Deep-Sea Research II

Oceanic oxygen minimum zones (OMZs) are thought to be regions of enhanced organic carbon burial, with high organic matter (OM) quality preservation resulting from low oxygen concentrations and faunal biomass both in the water column and at the seafloor. Here we use the Pakistan continental margin to assess the controls on OM deposition and quality along a transect through the OMZ from water depths of 140–1850 m. We sampled five sites spanning the OMZ during the inter- and latemonsoon seasons to address the role of (a) oxygen, (b) the benthic fauna, and (c) seasonality on the composition and quality of OM using lipid biomarkers. Organic carbon concentrations were highest within the OMZ and lipid biomarkers revealed that OM at the Pakistan Margin was primarily of marine origin. Oxygen concentrations in bottom waters played an indirect role in controlling OM quality, by influencing faunal community composition and biomass both in the water column and at the seafloor. Benthic fauna appear to influence OM quality via molecular filtration of certain compounds (lipid biomarkers). For example, foraminifera that peak in the OMZ core (300 m, 122 individuals 10 cm � 2 ,

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