[1] Calcite and dolomite solubilities in open weathering environments are proportional to pCO 2 and inversely proportional to temperature, and dolomite solubility is progressively greater than calcite below 25°C. The continent-scale weathering budget reveals the significance of the Northern Hemisphere (NH) to globally integrated riverine fluxes of Ca 2+ , Mg 2+ , and HCO 3 À . The NH contributes 70% of the global HCO 3 À flux while only 54% of the riverine discharge. We present results of a comparative hydrogeochemical study of carbonate mineral equilibria and weathering fluxes in two NH carbonaterich river basins. Surface water geochemistry and discharge were determined for headwater streams in Michigan and Slovenia within the St. Lawrence and Danube river basins. Michigan watersheds are established atop carbonate-bearing glacial drift deposits derived from erosion of Paleozoic strata with thick soil horizons (100-300 cm). Slovenia watersheds drain Mesozoic bedrock carbonates in alpine and dinaric karst environments with thin soil horizons (0-70 cm). Carbonate weathering intensity is a parameter that normalizes river runoff and HCO 3 À concentration to catchment area (meq HCO 3 À km À2 s À1 ), summing calcite and dolomite contributions, and is used to gauge the effects of climate, land use, and soil thickness on organic-inorganic carbon processing rates. Importantly, Michigan riverine discharge is one-tenth of Slovenian rivers, providing the opportunity to evaluate the kinetics of carbonate mineral equilibration.

We present here a fi eld geochemical study of controls on carbonate weathering within rapidly circulating, shallow groundwatersurface water systems in the glaciated midcontinent 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 signifi cantly 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 fi ve, 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 signifi cant 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 fl uxes of both Mg 2+ and HCO 3 -.

[1] We sought to determine the effect of elevated atmospheric CO 2 on mineral weathering reactions in midlatitude carbonate-bearing forest soils of differing nutrient availability. Increased plant growth and soil respiration under elevated atmospheric CO 2 suggest increased rates of carbon cycling, which may affect mineral weathering. A randomized complete block experiment was conducted, where aspen and maple saplings were grown in open top chambers under two levels of atmospheric CO 2 and soil N. Soil solution chemistry and soil gas PCO 2 profiles beneath aspen were collected from planting (1997) to harvest (1999). Carbonate mineral weathering products (Ca 2+ , Mg 2+ , HCO 3 À ) dominated solutions, which were saturated with respect to calcite. Soil PCO 2 values at 25 cm depth were 41% higher in high N soils, but CO 2 treatment was not significant. An ANOVA model tested treatment effects on spring 1998 solution chemistry. CO 2 treatment had a significant effect on DIC, which was 12% higher in elevated than ambient CO 2 chambers. Little effect of CO 2 treatment was observed in low N soils. In high N soils, solutions had higher concentrations of carbonate weathering products (DIC, 15%; HCO 3 À , 27%; Ca 2+ , 3%, not significant; Mg 2+ , 5%, not significant). Soil N availability had a significant, positive, effect on mean concentrations of Ca , and DOC. The soil N treatment difference in solutes may result from differences in PCO 2 and, additionally, NO 3 À from organic matter decomposition. Our results suggest that increased carbonate weathering may occur under increased atmospheric CO 2 and in fertile soils.

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