HR: 0800h
AN: H31D-1332    [Abstracts]
TI: Soil Solutions as the First Source of Divalent Cations in Rivers: Carbonate and Silicate Mineral Weathering in Two Michigan Watersheds (Tahquamenon and Huron)
AU: * Jin, L
EM: ljin@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Williams, E L
EM: erikalw@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Szramek, K
EM: kszramek@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Walter, L M
EM: lmwalter@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Hamilton, S K
EM: Hamilton@kbs.msu.edu
AF: Michigan State University, W. K. Kellogg Biological Station, Hickory Corners, MI 49060 United States
AB: Most studies attribute the dominant fraction of riverine Mg²⁺ flux to silicate, rather than carbonate, mineral weathering. We have followed the relative contributions of Mg²⁺ and Ca²⁺ to soil waters, shallow ground waters and streams in the Great Lakes region. Here, thick deposits of freshly eroded and reactive minerals in glacial drift provide an ideal natural laboratory to determine sequence and mass balances of carbonate versus silicate mineral weathering and to trace the evolution of soil waters into shallow ground waters and streams. Most glacial materials in Michigan originated from Canadian Shield granite/gneiss while Devonian/Silurian carbonates from Michigan Basin are present and significant for only Lower Peninsula of Michigan. This provides us with soil profiles derived primarily from either pure silicate or mixed silicate and carbonate in representative watersheds of the Upper Peninsula (Tahquamenon River watershed) versus the Lower Peninsula (Huron River watershed) of Michigan. Although silicate mineral weathering (e.g. amphibole and plagioclase) is apparent in the chemistry of soil waters in the Tahquamenon River watershed, the soil waters are also significantly contributed by wet precipitation to the major cation budgets. In contrast, amphibole and plagioclase are both more abundant in soils of the Huron River watershed, which allows assessment of their contributions to soil waters. As well, calcite and dolomite are present at 1m depth in these soils in nearly equal amounts. Here, soil waters are dominated by silicate weathering in the shallowest soil horizons where pH is low and dissolved organic carbon contents are elevated. By about 1m depth, soil waters become dominated by contributions from carbonate dissolution. The acquisition of Ca²⁺ and Mg²⁺ in soil waters in about a 3:1 mole ratio indicates that calcite and dolomite dissolve at similar rates, contrary to commonly held assumptions about the relative rates of calcite versus dolomite weathering. In fact, both dolomite and calcite are close to saturation implying carbonate minerals reach equilibrium well before reaching the water table. Studies on soils show that silicate and carbonate minerals have different Sr²⁺/Ca²⁺ mole ratios so that soil solution Sr²⁺ and Ca²⁺ concentrations can be applied as an indicator of mixing of dissolution between carbonate and silicate end-members. Because Mg²⁺ behaves conservatively in surface water systems, Mg²⁺ concentrations in streams reflect the intensity of dolomite and mafic mineral dissolution from these watersheds. Using stream discharge data, Mg²⁺ weathering intensity from these two watersheds can be compared to other watersheds in the world. We show that high runoff values coupled with intense carbonate weathering in mid-latitude, glaciated regions, allows them to be a significant Mg²⁺ source to the oceans.
DE: 1039 Alteration and weathering processes (3617)
DE: 1719 Hydrology
DE: 1806 Chemistry of fresh water
DE: 1879 Watershed
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005