THE WEATHERING OF SILICATE MINERALS IN THE GLACIATED MID-CONTINENT U.S.: RESULTS FROM NATURAL AND EXPERIMENTAL FIELD STUDIES

Few studies of silicate mineral weathering have been conducted in carbonate-bearing temperate forest soils. In northern Michigan, silicate weathering fluxes are among the highest of temperate U.S. watersheds. Global models of silicate weathering that consider only the effects of temperature and precipitation fail to predict these high rates. With climate and vegetation held constant, we compared the soil mineralogy and major element chemistry of soil waters from a carbonate-free temperate aspen forest site in northern Michigan with that from manipulated carbonate-containing soils from experimental tree-growth chambers (low- vs. high- fertility). All soils were well-drained sands composed of quartz, Na-rich plagioclase, and K-feldspar, with minor amounts of carbonate present only in the experimental soils. To quantify plagioclase feldspar weathering, the Na concentrations in soil waters were used after correction for atmospheric deposition, yielding Na*.

In natural soil water profiles, the maximum concentrations of Na*, Si, and dissolved organic carbon (DOC) were observed by a depth of 15 cm, a soil zone free of carbonate minerals. In both the low- and high-fertility experimental chambers, soil waters attained saturation with respect to carbonate minerals at 15-cm depth. Solubility and PCO₂ values were greater in the high fertility soils. Na* and DOC concentrations also were higher than those from natural soils and the DOC and Na* concentrations were slightly greater in the high- than low-fertility soil.

The presence of carbonate minerals in the experimental soils did not lower feldspar dissolution rates, in contrast with predictions from pH-dependent kinetics of silicates. In fact, the presence of carbonate minerals may enhance the effectiveness of DOC in solubilizing silicate minerals by increasing the pH and the complexing capacity of organic ligands for aluminum. The increased silicate weathering observed in the experimental soils may be the result of this latter process and suggests that silicate weathering rates will increase in disturbed soils where carbonate minerals are reworked from deeper soil zones into organic-rich surface soil zones. Further, generalized silicate weathering models may need to consider the enhancing effects of carbonate buffering and DOC concentrations.