Anaerobic microbial biogeochemistry in sediments from two Basins in the Gulf of Maine: Evidence for iron and manganese reduction

Abstract

Rates of sulphate reduction, denitrification and glucose turnover were determined in surface sediments collected from Jeffreys and Wilkinson Basins in the Gulf of Maine. These data were compared to porewater profiles of iron and manganese. Three distinct anaerobic biogeochemical situations were evident: (1) sulphate reduction was dominant and active near the sediment-water interface (deeper portions of Jeffreys Basin); (2) denitrification was active in the upper few centimetres and was underlain by rapid sulphate reduction (slope of Jeffreys Basin); (3) denitrification was dominant (Wilkinson Basin). An apparent iron and manganese reduction region was evident in all cores. Since glucose turnover was active and both denitrification and sulphate reduction were low within this metal reduction zone it is possible that microbial metal reduction is a significant process within the basin sediments. High concentrations of dissolved iron occurred within sediments deeper than those supporting active sulphate reduction. Sufficient iron was remobilized during iron reduction to not only precipitate all of the sulphide generated during sulphate reduction but to also allow for dissolved iron to diffuse into deeper sediments. Therefore, large vertical regions containing dissolved iron are not necessarily regions of iron reduction and iron reduction is restricted to a narrow sediment layer.