Evaluation of the diagenetic role of iron as a sulfide buffer at Cape Lookout Bight, North Carolina (USA)

Iron availability has critical impacts on primary productivity (micronutrient for cyanobacteria) and organic matter decomposition, as well as sedimentary diagenesis. This study is investigating the hypothesis that changes in iron concentration within marine sediments can control organic matter burial, via early diagenetic processes that impact pore water sulfide concentration (iron sulfidization), and phosphorus return flux to the water column (iron scavenging phosphorus and changing redox condition within sediments). The initial phase of this study is specifically focused on the diagenetic role of iron as a sulfide buffer in pore water, and its impact on bioturbation/bioirrigation. In this presentation, we outline a new approach to investigate the biogeochemistry of iron during early diagenesis, using controlled laboratory macrocosm experiments. Organic-rich coastal marine sediments were collected from Cape Lookout Bight (Outer Banks, North Carolina), a shallow coastal marine environment (depth < 8m) with an oxygenated water column, but organic-rich sediments dominated by sulfate reduction and methanogenesis. The uppermost portion of each sediment core was amended with synthetic sediment composed of kaolinite, variable amounts of hematite, and a geochemical tracer used to monitor bioturbation (samarium). The impact of iron concentration on oxygen penetration depth and bioturbation/bioirrigation is assessed using (1) detailed contour mapping of oxygen microelectrode measurements, and (2) X-ray fluorescence scanning of sub cores, which allows quantification for bioturbation induced samarium redistribution. The results from the experiments and employed statistical approaches (linear regressions and ANOVA) suggest that the oxygen penetration depth is determined by the number of the organisms in the sediments and the amount of iron addition. Future macrocosm study will develop quantitative diagenetic models that can provide insights for investigations of ancient organic matter burial episodes and global phosphorus cycle.