Jin, L. 2007. Mg- and Ca-carbonate versus silicate dissolution rates in mid-latitude, glaciated soil profiles: Implicatons for riverine weathering fluxes and global biogeochemical budgets. Ph.D. Dissertation, University of Michigan, Ann Arbor, Michigan, USA.
Chemical weathering of carbonate and silicate minerals in continental rocks is an important process in regulating global climate and ocean chemistry because of the interplay between organic/inorganic carbon reservoirs. Chemical weathering studies have dominantly focused on riverine chemistry and ion fluxes, with much less information available on soil zone processes. This dissertation investigates organic matter-mineral-water interactions in soil profiles developed on the recently deglaciated landscapes of Michigan. Importantly, these soils span a range of parent drift mineralogy and climate. As well, the regional geochemistry of surface and groundwaters are well characterized, so the contributions of early reactions in the soil zone can be determined. Study sites include natural soil profiles in forests as well as experimental soil monoliths which permit quantification of solute/water budget. At each site, soil solids, pore waters and gases were sampled with depth over the annual seasonal cycle and were characterized by geochemical analyses including elemental concentrations and isotope 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 Sr isotopes, C isotopes and elemental chemistry. Soil waters become saturated with calcite and dolomite within the carbonate layer suggesting that dissolved inorganic carbon transport is only limited by soil CO2-dependent carbonate solubility. Comparisons of soil water chemistries with regional surface waters/shallow groundwaters identify the soil zone as a key site of solute acquisition. Solute chemistry data, as well as endmember compositions identified by isotope analyses of soil minerals and pore waters, were utilized to calculate silicate versus carbonate weathering mass balance. These yield consistent results and show that silicate mineral sources of Ca2+ and Mg2+ contribute <10% of the divalent cations, with carbonate minerals dominating the weathering-derived cation fluxes from soil profiles. In particular, the soil solution data clearly suggest dolomite dissolution, rather than silicate dissolution, dominates as a riverine Mg2+ source. This places new constraints on variations of the global carbon cycle on short and long time scales.
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