PRIMARY-SECONDARY MINERAL RELATIONSHIPS AND CLAY FORMATION RATES DURING WEATHERING: WATERSHED GEOCHEMICAL MASS BALANCE AT COWEETA HYDROLOGIC LABORATORY, NORTH CAROLINA, USA

Studies of solute fluxes from watersheds permit application of chemical mass balance methods to the calculation of field-based mineral weathering rates, and to determining the contribution of elemental loss to landscape denudation. If mineral stoichiometries and primary-secondary mineral relationships are well characterized, mass balance methods are a reliable means for quantifying elemental transfers in landscapes. To date, the geochemical relationship between primary minerals that are weathering, and the formation of secondary clays, has not been systematically addressed in mass balance calculations. The results of this mass balance study demonstrate the need to properly characterize plagioclase-kaolinite and biotite-hydroxy-interlayered-vermiculite (HIV) relationships.

In previous watershed solute mass balance studies, the rate of kaolinite neoformation may or may not have been explicitly coupled with the plagioclase dissolution rate. For seven watersheds of Coweeta Hydrological Lab in North Carolina, we found that the decoupled kaolinite formation rate may differ by as much as 100% from the kaolinite formation rate coupled to the plagioclase dissolution rate. Microscopic observations of saprolite collected from the watersheds suggest that kaolinite formation rates should be decoupled from plagioclase formation rates.

When coupled with biotite weathering, the rate of HIV formation equals the biotite weathering rate, as the biotite silicate structure is preserved during the transformation. Using watershed Si flux data as a proxy for the intensity of chemical weathering, the HIV formation rates calculated from the coupled biotite-HIV scenario increase with increasing weathering, as would be predicted. HIV formation rates generated by decoupling from biotite failed to produce the predicted trend, supporting preliminary observations that suggest HIV formation should be coupled with biotite weathering. The kaolinite formation rates produced from the decoupled biotite-HIV mass balance calculations also failed to produce predicted trends when plotted against Si flux.

Additional applications of watershed mass balances to calculation of clay-mineral formation rates will relate chemical weathering rates to erosion rates, sediment composition, and landscape evolution.