Major ion pore-water chemistry evolution in Lake Michigan benthic sediments: Evidence for direct input from Michigan Basin saline groundwater

The Michigan Basin is composed of geological formations that contain brines and evaporites, and solutes from these geological sources have affected benthic sediment pore-water chemistry in Saginaw Bay (Lake Huron). We hypothesize that there exists similar potential for upward solute transport directly from the Michigan Basin into other Great Lakes areas. To test our hypothesis, we present here previously unpublished pore-water chemistry analyses from sediment cores collected during multiple Lake Michigan sampling events (spanning 1991−1999) and a new evaluation of previously published data. In several box cores, pore-water chloride concentrations increase with depth, and Cl:Br ratios are consistent with a geological formation brine source. In all gravity cores we collected from southern Lake Michigan, pore-water sodium concentrations increase with sediment depth. At one sample station, pore-water sodium concentrations exceed 2000 mg L−1 within 2 m of the sediment-water interface. Given the pore-water chemistry changes reported here, combined with information from previous studies of Lake Michigan bedrock geology, a Devonian formation brine is a plausible solute source. The presence of saline pore water within glaciolacustrine sediments underlying Lake Michigan indicates that this solute flux has been active during the past 10 k.y. However, the origins of this solute flux, including timing (onset) and contributions from advective and/or diffusive transport, are unknown. The specific geological source and solute transport process are important to resolve in order to evaluate potential effects of these Michigan Basin solute sources on the Great Lakes’ sediment biogeochemistry and water quality.