Award: OCE-1737749

Intellectual merit: The biological, chemical, and physical factors that affect the seabed following deposition of sediments (termed diagenetic processes) play an essential role in the cycling and burial of reactive elements such as carbon, nitrogen, and phosphorus in the ocean. The activities of bottom-dwelling animals which include feeding, burrowing, and ventilation of tubes and burrows that they construct, in aggregate termed bioturbation, dramatically impact sediment chemistry. Bioturbation alters the distributions and movements of dissolved and particulate material in deposits, the rates and pathways by which reactive components are biologically and chemically broken down, and the rates of exchange of nutrients and reactive metals with water overlying the seafloor. This project focused on the impacts of bioturbation on the oxidation of sulfur and nitrogen in sediments, and the effects of the resulting natural production of strong acids on the dissolution of carbonate minerals. We used multiple approaches to resolving and quantifying the complex compositional and material transport patterns associated with bioturbation and their impacts on acid production, particularly using geometric mimics to simulate animal activity. We also examined the role of cable bacteria, which we showed aggregate around burrows and tubes, in the oxidation of sulfur and carbonate dissolution. We discovered that the oxidation rate of the iron sulfide mineral pyrite in sediments is highly time-dependent due to changes in reactivity as oxidation proceeds. This new finding affects the quantitative model estimates of sulfuric acid production in sediments and its role in dissolving carbonate minerals such as bivalve shells. In order to better quantify natural nitric acid production, we developed a novel imaging sensor (termed planar optode) that allows real time in situ measurement and direct visualization of dissolved ammonia in 2 and 3 dimensions (Image 1). We also further explored the use of a planar carbonate saturometer that we invented to directly provide 2-D imaging of carbonate dissolution or precipitation in sediments. In studying these reactions, we discovered that silicate minerals such as clays can form rapidly during carbonate dissolution in the bioturbated zone, which affects the rate and extent of carbonate mineral reactions. An important implication is that the silicate and carbonate reaction cycles are highly coupled in the bioturbated zone of marine sediments, so that both acid – base balances and multiple elemental cycles are strongly affected by early diagenetic reactions and animal activity. Quantitative transport – reaction models were developed to improve predictions of these affects. Broader impacts: Because sedimentary deposits behave as open systems, early diagenetic reactions and the associated release of solutes, fluids, and particulate material, impact ocean water composition and ecosystem processes. Human activities are dramatically affecting ecosystems including coastal and continental boundary regions where bioturbation is most intense, and driving major global environmental changes. Many significant conceptual advances in understanding the biogeochemical reactions and cycling of reactive elements in sedimentary environments have come from the experimental and field studies in conjunction with advances in sensor technology, and theoretical modeling developed by our group. This research improves our ability to evaluate the impacts and predict future consequences of anthropogenic forcing on complex margin systems and thus global biogeochemical cycles. Last Modified: 04/06/2023 Submitted by: Robert C Aller