Bioturbation and manganese cycling in hemipelagic sediments

Abstract

The activities of infaunal macrobenthos have major influences on the types, rates and distributions of diagenetic reactions involving manganese in relatively carbon-rich deep-sea and nearshore sediments. In some non-sulphidic hemipelagic deposits of the eastern equatorial Pacific (Panama Basin) biogenic reworking drives internal cycles of manganese, which can apparently account for up to ca. 100% of organic carbon oxidation and reduction of 0 2 supplied (diffusively) to the sea floor. Heterotrophic (carbon-based) manganese reduction is stimulated by simultaneous mixing of reactive organic matter and manganese oxide into suboxic-anoxic deposits. In sulphidic sediments, biogenic reworking must also enhance a lithotrophic pathway (sulphur-based) pathway of manganese reduction by promoting contact of manganese oxides and iron sulphides. Particle reworking dramatically alters the balance between aerobic and anaerobic decomposition pathways, promoting the utilization of 0 2 in the reoxidaton of reduced metabolites rather than direct oxidation of carbon. Irrigated burrows create microenvironments, which increase manganese reduction—oxidation and deplete Mn2+ from deeper pore waters. This may increase net Mn 2+ production rates by removal of metabolites and potential co-precipitants with Mn 2+ . The occurrence and geometry of manganese oxide encrusted biogenic structures imply specific adaptations of infauna to manganese based microbial activity in hemipelagic sediments like the Panama Basin.