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 examine the general hydrogeochemical controls on calcite weathering and the specific role of trace pyrite oxidation in base cation export. White Rock Creek is a perennial gaining stream occupying a catchment underlain by expanding clay soils, and an uppermost fractured bedrock weathering zone (which acts as a surface aquifer) below which there is low permeability Austin Chalk. Seasonal variations in cation export are controlled predominantly by temperature effects on carbonate solubility, dilution by variable amounts of rainfall, and possibly organic activity. Creek water PCO2 is 7 to 10 times atmospheric PCO2 throughout the year, and does not correlate with surface temperature. Non-pyrite sulfate inputs are well-constrained; moreover, integrated runoff and solute export from the basin are similar to global mean values. Pyrite, although present only as a trace component, accounts for a disproportionate amount of bedrock weathering (∼40%) when oxidized to produce sulfuric acid. Irreversible dissolution of pyrite is insensitive to seasonal changes in temperature, runoff, and organic activity and probably reflects the slow movement of a weathering front at depth through the aquifer.