Erosional modulation of the balance between alkalinity and acid generation from rock weathering

Abstract

<p>The balance between alkalinity generation by carbonate and silicate weathering and <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/sulphuric-acid" title="Learn more about sulfuric acid from ScienceDirect's AI-generated Topic Pages">sulfuric acid</a> generation by sulfide weathering controls the effect of weathering on atmospheric <em>p</em>CO₂ over geologic timescales. How this balance varies across <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/environmental-gradient" title="Learn more about environmental gradients from ScienceDirect's AI-generated Topic Pages">environmental gradients</a> remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/headwater" title="Learn more about Headwater from ScienceDirect's AI-generated Topic Pages">Headwater</a> Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ³⁴S_SO₄, δ¹⁸O_SO₄, and δ¹⁸O_H₂O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/evaporite" title="Learn more about evaporites from ScienceDirect's AI-generated Topic Pages">evaporites</a>, and sulfides to the total weathering budget. Across the TRHR, riverine SO₄^2– is derived mainly from a mixture of an evaporite source with uniform values of δ³⁴S_SO₄ and δ¹⁸O_SO₄, and a sulfide source that contributes highly variable values of δ³⁴S (−12.2 ‰ to +4.1 ‰) and δ¹⁸O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO₄^2– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO₂ over timescales shorter than marine carbonate burial (∼10⁴ years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼10⁷ years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric <em>p</em>CO₂, are largely dependent on <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/petrography" title="Learn more about lithology from ScienceDirect's AI-generated Topic Pages">lithology</a>. However, within comparable lithologic settings, sulfide and carbonate <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/weathering-rate" title="Learn more about weathering rates from ScienceDirect's AI-generated Topic Pages">weathering rates</a> rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO₂ with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term <a href="https://www-sciencedirect-com.eproxy.lib.hku.hk/topics/earth-and-planetary-sciences/carbon-cycle" title="Learn more about carbon cycle from ScienceDirect's AI-generated Topic Pages">carbon cycle</a> during mountain building.<br></p>