Testing for supply-limited and kinetic-limited chemical erosion in field measurements of regolith production and chemical depletion

English Abstract:

Understanding the controls on silicate weathering rates is of wide interest because silicate weathering influences nutrient supply, landscape evolution, and Earth’s long-term climate. Some studies have suggested that silicate weathering rates are proportional to rates of mineral supply to the regolith, a condition known as supply-limited chemical erosion. Because this condition implies that silicate weathering rates should be strongly coupled to regolith production rates, and because regolith production rates are strongly coupled to rock uplift rates, this also implies that silicate weathering rates (and hence climate, via the carbon cycle) should be strongly coupled to tectonics over long timescales. Although theoretical considerations suggest that silicate weathering rates should scale with supply rates, it has been difficult to test whether chemical erosion rates are in fact supply-limited at any given field site. One obstacle is that the quantities that are most frequently used to test for supply limitation (i.e., rates of denudation and chemical erosion in regolith-based studies, and fluxes of sediment and solutes in river-based studies) are spuriously correlated, which complicates attempts to quantify effects of one variable on the other. Here we discuss a statistical method for testing for supply-limited chemical erosion, and we apply it to a number of published datasets. Our results suggest that chemical erosion rates in most of these datasets depend more strongly on supply rates than on dissolution kinetics, but they also suggest that the uncertainties in many datasets are too large to conclude whether regolith chemical erosion is or is not supply-limited. These analyses suggest that new measurements across a wide range of supply rates will be needed to quantify the strength of the coupling between chemical erosion and mineral supply, and, ultimately, to test hypotheses about the influence of tectonics on the long-term evolution of Earth’s climate.