LONG-TERM AND SHORT-TERM PLAGIOCLASE WEATHERING RATES IN SOILS DEVELOPED ON GLACIAL DRIFT DEPOSITS (SOUTHERN MICHIGAN, USA)

Silicate weathering has been intensively studied as a negative feedback to regulate the global atmospheric CO₂ on time scales of millions of years. Short-term weathering rates are generally derived from watershed export of cations. However, few studies have focused on watersheds with mixed silicate/carbonate mineralogy, because carbonate minerals when present dominate the water chemistry, making it difficult to evaluate the silicate contribution to the overall cation fluxes.

Soils in southern Michigan have developed over ca. 12,500 years on glacial drift deposits composed of both silicate and carbonate minerals. Plagioclase is the dominant and most reactive silicate mineral. Most streams draining glaciated watersheds in southern Michigan are too contaminated by road de-icing salts to study plagioclase weathering. In this work, we characterized mineral weathering rates in surface soils (to 2 m depth) at a Long Term Ecological Research site (LTER) in southern Michigan. Soil water was sampled for complete geochemical analyses over an annual cycle and water discharge out of the soils was determined from the difference between precipitation and evapotranspiration. Fluxes of Na* (corrected for precipitation input) were used to estimate reaction rates of plagioclase (318 mol ha^-1 yr^-1).

Long-term weathering rates were calculated from elemental loss integrated over the whole soil profile based on mass balances of Ti and Zr. Results suggest a significant volume loss due to the preferential leaching of carbonate minerals. The long-term plagioclase dissolution rate was about 2380 mol ha^-1 yr^-1, much higher than the short-term average value of 318 mol ha^-1 yr^-1. This is consistent with past studies of soil chronosequence that show strong decreases in mineral dissolution rates with increasing age due to loss of reactive minerals and surfaces. However, plagioclase weathering rates in southern Michigan, on both long- and short-term time scales, are much higher than data available in the literature. This is likely due to the high surface area produced by relatively recent glacial abrasion, the sedimentary source of much of the glacial drift, and mean annual temperature and precipitation.