The effect of lithology on the relationship between denudation rate and chemical weathering pathways

Abstract

Abstract. The denudation of rocks in mountain belts exposes a range of fresh minerals to the surface of the Earth that are chemically weathered by acidic and oxygenated fluids. The impact of the resulting coupling between denudation and weathering rates fundamentally depends on the types of minerals that are weathering. Whereas silicate weathering sequesters CO2, the combination of sulfide oxidation and carbonate dissolution emits CO2 to the atmosphere. Here, we combine the concentrations of dissolved major elements in stream waters with 10Be basin-wide denudation rates from 35 small catchments in eastern Tibet to elucidate the importance of lithology in modulating the relationships between denudation rate, chemical weathering pathways, and CO2 consumption or release. Our catchments span three orders of magnitude in denudation rate in low-grade flysch, high grade metapelites, and granitoid rocks. For each stream, we estimate the concentrations of solutes sourced from silicate weathering, carbonate dissolution, and sulfide oxidation using a mixing model. We find that for all lithologies, cation concentrations from silicate weathering are largely independent of denudation rate, but solute concentrations from carbonates and, where present, sulfides increase with increasing denudation rate. With increasing denudation rates, weathering may, therefore, shift from consuming to releasing CO2 in both (meta)sedimentary and granitoid lithologies. We find that catchments draining high grade metamorphic rocks have systematically higher concentrations of sulfate from sulfide weathering than catchments containing weakly metamorphosed sediments. Moreover, our data provide tentative evidence that sulfate concentrations in these catchments are potentially more sensitive to denudation rate. We propose that changes in the sulfur oxidation state during prograde metamorphism of pelites in the mid-crust could lead to sulfate reduction that is even more complete than in low-grade sediments and provides a larger sulfide source for oxidation upon re-exposure of the rocks. In this case, the elevated concentration of sulfate in catchments draining high-grade metapelites would suggest that exposure of an increasing fraction of metamorphic rocks during orogenesis could lead to a boost in the release of CO2 that is independent of denudation rate.

Detailed Chemical Analysis of Unusually Weathered Rocks

“…The breakdown of rocks into small pieces due to tectonic activity, freezethaw cycles, root expansion, and wet-dry cycles are referred to as physical weathering (5) . Lithophilous lichens, fungi, and bacteria induce the biological weathering process (7) while chemical weathering results from acidification, dissolution, and/or oxidation (8) . Chemical weathering takes a longer period compared to biological weathering but does occur in almost all types of rock and the mechanical strength of rocks significantly decreases when the degree of chemical weathering increases (9) .…”

“…Chemical weathering takes a longer period compared to biological weathering but does occur in almost all types of rock and the mechanical strength of rocks significantly decreases when the degree of chemical weathering increases (9) . The rate of rock weathering depends on several variables, for instance, parent rock type, size, climate, biological activity, topography and the chemical nature of interacting water (8,10) , including ionic potential and temperature (11) (12) . Chemical weathering can lead to changes in the mineralogical composition of rocks, forming Kaolinite, Vermiculite, Smectite, Montmorillonite, and Chlorite like minerals.…”