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

Abstract

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 -.

Mineralogical Transformations and Soil Development in Shale across a Latitudinal Climosequence

“…Some mineral weathering reactions, such as carbonate dissolution, pyrite oxidation or plagioclase feldspar dissolution, are described as profile initiating reactions that begin the process of disaggregating bedrock into regolith; weathering reactions such as clay dissolution, however, are likely more important in controlling the overall regolith thickness . Indeed, studies on various lithologies have documented the deepest weathering reactions as carbonate dissolution (White et al, 2005;Williams et al, 2007;Jin et al, 2010), biotite oxidation (Buss et al, 2008;Behrens et al, 2015;Bazilevskaya et al, 2015), or plagioclase feldspar dissolution (Brantley and White, 2009;Behrens et al, 2015) but these minerals sometimes constitute a small fraction of the overall parent mineralogical composition.…”

Anthropogenically enhanced fluxes of water and carbon from the Mississippi River

“…2). In transport limited systems, soil water ion concentrations are at or near saturation with soil minerals because of low water throughput and long water residence times [13][14][15] . As concentrations are at saturation and cannot be raised, this places a hydrologic constraint on the ability of additional acid generating processes (for example, soil respiration) to increase the bicarbonate F avgD .…”

Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and Soča Rivers, Slovenia)

“…The Alpine and Karst springs are at saturation with calcite, the Alpine springs have log pCO 2 values from -2.25 to -3.25 (bars), and the Karst springs have higher log pCO 2 values ranging from -2.4 to -1.7 (bars). Here, a decrease in pCO 2 leads to an increase in calcite saturation, a familiar pattern in groundwater fed streams (e.g., Williams et al 2007 andWalter 2004). Once these subsurface springs enter the surface steam environment, CO 2 either degasses to reach equilibrium with the warmer temperatures and with the atmosphere or is utilized for biological in-stream processes.…”

“…The majority of data available relating to the degree of dolomite weathering are from watersheds with mixed bedrock and relatively little global coverage (e.g., Meybeck 1987;Han and Liu 2004). Recent research in temperate zone glacial drift deposits of mixed carbonate and silicate mineralogy suggests that the contributions from dolomite relative to calcite weathering has been underestimated (Szramek and Walter 2004;Williams et al 2007;Jin et al 2009). Studies from glacial drift soil profiles show that soil water Mg/Ca ratios appear equal to or greater than that of the bulk carbonate soil carbonate (Jin et al 2009).…”

“…This is in part attributable to the relatively greater solubility of dolomite at lower temperatures compared with calcite and may also be controlled by the history of the carbonate surfaces during glacial erosion of bedrock (Jin et al 2009). Williams et al (2007) focuses on closely linked ground and surface water chemistry of the glaciated mid-continent of the United States and suggested that dolomite weathering (largely from glacial drift rather than bedded source areas) is an important contributor to these temperate environments. Jin et al (2008Jin et al ( , 2009 show that early reactions in the mixed mineralogy glacial drift soil column largely control the chemistry of shallow ground waters.…”