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

Erika L. Williams^,1, Kathryn J. Szramek^,1, Lixin Jin^,1, Timothy C.W. Ku^#,2 and Lynn M. Walter^,3

¹ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
² Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA
³ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

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 Ca²⁺-Mg²⁺-HCO₃^– 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_CO2) values, which increase significantly from north to south (log P_CO2 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_CO2 values produce HCO₃^– concentrations that vary by a factor of five, but the Mg²⁺/Ca²⁺ and Mg²⁺/HCO₃^– 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 Mg²⁺/HCO₃^– 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 Mg²⁺and HCO₃^–.

Key Words: carbon • rivers • groundwaters • Michigan • carbonate saturation • dolomite dissolution • carbon dioxide