Influence of oxygen on heterotrophic reworking of sedimentary lipids at the Pakistan margin

Abstract

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 late-monsoon 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 , 0.11 ml L −1 , 4.91 μM) efficiently removed polyunsaturated fatty acids from the sediments, whilst at the lower OMZ boundary, suspension feeding pennatulids (1200 m, 108 individuals hm −2 , 0.35 ml L −1 , 15.62 μM) stripped labile lipids from the water column particulates leading to the deposition of lipid-poor OM in the surficial sediments. Low infaunal and epifaunal biomass at the fully oxygenated deep site led to the persistence of phytodetritus and subsequently labile lipids in the sediments. Seasonal changes in OM quality and composition were noted at 140, 300 and 1850 m and were attributed to both particulate fluxes and faunal feeding patterns. Finally, lipid biomarker indices demonstrated that the OM at the Pakistan Margin was of low quality when compared to other similar environments.