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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 -.

The Devonian Antrim Shale is an organic-rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well-developed network of major, through-going vertical fractures controls reservoir-scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of d 13 C values (þ34 to À41% PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high d 13 C values (>þ15%) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low d 13 C values (<À32%) are solely associated with outcrop samples and record methane oxidation during cement precipitation. Fracture-fill calcite with d 13 C values between À10 and À30% can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of 13 C-rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins.It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture-fill cements. The d 18 O values in both formation waters and calcite cements increase with depth in the basin (À12 to À4% SMOW, and þ21 to þ27% PDB, respectively). Most fracture-fill cements from outcrop samples have d 13 C values between À41 and À15% PDB. In contrast, most cement in cores have d 13 C values between þ15 and þ34% PDB. Radiocarbon and 230 Th dating of fracture-fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.

[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.

Coordinated taphonomic, geochronologic, and geochemical studies of bivalve death assemblages and their sedimentary environments of San Blas, Caribbean Panama, permit us to identify the major factors controlling skeletal degradation in mixed carbonate-siliciclastic tropical shelf sediments. Ten sites were studied along environmental gradients including water nutrients, grain size, and sediment chemistry (carbonate, organic carbon, and reactive iron contents). Taphonomic data were derived from naturally occurring bivalve death assemblages and experimentally deployed specimens of Mytilus edulis and Mercenaria mercenaria to determine environmental controls on types and intensities of postmortem damage to skeletal hardparts and to quantify short-term rates of damage accrual. Death assemblage shells were dated using 14 C and amino acid racemization techniques to examine shell persistence, scales of time averaging, and long-term rates of damage accrual, including correlations between shell damage and shell age. Pore water and sediment geochemical analyses were used to determine the pathways and extent of early diagenetic change in the different sediment-pore water environments. We found that carbonate shell preservation is enhanced in dominantly siliciclastic sediments compared to dominantly carbonate sediments. The most important factors limiting the postmortem persistence of shell material are (1) exposure above the sediment-water interface, which is enhanced in coarser-grained carbonate sediments and permits attack by bioeroders and encrusters; (2) the availability of abundant reactive iron mineral phases in the sediments, which promotes supersaturated pore waters and limits acid production; and (3) shell microstructure (rather than mineralogy), particularly organic content that is the focus of intense microbial attack. Thus, there is significant potential for enhanced carbonate shell preservation in areas receiving ferric-rich tropical weathering products, which are common in much of the tropics today and are associated with subduction systems in the geologic past. This suggests that paleodiversity estimates from carbonate tropical settings are minima and that siliciclastic settings are probably underestimated regions for carbon burial, given the large proportion of tropical shelf area characterized by such conditions and the relatively high proportional capture there of local carbonate production.