Testing the “Sulfide Buffer/Phosphorus Trap Hypothesis” Using Anoxic Coastal Sediments from Cape Lookout Bight (North Carolina Outer Banks)

Iron availability has far-reaching consequences for marine biogeochemistry. In addition to serving as an essential micronutrient that can limit primary production and cyanobacterial nitrogen fixation, iron plays a fundamental role in organic matter decomposition and sedimentary diagenesis. We are presently 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 flux to the water column (sorption onto ironoxyhydroxides). These factors were recently proposed as possible mechanisms for promoting ancient organic matter burial episodes. The initial phase of this study is specifically concerned with the diagenetic role of iron as a buffer of pore water sulfide, and the impact of this process on bioturbation/bioirrigation and organic matter burial. In this presentation, we outline a new approach to investigate the hypothesis, using controlled laboratory macrocosm experiments. Marine sediments were collected from Cape Lookout Bight (North Carolina Outer Banks), a shallow coastal marine environment (< 8m) with an oxygenated water column, but organic-carbon-rich sediments dominated by sulfate reduction and methanogenesis. Following transportation to the laboratory, the uppermost portion of each 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) oxygen microelectrode measurements (yielding detailed contour maps of oxygen concentrations over time), and (2) X-ray fluorescence scanning of sub cores, which allows quantification of samarium redistribution by bioturbation. The results of the macrocosm work will be used to develop quantitative diagenetic models, which will provide a biogeochemical calibration for investigations of ancient organic matter burial episodes.