Mg- and Ca- carbonate versus silicate dissolution rates in mid-latitude, glaciated soil profiles: Implications for riverine weathering fluxes and global geochemical budgets

Chemical weathering of continental rocks is an important process in regulating global climate and ocean chemistry. Chemical weathering studies have dominantly focused on riverine chemistry, with much less information available on soil-zone processes. This dissertation investigates mineral-water interactions in soil profiles developed on the recently deglaciated landscapes of Michigan. At each study site, soil solids, pore waters and gases were sampled as a function of depth over the annual seasonal cycle, and were characterized by geochemical analyses including elemental concentrations and isotopic ratios (Sr and C).; Silicate-mineral weathering is largely restricted to the shallowest soil horizons where pH is low and dissolved organic carbon concentrations are high. The transition from silicate weathering to carbonate weathering in soil profiles is abrupt, reflected by large shifts in soil water pH and elemental concentrations. Soil waters become saturated with calcite and dolomite within the carbonate layer suggesting that dissolved inorganic carbon (DIC) transport is only limited by soil CO2-dependent carbonate solubility and discharge. Similar chemistry of soil waters and regional surface waters/groundwaters also identifies the soil zone as a key site of cation acquisition. Solute chemistry and Sr isotopic analyses were utilized to calculate silicate versus carbonate weathering mass balance. These yield consistent results and show that silicate mineral dissolution contributes <10% of Ca2+, with carbonate minerals dominating cation fluxes from soil profiles. More importantly, dolomite dissolution, rather than silicate dissolution, dominates as a riverine Mg2+ source (> 90%). If this applies to significant portions of continental surfaces, the global Mg cycle need to be re-visited.; C sources of soil water DIC were identified by carbon isotopes. At shallow soil horizons where only silicate weathering is present, soil CO2 is the only source of soil water DIC with carbon isotopic equilibrium between two carbon-bearing species. At deeper soil horizons where carbonate dissolution starts to dominate, soil water delta13CDIC is constant (= ½ delta13CCO2+ ½ delta 13CCaCO3), implying DIC are equally sourced from carbonate and soil CO2 in a closed system. This study correlates chemical weathering directly to CO2 consumption for the first time, and provides basis for riverine water 13C data interpretation.