Coupled carbonate dissolution and authigenic clay formation in the bioturbated zone

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Chemical reactions that occur in ocean sediments play an important role in global geochemical cycles, including the carbon cycle. Understanding these reactions is key to predicting the response of the ocean to ongoing disturbances like climate change. This project will combine field measurements, controlled laboratory experiments, and modeling to increase understanding of the interplay of sediment mixing by burrowing organisms and geochemical reactions, and how they affect the ocean’s carbon and alkalinity balance. The project provides for the training of graduate and undergraduate students in field work, experimental procedures, the use of sophisticated state-of-the-art analytical equipment, quantitative data analysis, and numerical modeling techniques. The educational goals are to encourage scientific discovery while fostering creativity, promote critical-thinking skills, enhance problem solving skills, and to encourage the development of the next generation of scientists to be able to address the many environmental challenges facing society. The general objectives of the project are to advance our conceptual understanding and predictive quantitative modeling of the impacts of complex patterns of oxidation-reduction and acid-base reactions on biogeochemical processes within the bioturbated zone. In continental margin deposits underlying oxygenated water, the reoxidation of reduced metabolic products such as ammonium, iron (II), manganese (II) and pyrite results in strong acid production. Sedimentary pyrite oxidation in particular can drive carbonate dissolution and silicate authigenesis. The resulting coupling of carbonate and silicate reactions remains poorly known, and may produce or consume carbonate alkalinity. These reactions are focused at boundaries between oxic and anoxic zones. Oxic-anoxic zonation and zonal interaction patterns in bioturbated deposits reflect the formation, scaling, and residence times of irrigated burrow structures as well as feeding and mobility activity, which dynamically exposes anoxic sediment and promotes reoxidation and oxic-anoxic oscillations. As shown by initial experiments, these processes result not only in undersaturation of carbonates and enhanced dissolution, but may also promote forward (alkalinity producing) and reverse (alkalinity consuming) silicate weathering reactions. The proposed research will utilize a combined approach of field measurements, controlled laboratory experimental manipulations and geometric mimics, 2-D imaging sensors, and theoretical modeling, to address how complex, dynamic biogenic transport-reaction geometries that result from bioturbation activities, mediate coupled redox reactions associated with carbonate dissolution and clay authigenesis, and affect net benthic alkalinity fluxes. This novel research will further constrain factors controlling net sediment-water alkalinity fluxes between lithogenic deposits and overlying water, and thus models of ocean acidification.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.