DEVELOPMENT OF WEATHERING RATES FROM FIELD DATA AND THE RELATIONSHIP BETWEEN THESE RATES AND CO2, SIERRA NEVADA CA

Weathering rates for plagioclase, alkali feldspar, biotite, and hornblende were calculated from field data collected in a series of canyons near Indian Wells Valley, CA.  Flow in these canyons is intermittent and most of the discharge is through alluvium that fills the bottoms of the canyons.  This alluvium is derived from the weathering of granite and granodiorite.  Carbon dioxide concentrations in the stream waters have been measured and a range of values for PCO₂ was found from 10^-1.3 to 10^-2.65.  Preliminary models of the system show a strong correlation between total dissolved solids (TDS) in the stream water and partial pressure of carbon dioxide (PCO₂).  Lithology, slope, vegetation, and recharge do not change dramatically between these canyons leading to the hypothesis that higher TDS is a result of faster weathering due to elevated CO2.  Equations to correct weathering rates for increased partial pressures of CO₂ have been developed from laboratory experiments (Berg and Banwart 2000, Laguache 1965).  These equations show an increase in weathering rates proportional to approximately 0.3PCO₂.

Paces (1983) approach to calculating weathering rates was employed for the data from the study area.  Initially no correlation could be made between the calculated rates and PCO₂.  Sensitivity studies of Paces equation showed that residence time and mineral stoichiometry have the largest effect on the calculated rates.  Residence times were recalculated using the same method for each canyon to reduce the uncertainty and mineral stoichiometries were determined through microprobe analysis of alluvium samples.  Weathering rates calculated with Paces (1983) equation were refined and the results show a stronger correlation between calculated rates and PCO₂.

Literature rates were adjusted to PCO₂ values observed in the field using the relationship of Berg and Banwart (2000).  These rates were used to develop a forward predictive model using PHREEQC and the rates calibrated to field observations.  Calibrated literature rates and rates calculated from Paces equation were compared.