CONTROLS ON CARBONATE AND SILICATE WEATHERING FLUXES IN TWO FORESTED WATERSHEDS, NORTHERN MICHIGAN

Mineral weathering rates in soil profiles can be influenced by soil water pH, PCO₂ of soil gas, and the organic acid anion fraction in dissolved organic carbon (DOC). Silicate and carbonate weathering fluxes were investigated in soil waters and streams of two temperate forested watersheds in northern Michigan that differ in carbonate content, topography, and vegetation. Soils in both watersheds (Cheboygan and Tahquamenon) have developed on permeable glacial drift (predominantly quartz, K-feldspar, and plagioclase), but Tahquamenon soils lack carbonates. Accordingly soil and stream waters from Tahquamenon have lower pH values, which might be expected to enhance silicate dissolution fluxes.

In these relatively pristine watersheds, dissolved Na is primarily derived from plagioclase weathering, with minor contributions from atmospheric deposition. Concentrations of Na adjusted for atmospheric deposition ([Na*]) were higher in soil waters in the Cheboygan watershed compared with those in the Tahquamenon. DOC contents are comparable in soils of the two watersheds (2 mM), but pH and alkalinity are higher in the Cheboygan (pH 6.5 vs. 5; Alk <2 mM vs. <0.05 mM), indicating carbonate dissolution and buffering.

Using our stream chemistry and USGS discharge data, we calculate weathering fluxes from the two watersheds. Area-normalized discharge from the Tahquamenon is 30% higher than from the Cheboygan. Despite lower pH in Tahquamenon waters, the Cheboygan watershed yields higher weathering fluxes from both silicates and carbonates (40% for Na*, and 50% for Alk and Ca+Mg). Cheboygan carbonate fluxes reflect the high solubility of carbonate minerals. In contrast, higher silicate weathering fluxes from the Cheboygan corresponds with the increasing negative charge on DOC with increasing pH, which may allow for increased complexing capacity and silicate dissolution. Thus in locations with abundant DOC and pH buffered by carbonate minerals, silicate weathering may be enhanced.