The role of iron in the diagenesis of organic carbon and nitrogen in sediments: A long-term incubation experiment

The burial and preservation of organic matter (OM) in marine sediments is tightly coupled to the diagenetic cycles of iron and manganese. Recently, it has been shown that approximately 20% of the sedimentary organic carbon (OC) may be bound to reducible iron oxides (Lalonde et al., 2012). These strong iron-OC complexes, formed within the oxic layer of the sediment, are transferred to the deeper anoxic sediment layers through sedimentation, physical reworking and bioturbation and are metastable over geological timescales. Using long-term (250-day) incubations under various redox and amendment conditions (Fe(II) and dissolved OM (DOM) additions), we examined the effect of iron on the early diagenetic transformations of OM in marine sediments. The fate of fresh, algal-derived DOM was monitored by tracking its stable carbon isotopic signature (delta C-13). We demonstrate the incorporation of the C-13-depleted tracer into the sediment through sorption (adsorption and co-precipitation). In the presence of iron oxides, we observed an increased transfer of the dissolved algal material to the solid phase, revealing the role of iron in shuttling DOM from sediment pore waters to sediment particles. Furthermore, we show that the presence of iron has a differential effect on OC and organic nitrogen (ON), with preferential preservation of OC and accelerated degradation of ON in the presence of reactive iron oxide surfaces. Hence, we propose that redox-sensitive metals may regulate the global redox balance through increased carbon preservation as well as exerting a control on the concentration of fixed nitrogen species in marine sediments. (C) 2014 Elsevier B.V. All rights reserved.