Chemical weathering and cation loss in a base-poor watershed

Weathering of minerals in soils provides important plant nutrients and consumes acidity, yet mineral weathering is difficult to observe and quantify. We derived present-day weathering flux estimates for soil minerals in a forested watershed in central New Hampshire using a geochemical mass balance. The Cone Pond watershed is characterized by low-alkalinity surface waters and acidic Spodosols developed on a thin mantle of locally derived till, making it susceptible to continued inputs of acid precipitation. Weathering reactions were developed on the basis of observed patterns of mineral abundance in the soil profile and measured mineral chemistry. The dissolution of approximately 171 mol ha^-1 yr^-1 of plagioclase feldspar is the predominant weathering reaction at Cone Pond. Weathering fluxes of hornblende, biotite, chlorite, and potassium feldspar ranged from 3 to 20 mol ha^-1 yr^-1, an order of magnitude lower. When normalized to their abundances in the soil, however, horn-blende and chlorite had the fastest weathering rates, as measured by cation release. Chemical weathering, mainly of plagioclase, resulted in the neutralization of only 52% of incoming acidity. Furthermore, silicate weathering could only account for the release of about 53 mol ha^-1 yr^-1 of Ca, compared to the observed net output (stream loss minus bulk precipitation input) of 75 mol ha^-1 yr^-1. On the basis of these observations and a companion study, we conclude that current acid inputs exceed the ability of Cone Pond soils to neutralize hydrogen ion, and that depletion of approximately 22-53 mol ha^-1 yr^-1 of Ca from labile soil pools is occurring in this area. The rate of Ca depletion is one-third to one-eighth of the rate estimated for a more base-rich watershed nearby.