of Mexico and Chesapeake Bay) highlights the dependency of sediment Mn partitioning on bottom water redox conditions. In sediments deposited in anoxic and sulfidic (euxinic) bottom waters, Mn is mostly present in Mn carbonates, pyrite and in non-reactive Mn forms, in approximately equal amounts. We find that in sediments deposited in periodically euxinic and hypoxic (oxygen <63μmolL−1) waters, Mn carbonates dominate over the two other fractions, and small amounts of Mn oxides are observed. In sediments deposited in oxygenated bottom waters, Mn oxides, Mn-rich vivianite-type minerals and Mn carbonates dominate and no pyrite-bound Mn is observed. A large advantage of the extraction scheme is that it quantifies sediment forms of Mn and Fe simultaneously. Given the role of Mn as a bottom water redox proxy, the separation of poorly ordered Mn oxides, carbonates and pyrite is of specific relevance.

Genomic evidence for sulfur intermediates as new biogeochemical hubs in a model aquatic microbial ecosystem

BackgroundThe sulfur cycle encompasses a series of complex aerobic and anaerobic transformations of S-containing molecules and plays a fundamental role in cellular and ecosystem-level processes, influencing biological carbon transfers and other biogeochemical cycles. Despite their importance, the microbial communities and metabolic pathways involved in these transformations remain poorly understood, especially for inorganic sulfur compounds of intermediate oxidation states (thiosulfate, tetrathionate, sulfite, polysulfides). Isolated and highly stratified, the extreme geochemical and environmental features of meromictic ice-capped Lake A, in the Canadian High Arctic, provided an ideal model ecosystem to resolve the distribution and metabolism of aquatic sulfur cycling microorganisms along redox and salinity gradients.ResultsApplying complementary molecular approaches, we identified sharply contrasting microbial communities and metabolic potentials among the markedly distinct water layers of Lake A, with similarities to diverse fresh, brackish and saline water microbiomes. Sulfur cycling genes were abundant at all depths and covaried with bacterial abundance. Genes for oxidative processes occurred in samples from the oxic freshwater layers, reductive reactions in the anoxic and sulfidic bottom waters and genes for both transformations at the chemocline. Up to 154 different genomic bins with potential for sulfur transformation were recovered, revealing a panoply of taxonomically diverse microorganisms with complex metabolic pathways for biogeochemical sulfur reactions. Genes for the utilization of sulfur cycle intermediates were widespread throughout the water column, co-occurring with sulfate reduction or sulfide oxidation pathways. The genomic bin composition suggested that in addition to chemical oxidation, these intermediate sulfur compounds were likely produced by the predominant sulfur chemo- and photo-oxidisers at the chemocline and by diverse microbial degraders of organic sulfur molecules.ConclusionsThe Lake A microbial ecosystem provided an ideal opportunity to identify new features of the biogeochemical sulfur cycle. Our detailed metagenomic analyses across the broad physico-chemical gradients of this permanently stratified lake extend the known diversity of microorganisms involved in sulfur transformations over a wide range of environmental conditions. The results indicate that sulfur cycle intermediates and organic sulfur molecules are major sources of electron donors and acceptors for aquatic and sedimentary microbial communities in association with the classical sulfur cycle.-hYdWkMRSgNRZXNRu4CdmLVideo abstract

Methane seepage patterns during the middle Pleistocene inferred from molybdenum enrichments of seep carbonates in the South China Sea

Seepage of methane-rich fluids, are widespread along the continental margins of the world. Intense seepages of methane may lead to the emission of this powerful greenhouse gas into the hydrosphere and atmosphere with implications in the understanding of past dramatic marine environment changes and global warming. However, euxinic environments with sulfidic bottom water as a result of vigorous seepages at ancient cold seeps remain unidentified. Here we report trace element geochemistry and associated mineralogical and carbon and oxygen isotopic data in seep carbonates collected from long drill cores in the South China Sea. Three distinct stages of methane seepages events probably caused by the dissociation of gas hydrates are discernible. Carbonates formed in stage 2 during the Marine Isotope Stage 5 (MIS5) are mainly occurred as aragonite and show extremely high molybdenum (Mo) contents (up to 118 μg/g), and strong enrichments of authigenic Mo in relative to uranium. These results suggest the presence of intermittent or even permanent euxinic conditions generated by sulfidic bottom water that result from intensive methane seepages. In contrast, mineral compositions and trace element patterns indicate that the carbonates before and after the MIS 5 (stages 1 and 3) form within sulfidic conditions restricted to pore water under relatively lower methane fluxes. Our results highlight extremely intense methane seepages that may potentially result in localized euxinic environments. The inferred euxinic environments imply more amount of massive methane release into the water column than a previous estimation, which was triggered by large-scale dissociation of gas hydrates in the South China Sea during MIS 5.

Inverse correlation between the molybdenum and uranium isotope compositions of Upper Devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations

Coupled Mo-U isotope data from the Upper Devonian Kettle Point Formation (Ontario, Canada) provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales. In the Gore of Chatham core, the Kettle Point black shales have high Mo concentrations (48–473 μg/g) and consistently high Mo/U ratios (≥3 times the Mo/U ratio of modern seawater), suggesting a euxinic depositional environment. These shales yield an inverse correlation (r = 0.89, p < 0.001) between authigenic U isotope (δ238U = −0.3‰ to +0.6‰ relative to CRM145 = 0‰) and Mo isotope (δ98Mo = +0.5‰ to +2.0‰ relative to NIST SRM 3134 = 0.25‰) compositions and a stratigraphic trend upsection towards lower δ98Mo and higher δ238U. These stratigraphic trends were probably not caused by global ocean redox variations, which should shift both isotope systems in the same direction. Instead, the inverse correlation between the δ98Mo and δ238U of the black shales indicates that changes to local depositional conditions in the Chatham Sag affected both isotope systems, consistent with the lithological and elemental evidence for variable paleohydrographic conditions (paleosalinity and sea-level). Black shales with the highest δ98Mo and lowest δ238U likely capture the most quantitative removal of Mo and U from more intensely sulfidic bottom waters, and thus most closely approximate global seawater isotope compositions. Lower δ98Mo and higher δ238U in the black shales likely record less quantitative Mo and U removal from weakly euxinic bottom waters and thus capture greater sediment-seawater isotope fractionation (lighter Mo isotopes and heavier 238U were preferentially removed from seawater to the sediments). Some samples with low δ98Mo (0.5–1.0‰) have high V and Mo enrichments, suggesting delivery of V and isotopically light Mo to the sediments by Fe-Mn particulates, but this process does not significantly affect U isotopes (because of weak adsorption of U onto Fe-Mn particulates) and is thus not the major driver of the inverse correlation. The highest δ98Mo (2.0‰) and lowest δ238U (−0.3‰) from the lower Kettle Point Formation represent minimum and maximum estimates of the Mo and U isotope compositions, respectively, of early Famennian global seawater. The slope of the inverse Mo-U isotope correlation for all Kettle Point black shales broadly parallels data from modern euxinic basins but is shifted to slightly lower isotopic compositions. Hence, Famennian seawater likely had δ98Mo (about 2.0–2.2‰) and δ238U (about −0.7‰ to −0.4‰) values that were slightly lower than modern global seawater (δ98Mo = 2.3‰; δ238U = −0.4‰), suggesting a mildly greater extent of euxinia (up to 5% of the seafloor) in the Famennian oceans. The opposite stratigraphic trends in δ98Mo and δ238U through the Kettle Point Formation may have caused misinterpretation of global ocean redox conditions if either isotope system was used individually. Hence, paired Mo and U isotope analyses are recommended to determine if stratigraphic trends through euxinic black shales are caused by local depositional changes or global ocean redox variations.

Brief oxygenation events in locally anoxic oceans during the Cambrian solves the animal breathing paradox

Oxygen is a prerequisite for all large and motile animals. It is a puzzling paradox that fossils of benthic animals are often found in black shales with geochemical evidence for deposition in marine environments with anoxic and sulfidic bottom waters. It is debated whether the geochemical proxies are unreliable, affected by diagenesis, or whether the fossils are transported from afar or perhaps were not benthic. Here, we improved the stratigraphic resolution of marine anoxia records 100–1000 fold using core-scanning X-Ray Fluorescence and established a centennial resolution record of oxygen availability at the seafloor in an epicontinental sea that existed ~501–494 million years ago. The study reveals that anoxic bottom-water conditions, often with toxic hydrogen sulfide present, were interrupted by brief oxygenation events of 600–3000 years duration, corresponding to 1–5 mm stratigraphic thickness. Fossil shells occur in some of these oxygenated intervals suggesting that animals invaded when conditions permitted an aerobic life style at the seafloor. Although the fauna evidently comprised opportunistic species adapted to low oxygen environments, these findings reconcile a long-standing debate between paleontologists and geochemists, and shows the potential of ultra-high resolution analyses for reconstructing redox conditions in past oceans.

Abundance and Biogeochemical Impact of Cable Bacteria in Baltic Sea Sediments.

Oxygen depletion in coastal waters may lead to release of toxic sulfide from sediments. Cable bacteria can limit sulfide release by promoting iron oxide formation in sediments. Currently, it is unknown how widespread this phenomenon is. Here, we assess the abundance, activity, and biogeochemical impact of cable bacteria at 12 Baltic Sea sites. Cable bacteria were mostly absent in sediments overlain by anoxic and sulfidic bottom waters, emphasizing their dependence on oxygen or nitrate as electron acceptors. At sites that were temporarily reoxygenated, cable bacterial densities were low. At seasonally hypoxic sites, cable bacterial densities correlated linearly with the supply of sulfide. The highest densities were observed at Gulf of Finland sites with high rates of sulfate reduction. Microelectrode profiles of sulfide, oxygen, and pH indicated low or no in situ cable bacteria activity at all sites. Reactivation occurred within 5 days upon incubation of an intact sediment core from the Gulf of Finland with aerated overlying water. We found no relationship between cable bacterial densities and macrofaunal abundances, salinity, or sediment organic carbon. Our geochemical data suggest that cable bacteria promote conversion of iron monosulfides to iron oxides in the Gulf of Finland in spring, possibly explaining why bottom waters in this highly eutrophic region rarely contain sulfide in summer.

Extreme sulfur isotope fractionation in the Late Devonian Dry Creek volcanogenic massive sulfide deposit, central Alaska

The Dry Creek Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, in east-central Alaska, occurs in a Late Devonian sequence of peralkaline rhyolite tuff, minor graphitic argillite, and local peralkaline quartz-porphyry rhyolite intrusions. Principal mineralized facies are semi-massive and massive sulfide in variably silicified and graphitic rhyolite tuff, massive sulfide in graphitic argillite, disseminated sulfides in graphitic and non-graphitic rhyolite tuff, and vein-hosted sulfides in a subvolcanic, peralkaline quartz-porphyry intrusion. In situ analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in δ34S values from −48.0 to 23.1‰. This remarkable 71.1‰ variation is more than twice the largest range known for sulfides in nearly all individual VMS deposits, both modern and ancient, which typically is <25‰. Based on results of geochemical reaction modeling presented here, extreme sulfur isotope systematics in the Dry Creek deposit is attributed to two main processes: low-temperature (<100 °C) kinetically controlled bacterial sulfate reduction (BSR) in anoxic to locally sulfidic bottom waters and pore fluids, and higher temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater sulfate during water-rock reactions. Deposition of organic-rich sediments prior to and during mineralization provided the key environment that promoted microbially mediated fractionations with Δ34Ssulfate-sulfide of up to 70‰. During BSR reaction progress modeling, residual sulfate evolved by Rayleigh distillation in pore water, producing δ34SSO4 values up to 58‰; normal Late Devonian seawater sulfate had δ34SSO4 = 22‰. BSR produced FeS precipitates with δ34S values from −50 to −15‰. Integrated textural and δ34S data also suggest that isotopically light H2S produced in pore fluids by BSR was incorporated into hydrothermal fluids and early disseminated sulfide minerals during the emplacement of peralkaline quartz-porphyry rhyolite intrusions in the shallow subsurface, broadly coeval with VMS mineralization. Modeling of moderate- to high-temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater-rock interaction predicts sulfide minerals with δ34S values from −8 to 19‰.High-precision CA-ID-TIMS U-Pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 206Pb/238U date of 363.02 ± 0.43 Ma (2σ total uncertainty). This date indicates that the formation of graphitic and sulfidic sediments at Dry Creek, and contemporaneous VMS mineralization, occurred at least 3.6 m.y. before, and hence are unrelated to, widespread black shale deposition during the Hangenberg Event at ca. 359 Ma.

Microbial Diversity in a Hypersaline Sulfate Lake: A Terrestrial Analog of Ancient Mars.

Life can persist under severe osmotic stress and low water activity in hypersaline environments. On Mars, evidence for the past presence of saline bodies of water is prevalent and resulted in the widespread deposition of sulfate and chloride salts. Here we investigate Spotted Lake (British Columbia, Canada), a hypersaline lake with extreme (>3 M) levels of sulfate salts as an exemplar of the conditions thought to be associated with ancient Mars. We provide the first characterization of microbial structure in Spotted Lake sediments through metagenomic sequencing, and report a bacteria-dominated community with abundant Proteobacteria, Firmicutes, and Bacteroidetes, as well as diverse extremophiles. Microbial abundance and functional comparisons reveal similarities to Ace Lake, a meromictic Antarctic lake with anoxic and sulfidic bottom waters. Our analysis suggests that hypersaline-associated species occupy niches characterized foremost by differential abundance of Archaea, uncharacterized Bacteria, and Cyanobacteria. Potential biosignatures in this environment are discussed, specifically the likelihood of a strong sulfur isotopic fractionation record within the sediments due to the presence of sulfate reducing bacteria. With its high sulfate levels and seasonal freeze-thaw cycles, Spotted Lake is an analog for ancient paleolakes on Mars in which sulfate salt deposits may have offered periodically habitable environments, and could have concentrated and preserved organic materials or their biomarkers over geologic time.

A molybdenum-isotope perspective on Phanerozoic deoxygenation events

The careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters. A synthesis and analysis of data from two Mesozoic ocean anoxic events and the Palaeocene-Eocene thermal maximum applies these techniques to constrain past ocean deoxygenation.

Isotopic composition of nitrate and particulate organic matter in a pristine dam reservoir of western India: implications for biogeochemical processes

Abstract. Isotopic composition of nitrate (δ15N and δ18O) and particulate organic matter (POM; δ15N and δ13C) were measured in the Tillari Reservoir, located at the foothills of the Western Ghats, Maharashtra, western India. The reservoir, which is stratified during spring–summer and autumn seasons but gets vertically mixed during the southwest monsoon (SWM) and winter, is characterized by diverse redox nitrogen transformations in space and time. The δ15N and δ18O values of nitrate were low (δ15N = 2–10 ‰, δ18O = 5–8 ‰) during normoxic conditions but increased gradually (the highest at δ15N = 27 ‰, δ18O = 29 ‰) when anoxic conditions facilitated denitrification in the hypolimnion during spring–early summer. Once nitrate was fully utilized and sulfidic conditions set in, NH4+ became the dominant inorganic N species, with δ15N ranging from 1.3 to 2.6 ‰. Low δ15N (∼ −5 ‰) and δ13C (−37 to −32 ‰) of POM co-occurring with high NH4+ and CH4 in sulfidic bottom waters were probably the consequence of microbial chemosynthesis. Assimilation of nitrate in the epilimnion was the major controlling process on the N isotopic composition of POM (δ15N = 2–6 ‰). Episodic low δ15N values of POM (−2 to 0 ‰) during early summer, coinciding with the absence of nitrate, might arise from N fixation, although further work is required to confirm the hypothesis. δ13C POM in the photic zone ranged between −29 and −27 ‰ for most parts of the year. The periods of mixing were characterized by uniform δ15N–NO3− and δ18O–NO3− at all depths. Higher POM (particulate organic carbon, POC, as well as particulate organic nitrogen, PON) contents and C ∕ N values with lower δ13C POM during the SWM point to allochthonous inputs. Overall, this study, the first of its kind in the Indian subcontinent, provides an insight into biogeochemistry of Indian reservoirs, using stable carbon and nitrogen isotopes as a tool, where the monsoons play an important role in controlling vertical mixing and dynamics of carbon and nutrients.

Organic geochemistry of the Lower Toarcian Posidonia Shale in NW Europe

A suite of core and outcrop samples of the Lower Toarcian Posidonia Shale and time-equivalent deposits was collected from multiple locations, including southern Germany (Swabian Alb and Franconian Alb), northern Germany, the east coast of Great Britain, the Netherlands, and Luxembourg, in order to study major differences and similarities with respect to depositional environment and redox potential based on molecular geochemical data. All samples are immature or in the early oil window with the highest thermal maturity occurring in the profile from Great Britain (about 0.7% vitrinite reflectance). Molecular geochemical maturity parameters were used to quantify maturation in detail.During shale deposition in the West Cleveland Basin, Great Britain, H2S was immediately consumed in the water column by terrigenous input, supplying abundant reactive iron to form pyrite. These samples are characterized by low dibenzothiophene/phenanthrene ratios (<0.2) and gammacerane indices (<0.1) and are accompanied by high sulfur contents and abundant tiny pyrites (>95% of<7μm in diameter), generated within the water column due to the immediate reaction of Fe2+ and H2S. In contrast, sulfidic (euxinic) bottom waters, possibly with enhanced salinity and stratified water column, prevailed due to a more restricted iron supply in Luxembourg, northern Germany and the Franconian Alb of southern Germany. A more oxygenated (dysoxic) bottom water column associated with anoxic sediments, i.e., anoxic/dysoxic boundary close to the sediment/water interface, prevailed in the Swabian Alb of southern Germany. Despite observed changes within each vertical profile at each site, large scale regional trends in depositional conditions can be clearly identified for the Early Toarcian.

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle

Understanding the global redox state of the oceans and its cause-and-effect relationship with periods of widespread organic-carbon deposition is vital to interpretations of Earth's climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency. Here, we present a compilation of new and published data from an organic-rich locality within the proto-North Atlantic Ocean during the Cenomanian–Turonian boundary event that shows a dramatic drawdown of redox-sensitive trace elements. Iron geochemistry independently suggests euxinic deposition (i.e., anoxic and sulfidic bottom waters) for the entire section, thus confirming its potential as an archive of global marine metal inventories. In particular, depleted molybdenum (Mo) and vanadium (V) concentrations effectively record the global expansion of euxinic and oxygen-deficient but non-sulfidic waters, respectively. The V drawdown precedes the OAE, fingerprinting an expansion of oxygen deficiency prior to an expansion of euxinia. Molybdenum drawdown, in contrast, is delayed with respect to V and coincides with the onset of OAE2. Parallel lipid biomarker analyses provide evidence for significant and progressive reorganization of marine microbial ecology during the OAE in this region of the proto-North Atlantic, with the smallest relative eukaryotic contributions to total primary production occurring during metal-depleted intervals. This relationship may be related to decreasing supplies of enzymatically important trace elements. Similarly, box modeling suggests that oceanic drawdown of Mo may have approached levels capable of affecting marine nitrogen fixation. Predictions of possible nitrogen stress on eukaryotic production, locally and globally, are consistent with the low observed levels of Mo and a rise in 2-methylhopane index values during the peak of the OAE. At the same time, the environmental challenge presented by low dissolved oxygen and euxinia coincides with increased turnover rates of radiolarian clades, calcareous nanofossils, and foraminifera, suggesting that the temporal patterns of anoxia/euxinia and associated nutrient limitation may have contributed to the fabric of OAE2-related turnover.

Sulfur isotope evidence for low and fluctuating sulfate levels in the Late Devonian ocean and the potential link with the mass extinction event

High amplitude positive carbon isotope excursions in the Late Devonian, the punctata and Kellwasser events, reflect major perturbations in the global carbon cycle that have been attributed to increased continental weathering and subsequent ocean eutrophication. Despite the comparable carbon isotope anomalies, however, a major extinction has been reported only for the Kellwasser Events, while the punctata Event is marked by low extinction intensity. This study presents multiple sulfur isotope records of carbonate-associated sulfate (CAS) and pyrite from Late Devonian sections in the Great Basin, USA, in order to document changes in the coupled (or decoupled) geochemical cycles of carbon and sulfur during the punctata and Upper Kellwasser events. A positive sulfur isotope shift in both CAS and pyrite accompanies the onset of the punctata Event, but to a larger extent in the latter. As a result, the sulfur isotope offset between CAS and pyrite (Δ34SCAS-py) dropped to less than 10‰. In the middle of the punctata Event, a sharp negative δ34SCAS excursion and negative Δ34SCAS-py values coincide with the Alamo impact. Unlike the rapid δ34Spy and δ34SCAS oscillations associated with the punctata Event, the Upper Kellwasser was a period of relative stability, except for a brief δ34SCAS drop before the event. Paired sulfur isotope data, aided by a simple box model, suggest that the geochemical cycle of sulfur may have been partly responsible for the contrasting biological responses that define these events. High stratigraphic δ34Spy and δ34SCAS variability, coupled with strong reservoir effect, demonstrates a relatively small oceanic sulfate pool existed during the punctata Event. Further, the Alamo impact likely triggered the rapid oxidation of microbially-produced sulfide within this event. The expansion of sulfidic bottom water thus may have been impeded during the punctata Event. In contrast, the lack of a positive shift in δ34SCAS and sizable Δ34SCAS-py values (>15‰) throughout the Upper Kellwasser Event imply higher relative sulfate levels. A larger seawater sulfate reservoir may have promoted the development of sulfidic bottom waters in the eutrophic epicontinental seas, increasing biological stress and potentially contributing to the mass extinction.

Morphological and molecular approaches reveal highly stratified protist communities along Baltic Sea pelagic redox gradients

The deep basins of the central Baltic Sea are characterized by anoxic and sulfidic bottom water and steep vertical pelagic redox gradients. The highly active prokaryotic assem- blages of this and other redox transition zones have been intensely studied, while knowledge on the protistan communities remains fragmentary. Thus, we conducted a multi-annual microscopy- based study, combined in one year with 18S rRNA gene and transcript-based DGGE fingerprints to identify the dominant protist taxa and to assess their vertical distribution. Both approaches, applied in high vertical resolution, demonstrated strong stratifications of the protist community composition along the redox gradient. The suboxic zone was dominated by dinoflagellates and oligotrichous ciliates related to Strombidium, whereas the interface and upper sulfidic zone were dominated by ciliates of the genera Mesodinium and Metacystis. Several flagellate taxa within the jakobids, euglenozoans and choanoflagellates occurred exclusively in sulfidic water. Our morpho- logical approach indicates that the pelagic redoxclines of the central Baltic Sea are inhabited by a stable and characteristic protist community. Incongruously, certain taxa (e.g. Mesodinium and Metacystis sp.) which remained undetected by the molecular fingerprinting technique could be identified and enumerated by microscopic observations, whereas small and virtually amorphous protists (especially flagellates) were detected only by sequencing DGGE bands. Fine-scaled assessment of dominant protists in distinct redox strata is a crucial step in understanding their impact and interactions with the prokaryotic world and the biogeochemical processes they medi- ate in these zones.

Redox-dependent changes in manganese speciation in Baltic Sea sediments from the Holocene Thermal Maximum: An EXAFS, XANES and LA-ICP-MS study

Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca–Mn–carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e.g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000–4000cal.yrBP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum (Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe–S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters.

Denitrification in the water column of the central Baltic Sea

Removal of fixed nitrogen in the water column of the eastern Gotland Basin, central Baltic Sea, was studied during two cruises in September 2008 and August 2010. The water column was stratified with anoxic sulfidic bottom water meeting oxic nitrate containing water at the oxic–anoxic interface. Anammox was never detected whereas denitrification was found in all incubations from anoxic depths and occurred immediately below the oxic–anoxic interface. Sulfide (H2S+HS−+S2−) was in most cases the only electron donor for denitrification but, in contrast to previous findings, denitrification was in some situations driven by organic matter alone. Nitrous oxide (N2O) became an increasingly important product of denitrification with increasing sulfide concentration and was >80% of the total N gas formation at 10μM sulfide. The potential rates of denitrification measured in incubations at elevated NO3- or sulfide concentrations were converted to in situ rates using the measured water column concentrations of NO3- and sulfide and the actual measured relations between NO3- and sulfide concentrations and denitrification rates. In situ denitrification ranged from 0.24 to 15.9nMN2h−1. Assuming that these rates were valid throughout the anoxic NO3- containing zone, depth integrated in situ denitrification rates of 0.06–2.11mmol Nm−2d−1 were estimated. The thickness of this zone was generally 3–6m, which is probably what can be maintained through regular turbulent mixing induced by internal waves at the oxic–anoxic interface. However, layers of up to 55m thickness with low O2 water (<10μM) were observed which was probably the result of larger scale mixing. In such a layer nitrification may produce NO3- and once the O2 has been depleted denitrification will follow resulting in enormous rates per unit area. Even with an active denitrification layer of 3–6m thickness the pelagic denitrification per unit area clearly exceeded sediment denitrification rates elsewhere in the Baltic Sea. When extrapolated to the entire Baltic Proper (BP) denitrification in the water column was in the range of 132–547ktonNyr−1 and was thus at least as important as sediment denitrification which has recently been estimated to 191ktonNyr−1. With a total external N-input of 773ktonNyr−1 it is clear that denitrification plays a significant role in the N-budget of the BP.