Spatial Variation of Chemical Weathering Rates in the Washington Cascades

Chemical weathering of silicate rocks acts as a net sink for atmospheric CO2 in the long-term global carbon cycle, and recent work suggests that both climate and tectonics is important in regulating long-term chemical weathering rates. We investigated the temporal and spatial variations of chemical fluxes across the Washington Cascades, where climate and tectonics spatially vary. In particular, long-term exhumation rates vary from 0.02 to 0.33 mm/yr and precipitation varies from 200 to 3500 mm/yr across the range. Previous studies in the western Cascades show that chemical weathering rates increase from west to east and are well correlated with long-term exhumation rates. We revisited sites of the western Cascades to provide a longer time-series of measurements, and established sites to the east where dryer portions of the range provide a semi-arid end-point for understanding climatic impacts on chemical weathering. We measured the dissolved major elements and stream discharge of fifteen catchments along the Skykomish in Washington Cascades; additionally, we measured major elements and stream discharge within eight catchments draining from the eastern Cascades which are more arid and tectonically less active compared to the western Cascades. We found that the dissolved silica fluxes in the western Cascades are ~ 80 (40-120) x 103 mol/km2 /yr, which are ~ 40 % higher than arid sites to the east. These higher Si fluxes are apparently due to the higher runoff in western Cascades, as major element concentrations do not significantly differ across the range. The maximum annual variation of Si fluxes is a factor of two in the western Cascades and it is mostly due to variation in stream discharges; stream discharges of sites differ 30 (6-60) % from previous studies while chemical data differ only 5 (3-30) %, excepting a single site. A more detailed time-series of major elemental fluxes and estimates of basin-wide erosion rates using cosmogenic radionuclide exposure ages will help to elucidate the controls that precipitation and erosion exert on dissolved load in these rivers.