CARBON BURIAL AND METAL ACCUMULATION IN A LOW-OXYGEN GREAT LAKES ENVIRONMENT

Near Alpena, Michigan, the Middle Island Sinkhole (MIS) sits below 23 m of water in Lake Huron. The water flowing out of MIS is compositionally distinct from Lake Huron, and more similar to the regional groundwater: low in oxygen and high in sulfur, giving rise to a unique eukaryote-free ecosystem dominated by cyanobacterial mats. While the biology of the mats has been studied to some degree, little is known about the underlying sediment and about how much carbon is preserved within the low-oxygen zones. Sediment cores throughout the sinkhole were analyzed for organic C and N content, and organic and inorganic carbon isotopes. The low-oxygen groundwater allows for enhanced accumulation of organic matter in MIS, distinguishing these sediments from other Great Lakes sites. MIS sediments have organic carbon contents greater than that of other Great Lake sediments, and C:N ratios more similar to the Redfield ratio of marine plankton and oceanic deepwater, than to C:N ratios of other Great Lake sediments or of the microbial mats themselves, suggesting enhanced preservation of pelagic carbon at MIS. High organic matter burial within MIS can potentially result in increased trace metal burial in the sediments, which can be examined using Fe speciation to look at whether conditions are primarily anoxic or euxinic. Fe speciation has been developed and calibrated primarily in modern sulfur-rich marine ecosystems, and is rarely applied to freshwater ecosystems. Measuring Fe speciation on the MIS sinkhole system sediments, in combination with other trace metal analyses (e.g., Mo), can be used to determine the degree of anoxia or euxinia at MIS— both at the water-sediment interface and in the sediment column—to provide a novel test of the applicability of Fe speciation to freshwater systems, and to propose specific pathways for the preservation of redox-sensitive metals in relation to organic C burial.