In this project, researchers at the University of Minnesota at Duluth will investigate the geochemistry of organic carbon, nitrogen, phosphorous, and iron in the sediments of Lake Superior and quantify their temporal and spatial variability. The geochemical budgets of nitrogen and phosphorus in Lake Superior are presently unbalanced, and major trends in nutrient concentrations and primary productivity remain unexplained. The researchers' preliminary data suggest that the sediments of Lake Superior have experienced large vertical migrations in the depth of oxygen penetration, analogous to those implicated in organic-poor sediments in the deep ocean. Given the importance of redox conditions for the sediment nutrient recycling, they hope to elucidate the seasonal and multi-decadal responses of these sediments to the temporal changes in water column chemistry and biological productivity.
The specific objectives are: (1) to characterize the geochemistry of organic carbon, nitrogen, phosphorus, and iron in Lake Superior sediments and quantify their temporal and spatial variability; (2) to simulate the diagenetic biogeochemical cycling in the lake and calculate the in situ rates of the organic carbon mineralization pathways and sediment-water nutrient fluxes at sites with different oxygen penetrations; (3) to elucidate seasonal and multi-decadal sediment responses to variations in the organic carbon sedimentation and bottom water concentrations of oxygen and nitrate, and quantify sediment contributions to the water column N and P budgets; and (4) to gain insights into the causes, consequences, and the dynamics of the increasing N:P imbalance in Lake Superior.
These objectives will be accomplished using direct measurements of geochemical parameters in the lake and sediment incubations. Results will be integrated into sediment reactive-transport models and mass-balance water column models for examination of in situ process rates, reconstructions of past dynamics, and future projections.
Broader Impacts: Arguably, study of these phenomena in Lake Superior will advance understanding of geochemical dynamics in other carbon-limited sediments. The sediment chemistry models developed will contribute to the ongoing system-wide modeling efforts in Lake Superior, will serve the large community of Lake Superior researchers, and will be transferable to other environments. Graduate and undergraduate students will be included on the research team.
