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  • 1
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    Sedimentary molybdenum cycling in the aftermath of seawater inflow to the intermittently euxinic Gotland Deep, Central Baltic Sea (2018)
    Elsevier
    In:  Chemical Geology, 491 . pp. 27-38.
    Details
    Publication Date: 2021-02-08
    Description: Molybdenum (Mo) concentrations and isotope compositions in sediments and shales are commonly used as proxies for anoxic and sulfidic (i.e., euxinic) conditions in the water column of paleo-marine systems. A basic assumption underlying this practice is that the proxy signal extracted from the geological record is controlled by long-term (order of decades to millennia) Mo scavenging in the euxinic water column rather than Mo deposition during brief episodes or events (order of weeks to months). To test whether this assumption is viable we studied the biogeochemical cycling of Mo and its isotopes in sediments of the intermittently euxinic Gotland Deep in the central Baltic Sea. Here, multiannual to decadal periods of euxinia are occasionally interrupted by inflow events during which well‑oxygenated water from the North Sea penetrates into the basin. During these events manganese (Mn) (oxyhydr)oxide minerals are precipitated in the water column, which are known to scavenge Mo. We present sediment and pore water Mo and Mo isotope data for sediment cores which were taken before and after a series of inflow events between 2014 and 2016. After seawater inflow, pore water Mo concentrations in anoxic surface sediments exceed the salinity-normalized concentration by more than two orders of magnitude and coincide with transient peaks of dissolved Mn. A fraction of the Mo liberated into the pore water is transported by diffusion in a downward direction and sequestered by organic matter within the sulfidic zone of the sediment. Diffusive flux calculations as well as a mass balance that is based on the sedimentary Mo isotope composition suggest that about equal proportions of the Mo accumulating in the basin are delivered by Mn (oxyhydr)oxide minerals during inflow events and Mo scavenging with hydrogen sulfide during euxinic periods. Since the anoxic surface sediment where Mo is released from Mn (oxyhydr)oxides are separated by several centimeters from the deeper sulfidic layers where Mo is removed, the solid phase record of Mo concentration and isotope composition would be misinterpreted if steady state Mo accumulation was assumed. Based on our observations in the Gotland Deep, we argue that short-term redox fluctuations need to be considered when interpreting Mo-based paleo-records.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.chemgeo.2018.04.031
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  • 2
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    Intense molybdenum accumulation in sediments underneath a nitrogenous water column and implications for the reconstruction of paleo-redox conditions based on molybdenum isotopes (2017)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 213 . pp. 400-417.
    Details
    Publication Date: 2020-02-06
    Description: The concentration and isotope composition of molybdenum (Mo) in sediments and sedimentary rocks are widely used proxies for anoxic conditions in the water column of paleo-marine systems. While the mechanisms leading to Mo fixation in modern restricted basins with anoxic and sulfidic (euxinic) conditions are reasonably well constrained, few studies have focused on Mo cycling in the context of open-marine anoxia. Here we present Mo data for water column particulate matter, modern surface sediments and a paleo-record covering the last 140,000 years from the Peruvian continental margin. Mo concentrations in late Holocene and Eemian (penultimate interglacial) shelf sediments off Peru range from ∼70 to 100 µg g−1, an extent of Mo enrichment that is thought to be indicative of (and limited to) euxinic systems. To investigate if this putative anomaly could be related to the occasional occurrence of sulfidic conditions in the water column overlying the Peruvian shelf, we compared trace metal (Mo, vanadium, uranium) enrichments in particulate matter from oxic, nitrate-reducing (nitrogenous) and sulfidic water masses. Coincident enrichments of iron (Fe) (oxyhydr)oxides and Mo in the nitrogenous water column as well as co-variation of dissolved Fe and Mo in the sediment pore water suggest that Mo is delivered to the sediment surface by Fe (oxyhydr)oxides. Most of these precipitate in the anoxic-nitrogenous water column due to oxidation of sediment-derived dissolved Fe with nitrate as a terminal electron acceptor. Upon reductive dissolution in the surface sediment, a fraction of the Fe and Mo is re-precipitated through interaction with pore water sulfide. The Fe- and nitrate-dependent mechanism of Mo accumulation proposed here is supported by the sedimentary Mo isotope composition, which is consistent with Mo adsorption onto Fe (oxyhydr)oxides. Trace metal co-variation patterns as well as Mo and nitrogen isotope systematics suggest that the same mechanism of Mo delivery caused the ‘anomalously’ high interglacial Mo accumulation rates in the paleo-record. Our findings suggest that Fe- and nitrate-dependent Mo shuttling under nitrogenous conditions needs to be considered a possible reason for sedimentary Mo enrichments during past periods of widespread anoxia in the open ocean.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2017.06.048
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  • 3
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    A model for microbial phosphorus cycling in bioturbated marine sediments: Significance for phosphorus burial in the early Paleozoic (2016)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 189 . pp. 251-268.
    Details
    Publication Date: 2019-09-23
    Description: Highlights • An empirical diagenetic model includes redox-dependent P storage by microorganisms. • Sediment mixing and burrowing by animals strongly enhances P burial. • Supporting evidence for a decrease in oceanic P inventory in the early Paleozoic. Abstract A diagenetic model is used to simulate the diagenesis and burial of particulate organic carbon (Corg) and phosphorus (P) in marine sediments underlying anoxic versus oxic bottom waters. The latter are physically mixed by animals moving through the surface sediment (bioturbation) and ventilated by burrowing, tube-dwelling organisms (bioirrigation). The model is constrained using an empirical database including burial ratios of Corg with respect to organic P (Corg:Porg) and total reactive P (Corg:Preac), burial efficiencies of Corg and Porg, and inorganic carbon-to-phosphorus regeneration ratios. If Porg is preferentially mineralized relative to Corg during aerobic respiration, as many previous studies suggest, then the simulated Porg pool is found to be completely depleted. A modified model that incorporates the redox-dependent microbial synthesis of polyphosphates and Porg (termed the microbial P pump) allows preferential mineralization of the bulk Porg pool relative to Corg during both aerobic and anaerobic respiration and is consistent with the database. Results with this model show that P burial is strongly enhanced in sediments hosting fauna. Animals mix highly labile Porg away from the aerobic sediment layers where mineralization rates are highest, thereby mitigating diffusive PO43− fluxes to the bottom water. They also expand the redox niche where microbial P uptake occurs. The model was applied to a hypothetical shelf setting in the early Paleozoic; a time of the first radiation of benthic fauna. Results show that even shallow bioturbation at that time may have had a significant impact on P burial. Our model provides support for a recent study that proposed that faunal radiation in ocean sediments led to enhanced P burial and, possibly, a stabilization of atmospheric O2 levels. The results also help to explain Corg:Porg ratios in the geological record and the persistence of Porg in ancient marine sediments.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2016.05.046
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  • 4
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    Modeling microbial reaction rates in a submarine hydrothermal vent chimney wall (2014)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 124 . pp. 72-97.
    Details
    Publication Date: 2017-06-19
    Description: The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction-transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation–reduction reactions associated with fluid mixing as a function of temperature. The metabolisms included in the reaction network are methanogenesis, aerobic oxidation of hydrogen, sulfide and methane and sulfate reduction by hydrogen and methane. Model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney (77–102 °C), and methane and sulfide oxidation peak near the seawater-side of the chimney. The fastest metabolisms are aerobic oxidation of H2 and sulfide and reduction of sulfate by H2 with maximum rates of 140, 900 and 800 pmol cm−3 d−1, respectively. The maximum rate of hydrogenotrophic methanogenesis is just under 0.03 pmol cm−3 d−1, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). These simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature. The model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the role of microbial activity in the deep subsurface.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2013.09.005
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  • 5
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    Controls on organic carbon and molybdenum accumulation in Cretaceous marine sediments from the Cenomanian–Turonian interval including Oceanic Anoxic Event 2 (2012)
    Elsevier
    In:  Chemical Geology, 324-325 . pp. 28-45.
    Details
    Publication Date: 2019-09-24
    Description: This study investigates the controls on organic carbon and molybdenum (Mo) accumulation in sediments deposited within the Western Interior Seaway across the Cenomanian–Turonian boundary interval (94.34–93.04 Ma) including Oceanic Anoxic Event 2 (OAE2). Carbon fluxes to the sediment–water interface (reflecting changes in primary productivity) and bottom-water oxygen concentrations (reflecting preservation effects) are reconstructed from field data and used to constrain a benthic model that simulates the geochemistry of unconsolidated sediments as they were deposited. The results show that increased availability of reactive iron prevents Mo sequestration as thiomolybdate (MoS42 −) during OAE2 (O2 ~ 105 μM) by (i) inhibiting sulfate reduction, and (ii) buffering any free sulfide that becomes available. In the post-OAE2 period (O2 ~ 50 μM), Mo accumulation is favored by a large reduction in iron flux. Importantly, this occurs in parallel with oxygenated bottom waters and high rates of aerobic carbon degradation in the surface sediments, implying that elevated Mo burial fluxes in ancient marine facies do not necessarily reflect euxinic or even anoxic conditions within the water column. Our findings suggest that both an increase in production and preservation lead to enrichment in organic carbon in the Western Interior Seaway. More generally, the results demonstrate that a careful consideration of the coupling between iron, carbon and oxygen cycles during the early stages of diagenesis is critical for interpreting geochemical proxies in modern and ancient settings.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.chemgeo.2011.10.004
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  • 6
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    Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) (2014)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 134 . pp. 234-256.
    Details
    Publication Date: 2019-09-23
    Description: We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO3−, NO2−, NH4+) and oxygen (O2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O2 concentrations at the shallowest station were hypoxic (42 μM) and increased to 125 μM at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (−10.3 mmol O2 m−2 d−1) and decreased quasi-exponentially with water depth to −3.2 mmol O2 m−2 d−1. Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2 mmol N m−2 d−1. Overall, the sediments acted as net sink for DIN. Observed increases in δ15NNO3 and δ18ONO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0‰ (15εapp) and 14.1‰ (18εapp). Measurements of δ15NNO2 further demonstrated that the sediments acted as a source of 15N depleted NO2−. These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH4+ and NO2−. The principal findings were that (i) net benthic 14N/15N fractionation (εDEN) was 12.9 ± 1.7‰, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO2− (−22 ± 1.9‰), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported εDEN for fine-grained sediments are much lower (4–8‰). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater–seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2014.02.026
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  • 7
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    Sulfate reduction and methane oxidation activity below the sulfate-methane transition zone in Alaskan Beaufort Sea continental margin sediments: Implications for deep sulfur cycling (2014)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 144 . pp. 217-237.
    Details
    Publication Date: 2017-06-19
    Description: Two ∼6 m long sediment cores were collected along the ∼300 m isobath on the Alaskan Beaufort Sea continental margin. Both cores showed distinct sulfate-methane transition zones (SMTZ) at 105 and 120 cm below seafloor (cmbsf). Sulfate was not completely depleted below the SMTZ but remained between 30 and 500 μM. Sulfate reduction and anaerobic oxidation of methane (AOM) determined by radiotracer incubations were active throughout the methanogenic zone. Although a mass balance could not explain the source of sulfate below the SMTZ, geochemical profiles and correlation network analyses of biotic and abiotic data suggest a cryptic sulfur cycle involving iron, manganese and barite. Inhibition experiments with molybdate and 2-bromoethanesulfonate (BES) indicated decoupling of sulfate reduction and AOM and competition between sulfate reducers and methanogens for substrates. While correlation network analyses predicted coupling of AOM to iron reduction, the addition of manganese or iron did not stimulate AOM. Since none of the classical archaeal anaerobic methanotrophs (ANME) were abundant, the involvement of unknown or unconventional phylotypes in AOM is conceivable. The resistance of AOM activity to inhibitors implies deviation from conventional enzymatic pathways. This work suggests that the classical redox cascade of electron acceptor utilization based on Gibbs energy yields does not always hold in diffusion-dominated systems, and instead biotic processes may be more strongly coupled to mineralogy.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2014.08.018
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  • 8
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    Thermodynamic limitations on microbially catalyzed reaction rates (2012)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 90 . pp. 96-109.
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    Publication Date: 2017-09-26
    Description: Quantification of global biogeochemical cycles requires knowledge of the rates at which microorganisms catalyze chemical reactions. In order for models that describe these processes to capture global patterns of change, the underlying formulations in them must account for biogeochemical transformations over seasonal and millennial time scales in environments characterized by different energy levels. Building on existing models, a new thermodynamic limiting function is introduced. With only one adjustable parameter, this function that can be used to model microbial metabolism throughout the range of conditions in which organisms are known to be active. The formulation is based on a comparison of the amount of energy available from any redox reaction to the energy required to maintain a membrane potential, a proxy for the minimum amount of energy required by an active microorganism. This function does not require species- or metabolism-specific parameters, and can be used to model metabolisms that capture any amount of energy. The utility of this new thermodynamic rate limiting term is illustrated by applying it to three low-energy processes: fermentation, methanogenesis and sulfate reduction. The model predicts that the rate of fermentation will be reduced by half once the Gibbs energy of the catalyzed reaction reaches −12 kJ (mol e−)−1, and then slowing exponentially until the energy yield approaches zero. Similarly, the new model predicts that the low energy yield of methanogenesis, −4 to −0.5 kJ (mol e−)−1, for a partial pressure of H2 between 11 and 0.6 Pa decreases the reaction rate by 95–99%. Finally, the new function’s utility is illustrated through its ability to accurately model sulfate concentration data in an anoxic marine sediment.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.gca.2012.05.011
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  • 9
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    Estimating the time of pockmark formation in the SW Xisha Uplift (South China Sea) using reaction-transport modeling (2015)
    Elsevier
    In:  Marine Geology, 364 . pp. 21-31.
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    Publication Date: 2019-09-23
    Description: Carbon cycling and fluid seepage in marine sediments over the late Quaternary were investigated at a now-extinct pockmark located in a mega-pockmark field in the SW Xisha Uplift (NW South China Sea). Measured particulate organic carbon (POC) content, and porewater sulfate (SO4 2-), dissolved inorganic carbon (DIC) concentrations and δ34S-SO4 2- distributions were used to constrain a non-steady-state reaction-transport model and quantify POC mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark. An increase in POC content and δ34S-SO4 2- and a decrease in sulfate concentrations in the upper ca. 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early Holocene around 10kyrB. P. caused by enhanced primary productivity during the strengthened southwestern summer monsoon. These features were simulated with the model assuming a Holocene increase in POC flux and reactivity. Subsequently, starting from an initial condition reminiscent of a modern active cold seep (Hydrate Ridge), hindcast simulations showed that fluid seepage at the pockmark ceased ca. 39kyr ago. This corresponds to a relative sea level high-stand, which is believed to be associated with gas hydrate stabilization. The non-steady-state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.margeo.2015.03.006
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  • 10
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    Biological nitrate transport in sediments on the Peruvian margin mitigates benthic sulfide emissions and drives pelagic N loss during stagnation events (2016)
    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 112 . pp. 123-136.
    Details
    Publication Date: 2019-09-23
    Description: Highlights • Very high rates of dissimilatory nitrate reduction to ammonium by Thioploca. • Non-steady state model predicts Thioploca survival on intracellular nitrate reservoir. • Ammonium release by Thioploca may be coupled to pelagic N loss by anammox. • Thioploca may contribute to anammox long after bottom water nitrate disappearance. • Model indicates that benthic foraminifera account for 90% of benthic N2 production. Abstract Benthic N cycling in the Peruvian oxygen minimum zone (OMZ) was investigated at ten stations along 12oS from the middle shelf (74 m) to the upper slope (1024 m) using in situ flux measurements, sediment biogeochemistry and modelling. Middle shelf sediments were covered by mats of the filamentous bacteria Thioploca spp. and contained a large ‘hidden’ pool of nitrate that was not detectable in the porewater. This was attributed to a biological nitrate reservoir stored within the bacteria to oxidize sulfide to sulfate during ‘dissimilatory nitrate reduction to ammonium’ (DNRA). The extremely high rates of DNRA on the shelf (15.6 mmol m-2 d-1 of N), determined using an empirical steady-state model, could easily supply all the ammonium requirements for anammox in the water column. The model further showed that denitrification by foraminifera may account for 90% of N2 production at the lower edge of the OMZ. At the time of sampling, dissolved oxygen was below detection limit down to 400 m and the water body overlying the shelf had stagnated, resulting in complete depletion of nitrate and nitrite. A decrease in the biological nitrate pool was observed on the shelf during fieldwork concomitant with a rise in porewater sulfide levels in surface sediments to 2 mM. Using a non-steady state model to simulate this natural anoxia experiment, these observations were shown to be consistent with Thioploca surviving on a dwindling intracellular nitrate reservoir to survive the stagnation period. The model shows that sediments hosting Thioploca are able to maintain high ammonium fluxes for many weeks following stagnation, potentially sustaining pelagic N loss by anammox. In contrast, sulfide emissions remain low, reducing the economic risk to the Peruvian fishery by toxic sulfide plume development.
    Type: Article , PeerReviewed
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    DOI: 10.1016/j.dsr.2016.02.013
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