Iron redox cycling, sediment resuspension and the role of sediments in low oxygen environments as sources of iron to the water column

Abstract

Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To investigate this problem, we examined iron redox cycling in the sediments of Santa Monica Basin (SMB), CA as it relates to benthic sources of iron to the water column. Our results show that iron redox cycling in SMB sediments results in the formation of a thin layer at the sediment surface (<2 cm thick) that is highly enriched in amorphous, reactive iron oxides. Such oxides, when resuspended into the water column, may be a source of bioaccessible iron to primary producers. Calculations show that the resuspension flux of reactive amorphous iron oxides from SMB sediments may be up to two orders of magnitude greater than benthic fluxes of soluble dissolved iron from most continental margin sediments. The factors which favor the formation of this surface sediment layer enriched in reactive amorphous iron oxides appear to be related to the dissolved O2 level in SMB bottom waters, and its impact on iron redox cycling and the occurrence of sediment bioturbation. These factors are not necessarily unique to SMB, and other sediment sites on the open continental margin that lie within the O2 minimum zone may also have similar properties. Furthermore, expansion of O2 minimum zones due to global warming could possibly increase the areal extent of such environments, and potentially act as a negative feedback on rising atmospheric CO2 by providing additional iron to stimulate primary production. Further work examining iron biogeochemistry in low O2 continental margin settings will be needed to more critically examine these possibilities, along with studies that better define the role sediment resuspension fluxes may play as an iron source to surface waters. Such efforts will ultimately lead to an improved understanding of the role that ocean deoxygenation may play in enhancing this sediment iron source to support surface ocean primary production.