The Role of Iron in the Diagenesis of Organic Carbon and Nitrogen in Sediments: A Long-term Incubation Experiment

English Abstract:

Marine sediments have long been proposed as a major sink of organic matter (OM) on the planet with the bulk of this OM being sequestered within coastal and deltaic sediments. The long-term storage of reduced organic compounds within these environments helps maintain the global redox balance at the surface of the planet. As a dominant constituent of the sediment mineral matrix, redox-sensitive nanophase iron (Fe) oxides play an important role in the preservation of OM through the formation of metastable Fe-OM complexes that sequester approximately 20% of the total sediment OM pool upon burial. Using a long-term sediment slurry incubation approach we examined the effect of iron on the preservation and degradation of OM. The fate of freshly deposited algal dissolved OM was monitored by tracking its depleted stable carbon isotopic signature (δ¹³C) relative to the native sediment OM. We demonstrate the incorporation of the tracer’s depleted δ¹³C signature into the sediment within the first hour of the incubation; this is consistent with the timeframe for sorptive processes and co-precipitation. In the presence of iron (oxy)hydroxides we see increased incorporation of the algal tracer’s depleted isotopic signature within the solid phase demonstrating a shuttling effect of OM from the sediment pore waters into sediment particles. The effect of iron oxides on the fate of organic matter was found to be twofold, showing increased preservation of organic carbon species while increasing the rate of nitrogen removal through a process known as Feammox, short circuiting traditional nitrogen removal pathways. This study is the first study of its kind that demonstrates the dual role of iron oxides, by decoupling the elemental cycles of carbon and nitrogen within sediments originating from coastal regions.