Production and preservation of organic matter: The significance of iron

Abstract

“Iron fertilization” has been previously recognized as a potential mechanism for enhanced organic matter burial in marine sediments. However, the singular view of iron as a control on primary production overlooks its role in sedimentary diagenesis, a factor that must be evaluated when considering organic matter accumulation. This study examines the role of iron as a buffer of pore water sulfide and its implications for marine organic matter burial. Biogeochemical model experiments indicate that dissolved sulfide buildup in surficial marine sediments is highly sensitive to reactive iron concentration. A reduction in reactive iron concentration can initiate dissolved sulfide accumulation, the consequences of which include inhibition of bioturbation/bioirrigation, a decrease in oxygen exposure time, and enhanced organic matter burial. Alternatively, an increase in reactive iron concentration can serve to decrease organic matter burial. The coupling of iron and phosphorous cycling within marine sediments provides an important positive feedback, and therefore this mechanism is designated the “sulfide buffer/phosphorous trap hypothesis.” Given sufficient organic carbon supply, carbonate‐rich and opal‐rich sediments should be especially prone to the development of sulfidic conditions because of a deficiency in terrigenous iron. Widespread chalk and marl deposition during the Cretaceous, in association with the evolutionary expansion of calcareous plankton, may have predisposed many benthic marine environments to the accumulation of toxic hydrogen sulfide and fostered the development of “oceanic anoxic events.” Comparison of model results with proxy data from oceanic anoxic event II (middle Cretaceous) suggests a complex role of iron as a control on both organic matter production and preservation.