Kathryn Szramek

The carbonate system geochemistry of shallow groundwater–surface water systems in temperate glaciated watersheds (Michigan, USA): Significance of open-system dolomite weathering

We present here a field geochemical study of controls on carbonate weathering within rapidly circulating, shallow groundwater–surface water systems in the glaciated mid-continent region. Groundwaters and surface waters in three watersheds spanning the Upper to Lower Peninsulas of Michigan consist of Ca 2+ -Mg 2+ -HCO 3 − solutions derived from the open-system dissolution of calcite and dolomite in soils developed on mixed mineralogy glacial drift. The thermodynamic stabilities of calcite and dolomite both decrease with decreasing temperature, with dolomite more strongly affected. Thus, the low mean annual temperature of these temperate weathering environments maximizes the absolute solubility of dolomite as well as its solubility relative to calcite. Many groundwaters in the study area approach equilibrium with respect to the more soluble dolomite and are moderately supersaturated with respect to calcite. Groundwaters in each watershed have distinct and relatively narrow ranges of carbon dioxide partial pressure ( P CO 2 ) values, which increase significantly from north to south (log P CO 2 of −3.0 to −2.2 atm), suggesting that there are landscape-level differences in carbon transformation rates in soil weathering zones. Increases in weathering-zone P CO 2 values produce HCO 3 − concentrations that vary by a factor of five, but the Mg 2+ /Ca 2+ and Mg 2+ /HCO 3 − ratios of all groundwaters are similar, suggesting relatively constant weathering input ratios of calcite and dolomite. Although surface waters commonly are between 2 and 10 times supersaturated with respect to calcite, the Mg 2+ /HCO 3 − ratios of surface waters are very close to initial groundwater values, suggesting that back precipitation of calcite is not a significant process in these systems. The enhanced solubility of dolomite at low temperatures coupled with the landscape-level differences in carbon cycling suggest that temperate-zone weathering reactions in glaciated terrains are significant contributors to continent-scale fluxes of both Mg 2+ and HCO 3 − .