SURFACE WATER DISSOLVED LOADS AND SOIL CHEMICAL WEATHERING RATES: RESULTS FROM THE SANTA CRUZ TERRACES

Efforts to quantify rates of global silicate weathering and related feedbacks to global climate systems often rely on dissolved loads in surface waters for estimates of basin weathering fluxes. This approach can utilize extensive historical data, minimize sampling, and simplify accounting for complicating factors such as mixed basin lithology. However, this approach does not allow identification of solute sources within the watershed, nor does it capture fluxes in cases where weathering products are retained within the basin, as might happen where evapotranspiration rates are high. Basin flow paths and rainfall-runoff response must be understood to answer these questions. A failure to address these processes could lead to an incorrect understanding of basin scale weathering and global geochemical systems.

This study builds on an existing characterization of soil silicate weathering along a chronosequence of marine terraces near Santa Cruz (CA, USA). The Wilder Creek watershed drains these terraces, is underlain by a mixed bedrock lithology, and is subject to high rates of evapotranspiration. Soil water chemistry data and stream dissolved load based weathering rates from a series of nested watersheds in the Wilder basin indicate that basin-scale weathering rates are <20% of measured contemporary and long-term weathering rates determined for terrace soils. The soil data is representative of about 40% of the watershed area, with actively eroding, moderate to steep hillslopes making up the remainder. The discrepancy in basin and soil weathering fluxes suggests that additional factors need to be considered: saturation state inhibition of weathering within ground water, efflux during large periodic events associated with El Niño circulation patterns, the "flashiness" of the hydrograph, water residence times in different components of the system, and retention of weathering products within the soil/saprolite. Identifying the reasons for the discrepancy between soil and stream dissolved load weathering fluxes is needed for a full understanding of chemical weathering at landscape scales.