Quantitative Feedback Between the Rates of Soil Chemical Weathering and Colluvial Sediment Transport along a Hillslope Transect

The notion that chemical weathering on the Earth's surface is limited by mineral supply underlies the hypothesis that accelerated tectonic uplift leads to accelerated chemical weathering. To a soil on a hillslope, however, mineral supply operates via two physical mechanisms; in-situ soil production from the underlying saprolite or bedrock and sediment input from upslope. Thus, we hypothesize that soil chemical weathering rates systematically vary along a hillslope transect in response to the topography-dependent variations in the rates of soil production and colluvial transport. Likewise, as the physical processes change over time, so does soil chemical weathering rates. We have combined two successful mass balance models from the fields of geochemistry and geomorphology and merged the new model with empirical data from a grass covered hillslope transect in Coastal California. The results support the hypotheses. In the studied hillslope with greywacke bedrock, the formation of colluvial soils in the upslope is largely mediated via physical breakdown of in-situ saprolite. However, the soils in the lower depositional slope are geochemically decoupled from the in-situ saprolite. As the colluvial soils move downslope, they are chemically weathered, and the strength of the feedback between the colluvial sediment transport and soil chemical weathering is an order of magnitude larger than that between the soil production and chemical weathering in the lower part of the hillslope. Furthermore, we suggest that the past transient history of colluvial sediment flux may leave significant geochemical signature along the studied hillslope transect.