Is groundwater weathering in regions of high topographic relief important?
insights from surface heat flow and a model linking chemical weathering to soil formation, creep and mass wasting

Mark Gabriel Little* and Cin-Ty Aeolus Lee

Abstract

The magnitude and nature of chemical weathering is fundamental to understanding CO2 drawdown and the marine budget of elements of paleo-oceanographic significance. The relationship between chemical weathering and erosion is unclear because chemical weathering, occurring during soil formation on hillslopes, is a quasi-continuous process but erosion can be episodic, especially in rapidly uplifting regions where mass wasting is undoubtedly the dominate form of physical erosion. Here the link between chemical weathering associated with soil formation and erosion associated with mass wasting (dissolution of suspended load and river bedload) is investigated. It is assumed from the outset that landform evolution in regions of high topographic relief is controlled by a cyclic process in which a soil mantle gradually forms on a hillslope, but when it reaches a critical thickness such that the basal shear stress exceeds the Coulomb-failure criterion, mass wasting occurs. Averaged over 10+ kyrs, the periodicity (0.5-10 kyr for steep slopes) of mass wasting is controlled by the rate of soil formation. These results are combined with simple, empirically constrained dissolution models to predict the ratio of suspended to dissolved load in rivers. The predicted ratios, however, are found to be higher than observed in rivers, the discrepancy worsening with increasing topographic relief. This discrepancy arises from the fact that in regions of high relief, physical erosion rates (mass wasting) are so high that soil mantles do not reside on hillslopes long enough to allow for significant chemical weathering. This conclusion is at odds with the widely held view that tectonic uplift, erosion and chemical weathering are intimately linked. Thus, our model is either wrong or enhanced physical erosion does not directly equate with enhanced chemical weathering. In order to make up for this discrepancy, a component rich in dissolved load compared to suspended load is required. One possibility is groundwater weathering. Indeed, apparent surface heat flows measured in mountainous regions (Cascades, Sierra Nevada, and Peninsular Ranges in western North America), are anomalously low compared to what is expected from the young tectonothermal ages of these regions. These low values can be explained as an artifact of a strong downward or lateral advective component associated with groundwater flow, which itself is replenished by precipitation at high elevations and driven by hydraulic head imposed by the topography. We suggest that it may be worth entertaining the possibility that groundwater flow in mountainous regions may be an important component of solute and heat transport. If so, it may also be worth considering the role of groundwater weathering in transporting solutes directly into the marine environment.

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