Megafauna Can Control the Quality of Organic Matter in Marine Sediments
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Keywords:
Sedimentary organic matter
Bioturbation
Detritivore
Oxygen minimum zone
Continental Margin
Organic matter recycling at the sea floor directly relates to the functioning of benthic ecosystem through dictating the energy flow of the environment. Sediments from the Arabian Sea Oxygen Minimum Zone (OMZ) typically contain high concentrations of organic matter and are thus potentially capable of sustaining an active ecosystem. However, faunal abundances within the OMZ are typically low, a feature which has previously been related to the low oxygen content of the environment. Alternatively, these low abundances within (and outside) the OMZ may be due to low organic matter quality rather than oxygen availability. To investigate this, the bioavailability and fate of organic matter in the surface sediments of the Arabian Sea Oxygen Minimum Zone were studied through a series of incubations and analyses of intact sedimentary phytopigments. The results suggest that, indeed, the sedimentary organic matter accumulating in the OMZ has a very low quality. The slurry incubations of surficial sediments yielded OM remineralisation rates between 77-42 nmol C ml-1d-1 with a residence time of up to 79 yrs. The low OM quality is also clearly demonstrated by the sedimentary pigments, which were strongly dominated by degraded phaeopigments, chlorophyll a being nearly absent already in the top half centimeter of the sediment. In addition, a short-term experiment was performed where 13C-labeled (particulate and dissolved) organic matter was added on sediment from a low oxygen site (bottom water oxygen content 45 m), to investigate potential turnover of labile carbon. The deliberate tracer experiment focused on uptake by bacteria and foraminifera, key players of organic matter recycling in these low oxygen settings. In addition, Gromia, large eukaryotes commonly found in the lower boundary of the Arabian Sea OMZ, were targeted in the tracer study.
Oxygen minimum zone
Sedimentary organic matter
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This study suggests that varying concentration of dissolved oxygen of overlying bottom water influences geochemical fractionation, speciation, and oxidation state of sedimentary Cr in a marine system. The nature of sedimentary organic matter (labile or nonlabile) and Fe speciation also controls the geochemical fractionation and oxidation state of Cr in the sediment from the continental margin across the oxygen minimum zone of the Arabian Sea. Increasing concentration of sedimentary organic matter (under hypoxic conditions) increased association of Cr with sedimentary organic binding phases. The association of Cr with Fe‐Mn‐oxyhydroxide phase gradually decreased with decreasing dissolved oxygen concentration due to the reduction and dissolution of Fe (III). The sedimentary organic matter was found to be the major hosting phase for Cr (VI) in the shelf sediments. The decrease in sedimentary Fe (III) concentration under hypoxic conditions also prevented Cr (VI) reduction. This study suggests that the type of sedimentary organic matter and Fe (III) concentration may influence benthic exchange fluxes of Cr under variable redox conditions in many coastal/margin systems.
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Oxygen minimum zone
Continental Margin
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The flux of manganese from continental margin sediments to the ocean was measured with a free-vehicle, benthic flux chamber in a transect across the continental shelf and upper slope of the California margin. The highest fluxes were observed on the shallow continental shelf. Manganese flux decreased linearly with bottom water oxygen concentration, and the lowest fluxes occurred in the oxygen minimum zone (at a depth of 600 to 1000 meters). Although the flux of manganese from continental shelf sediments can account for the elevated concentrations observed in shallow, coastal waters, the flux from sediments that intersect the oxygen minimum cannot produce the subsurface concentration maximum of dissolved manganese that is observed in the Pacific Ocean.
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Oxygen minimum zone
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Abstract Continental margin sediments are important sites of marine nitrogen cycling and potential contributors to atmospheric N 2 O emissions. We employed trace‐level N 2 O microsensors to measure vertical N 2 O profiles at submillimeter resolutions in intact cores from outer continental margin sediments underlying the NE Pacific oxygen minimum zone. We used mathematical modeling to estimate depth‐dependent rates of N 2 O production and fluxes to the overlying water along a transect of diminishing bottom water oxygen concentrations. Net sediment efflux was observed at all sites on the outer continental margin, with a mean value of 524 nmol m −2 d −1 . N 2 O efflux increased with decreased oxygen penetration depth in sediments. Enhanced N 2 O production and efflux were obtained when outer continental shelf sediments were experimentally exposed to lower bottom‐water O 2 concentrations, to simulate upwelling conditions. Our results underline the need for further investigation of the drivers of N 2 O production in continental margin sediments, and the relative importance of these environments to the global N 2 O budget.
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Oxygen minimum zone
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Bioturbation
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