Biogeochemical effects of volcanic degassing on the oxygen-state of the oceans during the Cenomanian/Turonian Anoxic Event 2

ABSTRACT FINAL ID: PP11A-1769 Cretaceous anoxic events may have been triggered by massive volcanic CO2 degassing as large igneous provinces (LIPs) were emplaced on the seafloor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seafloor, benthic fluxes of dissolved species across the seabed, and burial of particulates in marine sediments. The ocean is represented by twenty-seven boxes. To estimate horizontal and vertical fluxes between boxes, a coupled ocean–atmosphere general circulation model (AOGCM) is run to derive the circulation patterns of the global ocean under Late Cretaceous boundary conditions. The AOGCM modeling predicts a strong thermohaline circulation and intense ventilation in the Late Cretaceous oceans under high pCO2 values. With an appropriate choice of parameter values such as the continental input of phosphorus, the model produces ocean anoxia at low to mid latitudes and changes in marine δ13C that are consistent with geological data such as the well established δ13C curve. The spread of anoxia is supported by an increase in riverine phosphorus fluxes under high pCO2 and a decrease in phosphorus burial efficiency in marine sediments under low oxygen conditions in ambient bottom waters. Here, we suggest that an additional mechanism might contribute to anoxia, an increase in the C:P ratio of marine plankton which is induced by high pCO2 values. According to our AOGCM model results, an intensively ventilated Cretaceous ocean turns anoxic only if the C:P ratio of marine organic particles exported into the deep ocean is allowed to increase under high pCO2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redfield ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO2 emissions. The formation of C-enriched marine organic matter may also explain the frequent occurrence of global anoxia during other geological periods characterized by high pCO2 values

The physicochemical habitat of Sclerolinum sp., at Hook Ridge hydrothermal vent, Bransfield Strait, Antarctica

At Hook Ridge hydrothermal vent, a new species of Sclerolinum (Monilifera, Siboglinidae) was found at a water depth of 1,045 m. On the basis of investigations of multicores and gravity cores, the species habitat is characterized. Sclerolinum does not occur in sediments that are most strongly influenced by hydrothermal fluids, probably because of high temperature (up to 49°C) and precipitation of siliceous crusts. About 800 individuals m-2 occur in sediments that are only weakly exposed to hydrothermal flow and have the following characteristics: 20°C (15 cm sediment depth) to 21.5°C (bottom water), 18-40 cm yr-1 advection rates, pH 5.5, <25 µmol L-1 methane, <170 µmol L-1 sulfide, and <0.0054 mol m-2 yr-1 sulfide flux. Comparison with geochemical data from other reducing sediments indicates that the two groups of Siboglinidae, Monilifera and Frenulata, occur in sediments with low sulfide concentration and flux. In contrast, sulfurbased chemosynthetic organisms that typically occur at hydrothermal vents and cold seeps (e.g., Vestimentifera, vesicomyid clams, and bacterial mats) occur in sediments with higher sulfide availability; threshold values are around 500 µmol L-1 sulfide and 0.1 mol m-2 yr-1 sulfide fluxes. We did not find typical hydrothermal vent species at Hook Ridge hydrothermal vent, which might be explained by the unfavorable physicochemical habitat: At sites inhabited by Sclerolinum, sulfide availability appears to be too low, whereas at sites with higher sulfide availability, the temperatures might be too high, siliceous crust precipitation could preclude their occurrence, or both

Methanhydrate in arktischen Sedimenten – Einfluss auf Klima und Stabilität der Kontinentalränder

Methane hydrates in marine sediments – Impact on climate and stability of continental slopes: The Arctic Ocean increasingly gets into the focus of methane hydrate research with respect to Global Warming. In the cold Arctic Ocean, hydrates are stable at relatively shallow water depths, and due to rapidly increasing water temperatures this region is considered to become a major source of atmospheric methane in the near future. But many factors, which are essential to make solid predictions about the fate and consequences of hydrate-related methane in the Arctic, still remain unclear. Uncertainties range from the size of the Arctic methane hydrate inventory to the efficiency of microbes to consume methane that is liberated in sediments and migrating through the water column. A potential collateral impact of massive gas hydrate destabilization could be failures of Arctic continental slopes with resulting mass wasting and tsunami formation. Although the correlation between hydrates and mass wasting are still a matter of debate, historic events have been identified and their causes are part of ongoing research. This book chapter will provide an overview of most recent research and discussions about Arctic gas hydrates and its fate in the light of Global Warming

Sediment release of dissolved organic matter in the oxygen minimum zone off Peru

In combination to sluggish ventilation by ocean currents, the nutrient upwelling and high surface productivity, followed by organic matter remineralization, leads to a pronounced oxygen minimum zone (OMZ) in the eastern tropical South Pacific (ETSP). There, oxygen concentrations drop below 1 �mol/kg at a water depth <80 m. The high productivity results in the supply of organic matter (OM) to the anoxic sediments and its utilization by heterotrophic communities. The microbial utilization of OM under anoxia leads to nitrogen loss processes, and an accumulation of sulphide and methane. The proximity of the OMZ to the ocean surface in the ETSP may lead to an active outgassing of climate relevant products of the anoxic OM remineralization. The degradation of OM in sediments is associated with production of dissolved organic matter (DOM) from organic particles (POM) that is further remineralized into inorganic nutrients and dissolved inorganic carbon, which then can be released back to the water column, fuelling productivity. Part of the DOM pool may be released to the overlying water column and serve as ligands for micronutrients, such as iron, or provide an additional substrate for microbial communities to respire, affecting overlying water column biogeochemistry. Despite the potential relevance for biogeochemical processes, the quality of the DOM in the pore waters that may be released to the overlying water column has been barely studied in the ETSP off Peru. High spatial resolution measurements of DOM fluorescence (FDOM) during the research cruise M93 (Feb-March 2013) indicated elevated intensities near the sediments in the ETSP off Peru. Those intensities were interpreted as a sediment release of DOM, the quantification of dissolved organic carbon (DOC) flux, however, was not possible at the time. To estimate DOM fluxes and DOM quality, DOC and DOM samples were collected from the sediment pore waters and from benthic incubation chambers from six stations along the 12°S transect in the Peruvian upwelling in 2017 (cruises M136, M137). Samples were collected using a multiple-corer and by Biogeochemical Observatories, respectively. Here, we evaluate DOC fluxes from the sediments and relate them to the measurements of FDOM. We evaluate the quality of DOM by Excitation Emission spectroscopy, followed by parallel factor analysis. The possible implications of the DOM release for water column biogeochemistry are discussed

Ocean phosphorus inventory: large uncertainties in future projections on millennial timescales and their consequences for ocean deoxygenation

Previous studies have suggested that enhanced weathering and benthic phosphorus (P) fluxes, triggered by climate warming, can increase the oceanic P inventory on millennial timescales, promoting ocean productivity and deoxygenation. In this study, we assessed the major uncertainties in projected P inventories and their imprint on ocean deoxygenation using an Earth system model of intermediate complexity for the same business-as-usual carbon dioxide (CO2) emission scenario until the year 2300 and subsequent linear decline to zero emissions until the year 3000. Our set of model experiments under the same climate scenarios but differing in their biogeochemical P parameterizations suggest a large spread in the simulated oceanic P inventory due to uncertainties in (1) assumptions for weathering parameters, (2) the representation of bathymetry on slopes and shelves in the model bathymetry, (3) the parametrization of benthic P fluxes and (4) the representation of sediment P inventories. Considering the weathering parameters closest to the present day, a limited P reservoir and prescribed anthropogenic P fluxes, we find a +30 % increase in the total global ocean P inventory by the year 5000 relative to pre-industrial levels, caused by global warming. Weathering, benthic and anthropogenic fluxes of P contributed +25 %, +3 % and +2 %, respectively. The total range of oceanic P inventory changes across all model simulations varied between +2 % and +60 %. Suboxic volumes were up to 5 times larger than in a model simulation with a constant oceanic P inventory. Considerably large amounts of the additional P left the ocean surface unused by phytoplankton via physical transport processes as preformed P. In the model, nitrogen fixation was not able to adjust the oceanic nitrogen inventory to the increasing P levels or to compensate for the nitrogen loss due to increased denitrification. This is because low temperatures and iron limitation inhibited the uptake of the extra P and growth by nitrogen fixers in polar and lower-latitude regions. We suggest that uncertainties in P weathering, nitrogen fixation and benthic P feedbacks need to be reduced to achieve more reliable projections of oceanic deoxygenation on millennial timescales

Simple transfer functions for calculating benthic fixed nitrogen losses and C:N:P regeneration ratios in global biogeochemical models

Empirical transfer functions are derived for predicting the total benthic nitrate loss(LNO3) and the net loss of dissolved inorganic nitrogen (LDIN) in marine sediments,equivalent to sedimentary denitrification. The functions are dynamic vertically integratedsediment models which require the rain rate of particulate organic carbon to the seafloor(RRPOC) and a proposed new variable(O2-NO3)bw (bottom water O2 concentration minus NO3-concentration) as the only input parameters. Applied globally to maps of RRPOC and(O2-NO3)bw on a 1° x 1° spatial resolution, the models predict a NO3- drawdown of 196 Tg yr-1 (LNO3)of which 153 – 155 Tg yr-1 is denitrified to N2 (LDIN). This is in good agreement with previous estimates using very different methods. Our approach implicitly accounts for fixed N loss via anammox, such that our findings do not support the idea that the relatively recent discovery of anammox in marine sediments might require current estimates of the global benthic marine N budget to be revised. The continental shelf (0 – 200 m) accounts for >50% of global LNO3 and LDIN, with slope (200 – 2000 m) and deep-sea (>2000 m) sediments contributing ca. 30% and 20%, respectively. Denitrification in high-nitrate/low-oxygen regions such as oxygen minimum zones is significant (ca. 15 Tg N yr-1; 10% of global) despite covering only 1% of the seafloor. The data are used to estimate the net fluxes of nitrate (18 Tg N yr-1) and phosphate(27 Tg P yr-1) across the sediment-water interface. The benthic fluxes strongly deviate from Redfield composition, with globally averaged N:P, N:C and C:P values of 8.3, 0.067 and 122, respectively, indicating world-wide fixed N losses (by denitrification) relative to C and P. The transfer functions are designed to be coupled dynamically to general circulation models to better predict the feedback of sediments on pelagic nutrient cycling and dissolved O2 distributions

Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming

Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirspublishersversionPeer reviewe