CATCHMENT-SCALE COUPLING BETWEEN PYRITE OXIDATION AND CALCITE WEATHERING

Integrated time series of major element concentrations and flow rates from a small watershed (White Rock Creek, Dallas, TX) are used to exam ine the general hydrogeochemical controls on calcite weathering and th e specific role of trace pyrite oxidation in base cation export. White Rock Creek is a perennial gaining stream occupying a catchment underl ain by expanding clay soils, and an uppermost fractured bedrock weathe ring zone (which acts as a surface aquifer) below which there is low p ermeability Austin Chalk. Seasonal variations in cation export are con trolled predominantly by temperature effects on carbonate solubility, dilution by variable amounts of rainfall, and possibly organic activit y. Creek water P-CO2 is 7 to 10 times atmospheric P-CO2 throughout the year, and does not correlate with surface temperature. Non-pyrite sul fate inputs are well-constrained; moreover, integrated runoff and solu te export from the basin are similar to global mean values. Pyrite, al though present only as a trace component, accounts for a disproportion ate amount of bedrock weathering (similar to 40%) when oxidized to pro duce sulfuric acid. Irreversible dissolution of pyrite is insensitive to seasonal changes in temperature, runoff, and organic activity and p robably reflects the slow movement of a weathering front at depth thro ugh the aquifer. (C) 1998 Elsevier Science B.V. All rights reserved.