EVIDENCE OF DISSOLVED SOLIDS FLUX UPWARD FROM THE MICHIGAN BASIN INTO LAKE MICHIGAN BENTHIC SEDIMENTS – A NEW LOOK AT OLD DATA

An understanding of anthropogenic and natural sources of dissolved solids, i.e., sodium and chloride, is needed for underpinning long-term projections of Great Lakes’ water quality. The Michigan Basin, which underlies a substantial portion of the Great Lakes’ watershed, comprises geological formations that contain brine and(or) evaporite (salt) deposits, which are potential solute sources. We have documented previously within Saginaw Bay (Lake Huron) benthic sediments the presence of upward solute fluxes from these Michigan Basin sources. We hypothesize that similar fluxes are occurring in other areas of the Great Lakes overlying the Michigan Basin.

To test this hypothesis, we examined porewater chemistry analyses from sets of sediment cores collected from southern Lake Michigan, and compared these analyses with variations in sediment type and in regional bedrock geology. This comparison provided the basis for evaluating potential source(s), timing, spatial extent, and geological factors affecting solute exchange between the Michigan Basin and Lake Michigan benthic sediments. In several shallow box cores, dissolved chloride concentrations (maximum 40 mg L^-1) increase with depth; Cl:Br ratios are consistent with those of formation brine. In all gravity cores (analyzed only for major cations), dissolved sodium concentrations increase with depth. At one station, dissolved sodium concentrations exceeding 2,000 mg L^-1 are measured within 2 m of the sediment-water interface. The presence of saline porewater within glaciolacustrine sediments underneath Lake Michigan indicates that this solute flux commenced sometime within approximately the past 10 ka. Combined with a new evaluation of porewater chemistry data from Callender (1969), we infer that dissolved solids from Michigan Basin sources are affecting porewater chemistry, to varying degrees, across substantial portions of southern Lake Michigan. The relative importance of advective versus diffusive solute transport, and the potential for transfer to the overlying water column, currently is unclear. What is clear is that a better understanding of the geological source(s) and transport mechanism are needed to resolve the potential influence of these saline fluids on Lake Michigan water quality and sediment biogeochemistry.