Sediments in Low Oxygen Environments as a Potential Source of Iron to the Water Column: The Role of Iron Redox Cycling and Sediment Resuspension

Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To address this issue, we examined iron redox cycling in the sediments of Santa Monica Basin (SMB), CA, as it relates to benthic iron sources to the water column. Our results show that sediment iron redox cycling results in the formation of a thin surface layer (<2 cm thick) in SMB sediments that is highly enriched in reactive iron oxides (up to ~4 wt% Fe as these oxides). Such oxides, when resuspended into the water column, may be a highly bioavailable source of iron to primary producers. Calculations show that this resuspension flux may be up to 100 times greater than the benthic flux of soluble dissolved iron from most continental margin sediments. Iron redox cycling in SMB sediments also has the net effect of transforming iron that is initially deposited in the sediments in less reactive phases (most likely “structural” iron in octahedral and tetrahedral sites in clays minerals) into reactive iron oxides. The factors which favor the formation of these reactive iron oxides, and their enrichment in surface sediments, appear to be related to dissolved O₂ levels in SMB bottom waters and its impact on both iron redox cycling and the occurrence of sediment bioturbation. These factors are not necessarily unique to SMB, and other sedimentary environments 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 this could potentially act as a negative feedback on atmospheric CO₂ by providing additional iron to stimulate primary production. More work examining iron biogeochemistry in low-O₂ continental margin settings will be needed to more critically examine these possibilities and better define the role that ocean deoxygenation may play in enhancing this sediment iron source to surface waters.