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

Abstract:

The impact of chemical weathering on the global carbon cycle depends on the rate and style of weathering, especially in rapidly eroding mountain belts, where chemical weathering may be influenced by both physical erosion rate and lithology. To investigate the importance of lithology in relationships between denudation rate, chemical weathering pathways and CO₂ consumption or release, we turn to the eastern margin of the Tibetan Plateau. In this region, denudation rates vary over several orders of magnitude across an area with relatively little variation in mean annual precipitation or mean basin elevation. We combine water chemistry data with ¹⁰Be basin-wide denudation rates from 35 small basins that span two orders of magnitude in denudation rate and include three different lithologies – flysch metamorphosed to very low greenschist grade, high grade metamorphic rocks, and granitic rocks. We find that for all lithologies, the concentration of dissolved silicate weathering products is insensitive to denudation rate, but carbonate- and sulfuric acid driven weathering increase with increasing denudation rate. High grade metamorphic rocks have systematically higher proportions of weathering via sulfuric acid than other lithologies and. As a consequence, they show the steepest relationship between denudation rate and CO₂ release by sulfuric acid-driven weathering of carbonates. We suggest that the availability of sulfides driving this high proportion of sulfuric acid-driven weathering in metamorphic rocks may be related to the thermal reduction of sulfates at high temperatures during metamorphism. This suggestion implies that as orogenesis proceeds and deeper rocks become exposed, weathering may shift from consumption of CO₂ to its release.