The effect of lithology on the relationship between denudation rate and chemical weathering pathways – evidence from the eastern Tibetan Plateau

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 3 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. For a given denudation rate, we report dissolved solid concentrations and inferred weathering fluxes in catchments underlain by (meta)sedimentary rock that are 2–10 times higher compared to catchments containing granitoid lithologies, even though climatic and topographic parameters do not vary systematically between these catchments. Thus, varying proportions of exposed (meta)sedimentary and igneous rocks during orogenesis could lead to changes in the sequestration and release of CO2 that are independent of denudation rate.

Erosion and weathering in carbonate regions reveal climatic and tectonic drivers of carbonate landscape evolution

“…However, there are observations that suggest that this feedback does not occur in carbonates. Long-term weathering rates of carbonates in experiments were independent of grain size and texture (Levenson et al, 2015), which corroborates water chemistry measurements that suggest carbonate weathering is generally limited by water and acid availability (Bufe et al, 2022).…”

CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River: Effect of Erosion and Lithology on Chemical Weathering

“…On the contrary, the cations sourced from SSW ([Cat] SSW ) do not display a direct link with [SO 4 2− ] owp . The high [Cat] CO2SilW /([Cat] CO2SilW + [Cat] SSW ) ratios indicate that most of the generated sulfuric acid from OWP is buffered by carbonate minerals, which is expected in carbonate‐dominated terrains (Bufe et al., 2022). Because of the difference in reactivity among minerals and the position of weathering fronts, the reactive SulfCarbW is closely associated with OWP, by contrast, the less reactive SSW seems to be important only when the acid production exceeds the buffering capacity of carbonate (Bufe et al., 2021; Calmels et al., 2011; Torres et al., 2016, 2017).…”

“…Because the reactive carbonate and sulfide minerals have much fast reaction kinetics compared with silicate minerals (Blum et al., 1998; Calmels et al., 2007), carbonate and sulfide weathering are supposed to be under supply limitation across several orders of magnitude of erosion rates while silicate weathering is under kinetic limitation at rapidly‐eroding mountain belts (Bufe et al., 2021; Gu et al., 2020; Kemeny et al., 2021; West et al., 2005). Due to abundant minerals remaining in the regolith, the increased erosion may not always have a positive effect on the weathering under kinetic limitation (Bufe et al., 2021, 2022; Emberson et al., 2018; Gu et al., 2020). The higher erosion rate could lead to a high instantaneous weathering rate but also a thin regolith at the same time (Dixon & von Blanckenburg, 2012; Gabet & Mudd, 2009), indicating that the increase in silicate weathering rate could be offset by the decrease in regolith thickness.…”

Drivers of landscape evolution in eastern Tibet

“…The presence of this signal along the TR but not adjacent river basins (reflected by trends in steepness indices and disequilibrium at drainage divides) could result from a local tectonic driver affecting only the western branch of the Yangtze River, or from the diachronous arrival of a more regional signal due to differences in river network structure and/or rock resistance to erosion that occur downstream of our three studied river systems. Contrasts in rock resistance to erosion can slow down or accelerate the propagation of signals (e.g., Zondervan et al, 2020b), leading to different arrival times in adjacent basins characterized by different lithologies; such a scenario would appear plausible, as terranes downstream of the study area are known to be geologically diverse (e.g., Bufe et al, 2022;Roger et al, 2004).…”