The Role of Disseminated Calcite in Rates of Calcium Release and CO2 Consumption During the Weathering of Granitoid Rocks

Abstract

Although Ca and Na reside predominately in plagioclase feldspar in granitoid rocks, recent studies have demonstrated that excess Ca often exits in surface and ground waters relative to plagioclase stochiometry. Explanations advanced to explain the Ca excess have included selective loss of Ca in soils due to watershed acidification (Hyman et al., 1998), selective weathering of the anorthite component of plagioclase (Clayton 1986), and the rapid weathering of trace mineral phases such as calcite (Mast and Drever 1990). Chemical hydrolysis of Ca-silicate rocks is recognized as impacting long term atmospheric CO2 consumption and global climate through silicate hydrolysis reactions (Bemer and Caldeira 1997). In contrast, carbonate mineral dissolution does not have a corresponding impact because the CO2 consumed in weathering is reintroduced back into the atmosphere by precipitation of carbonates in the ocean. Thus, the solute Ca/Na ratio in watersheds and rivers, reflecting proportions of silicate and carbonate weathering, is important in understanding long term global CO2 balances.