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Organic carbon (OC) burial efficiency, which relates the OC burial rate to respiration in the seafloor, is a critical parameter in the reconstruction of past marine primary productivities. The current accepted theory is that sediments underlying oxygen-deficient (anoxic) bottom waters have low respiration rates and high OC burial efficiencies. By combining novel in situ measurements in anoxic basins with reaction-transport modelling, we demonstrate that sediments underlying anoxic bottom waters have much higher respiration rates than commonly assumed. A major proportion of the carbon respiration is concentrated in the top millimeter—the so-called 'reactive surface layer'—which is likely a feature in approximately 15% of the coastal seafloor. When re-evaluating previously published data in light of our results, we conclude that the impact of bottom-water anoxia on OC burial efficiencies in marine sediments is small. Consequently, reconstructions of past marine primary productivity in a predominantly anoxic ocean based on OC burial rates might be underestimated by up to an order of magnitude. [ABSTRACT FROM AUTHOR]

It has been hypothesized that the evolution of animals during the Ediacaran–Cambrian transition stimulated the burial of phosphorus in marine sediments. This assumption is centrally based on data compilations from marine sediments deposited under oxic and anoxic bottom waters. Since anoxia excludes the presence of infauna and sediment reworking, the observed differences in P burial are assumed to be driven by the presence of bioturbators. This reasoning however ignores the potentially confounding impact of bottom-water oxygenation on phosphorus burial. Here, our goal is to test the idea that bioturbation increases the burial of organic and inorganic phosphorus (P org and P inorg , respectively) while accounting for bottom-water oxygenation. We present solid-phase phosphorus speciation data from salt marsh ponds with and without bioturbation (Blakeney salt marsh, Norfolk, UK). In both cases, the pond sediments are exposed to oxygenated bottom waters, and so the only difference is the presence or absence of bioturbating macrofauna. Our data reveal that the rate of P org and P inorg burial are indistinguishable between bioturbated and non-bioturbated sediments. A large terrestrial fraction of organic matter and higher sedimentation velocity than generally found in marine sediments (0.3 ± 0.1 cm yr -1) may partially impact these results. However, the absence of a clear effect of bioturbation on total P burial puts into question the presumed importance of bioturbation for phosphorus burial. [ABSTRACT FROM AUTHOR]

It has been hypothesised that the evolution of animals during the Ediacaran-Cambrian transition had a major impact on atmospheric O2 and CO2 concentrations. The models upon which this hypothesis rests, critically assume that bioturbation by the newly evolved fauna increased the burial of organic phosphorus (Porg) within the seafloor, relative to organic carbon (Corg) and that inorganic phosphorus (Pinorg) burial was not affected by bioturbation. This assumption is centrally based on data compilations from marine sediments deposited under oxic and anoxic bottom waters. Since anoxia excludes the presence of infauna and sediment reworking, the observed differences in P burial are assumed to be solely driven by the presence of bioturbators. This reasoning however ignores the potentially confounding impact of bottom water oxygenation on phosphorus burial. Here, our goal is to provide a field verification for the idea that bioturbation increases the relative burial of organic phosphorus, while accounting for bottom water oxygenation. We present solid-phase phosphorus speciation data from salt marsh ponds with and without bioturbation (Blakeney salt marsh, Norfolk, UK). In both cases, the pond sediments are exposed to oxygenated bottom waters and so the only difference is the presence/absence of bioturbating macrofauna. Our data reveal that both the Corg : Porg ratio of buried organic matter and the rate of Pinorg burial are indistinguishable between bioturbated and non-bioturbated sediments. The absence of a clear effect of bioturbation on total P burial implies that previous studies may have overestimated the impact of the rise of bioturbation on atmospheric O2 and CO2 concentrations in the early Cambrian. [ABSTRACT FROM AUTHOR]

The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence. Here, we quantify the geochemical cycling in both pond types, using pore-water analyses and solid-phase speciation. Our results demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, and so these cannot act as the direct driver of the observed redox dichotomy. Instead, our results suggest that the presence of bioturbation plays a key role in the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Overall, we propose that the observed dichotomy in pond geochemistry is due to alternative stable states, which result from non-linear interactions in the sedimentary iron and sulphur cycles that are amplified by bioturbation. This way, small differences in solid phase input can result in very different regimes of redox cycling due to positive feedbacks. This non-linearity in the iron and sulphur cycling could be an inherent feature of marine sediments, and hence, alternative stable states could be present in other systems. [ABSTRACT FROM AUTHOR]