Description: Anthropogenic impacts are perturbing the global nitrogen cycle via warming effects and pollutant sources such as chemical fertilizers and burning of fossil fuels. Understanding controls on past nitrogen inventories might improve predictions for future global biogeochemical cycling. Here we show the quantitative reconstruction of deglacial bottom water nitrate concentrations from intermediate depths of the Peruvian upwelling region, using foraminiferal pore density. Deglacial nitrate concentrations correlate strongly with downcore δ13C, consistent with modern water column observations in the intermediate Pacific, facilitating the use of δ13C records as a paleo-nitrate-proxy at intermediate depths and suggesting that the carbon and nitrogen cycles were closely coupled throughout the last deglaciation in the Peruvian upwelling region. Combining the pore density and intermediate Pacific δ13C records shows an elevated nitrate inventory of 〉10% during the Last Glacial Maximum relative to the Holocene, consistent with a δ13C-based and δ15N-based 3D ocean biogeochemical model and previous box modeling studies.

Description: Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3D seismic data of the CNS indicating that about one third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (~42 %) may reach the atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant.

Description: Mud Volcanism and fluid seepage are widespread phenomena in the Gulf of Cadiz (SW Iberian Margin). In this seismically active region located at the boundary between the African and Eurasian plates, fluid flow is typically focused on deeply rooted active strike-slip faults. The geochemical signature of emanating fluids from various mud volcanoes (MVs) has been interpreted as being largely affected by clay mineral dehydration and recrystallization of Upper Jurassic carbonates. Here we present the results of a novel, fully-coupled 1D basin-scale reactive-transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time, compaction of sediments, diffusion and advection of fluids, as well as convective and conductive heat flow. The outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the Gulf of Cadiz. Combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization, we were able to reproduce measured concentrations of Cl, strontium and 87Sr/86Sr of emanating mud volcano fluids. These results support previously made qualitative interpretations and add further constraints on fluid forming processes, reaction rates and source depths. The geochemical signature at Porto MV posed a specific problem, because of insufficient constraints on non-radiogenic 87Sr/86Sr sources at this location. We favour a scenario of basement-derived fluid injection into basal Upper Jurassic carbonate deposits (Hensen et al., 2015). Although the mechanism behind such basement-derived flow, e.g. along permeable faults, remains speculative at this stage, it provides an additional source of low 87Sr/86Sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding MVs at the seafloor. The dynamic reactive-transport model presented in this study provides a new tool addressing the combined simulation of complex physical-geochemical processes in sedimentary systems. The model can easily be extended and applied to similar geological settings, and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins.

Description: The kinetics of particulate organic carbon (POC) mineralization in marine surface sediments is not well constrained. This creates considerable uncertainties when benthic processes are considered in global biogeochemical or Earth system circulation models to simulate climate-ocean interactions and biogeochemical tracers in the ocean. In an attempt to improve our understanding of the rate and depth distribution of organic carbon mineralization in bioturbated (0–10 cm) sediments, we parameterized a 1-D diagenetic model that simulates the reactivity of three discrete POC pools at global scale (a "multi-G" model). The rate constants of the three reactive classes (highly reactive, reactive, refractory) are fixed and determined to be 70 yr−1, 0.5 yr−1, and ~0.001 yr−1, respectively, based on the Martin curve model for pelagic POC degradation. In contrast to previous approaches, the reactivity of the organic material degraded in the seafloor is continuous with, and set by, the apparent reactivity of material sinking through the water column. The model is able to simulate a global database (185 stations) of benthic oxygen and nitrate fluxes across the sediment-water interface in addition to porewater oxygen and nitrate distributions and organic carbon burial efficiencies. It is further consistent with degradation experiments of fresh phytoplankton. We propose that an important yet mostly overlooked consideration in previous upscaling approaches is the proportion of the relative reactive POC classes reaching the seafloor in addition to their reactivity. The approach presented is applicable to both steady-state and non-steady state scenarios, and links POC degradation kinetics in sedimentary environments to water depth and the POC rain rate to the seafloor.

Description: 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 reservoirs.

Description: Iron speciation was determined in hemiplegic sediments from a high productivity area to investigate systematically the early diagenetic reactivity of Fe. A combination of various leaching agents (1 M HCI, dithionite buffered in citrate/acetic acid, HF/H2SO4, acetic Cr(II)) was applied to sediment and extracted more than 80% of total Fe. Subsequent Fe species determination defined specific mineral fractions that are available for Fe reduction and fractions formed as products of Fe diagenesis. To determine the Fe speciation of (sheet) silicates we explored an extraction procedure (HF/H2SO4) and verified the procedure by application to standard rocks. Variations of Fe speciation of (sheet) silicates reflect the possible formation of Fe-bearing silicates in near surface sediments. The same fraction indicates a change in the primary input at greater depth, which is supported by other parameters. The Fe(II)/ Fe(III) -ratio of total sediment determined by extractions was compared with Mössbauer-spectroscopy ] at room temperature and showed agreement within 10%. M6ssbauer-spectroscopy indicates the occurrence of siderite in the presence of free sulfide and pyrite, supporting the importance of microenvironments during mineral formation. The occurrence of other Fe(II) bearing minerals such as ankerite (Ca-, Fe-, Mg-carbonate) can be presumed but remains speculative.

Description: Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P at six stations, including the analysis of particles from the water column, surface sediments, and pore fluids, as well as in situ benthic flux measurements. A major fraction of solid-phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15). This suggests that the relative burial efficiencies of POC and TPP are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1), however, showing that a lack of oxygen promotes the intensified release of dissolved P from sediments, whilst preserving the POC / TPP burial ratio. Benthic dissolved P fluxes were always higher than the TPP rain rate to the seabed, which is proposed to be caused by transient P release by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments, indicating ongoing phosphorite formation. This is further supported by decreasing porewater phosphate concentrations with sediment depth, whereas solid-phase P concentrations were comparatively high.

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.

Description: Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 μmol Si cm−2 yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Description: During the last glaciation the Marmara Sea was isolated from the Mediterranean Sea because global sea level was below the depth of the Dardanelles sill. Prior to the postglacial reconnection to the Mediterranean Sea (~14.7 cal kyr BP), the surface waters of the Marmara Sea were brackish (Marmara Lake). Freshening of a previously saline Marmara Sea could have happened via spill-out of brackish to fresh water from the surface water of the Black Sea through the Bosphorus Strait. This hypothesis has not been tested against alternative possibilities (salt flushing by river run-off and precipitation). Here we use the dissolved Cl- and stable isotope composition (δO18 and δD) of Marmara Sea sediment pore water to estimate the salinity and stable isotope composition of Marmara Lake bottom water and to evaluate possible freshening scenarios. We use a transport model to simulate pore water Cl-, δO18 and δD in Marmara Sea sediments in the past 130 kyr, which includes the last interglacial (130-75 cal kyr BP), the last glacial (75-14 cal kyr BP) and the current postglacial period. Our results show that the bottom waters of the Marmara Lake were brackish (~4‰ salinity) and isotopically depleted (δO18~-10.2‰ and δD~-70‰, respectively) compared to modern seawater. Their salinity and stable isotope ratios show that they are a mixture of Mediterranean waters and Danube-like waters implying that the freshening took place via spill-out of freshwater through the Bosphorus. Our modelling approach indicates that the transit of fresh water from glacial Eurasia to the Mediterranean via the Marmara Sea started at least 50 cal kyr BP, was continuous throughout most of the last glaciation and persisted up to the post glacial reconnection to the Mediterranean through the Dardanelles sill (14.7 cal kyr BP). These results are consistent with previously published micropaleontological and geochemical investigations of sediment cores that indicate lacustrine conditions in the Marmara Sea from about 75 to 14.7 cal kyr BP.