Rock weathering tells a new story: more erosion does not always mean more carbon consumption
A study of the Lancang River shows that silicate, carbonate, and sulfide weathering respond differently to erosion
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All data are corrected for runoff using θsil = 0 and θsulf,carb = −0.75. Green indicates carbonate, yellow indicates silicate, and blue indicates sulfide. The dashed lines represent power-law fits to erosion sensitivity. Sites with strong secondary carbonate precipitation were excluded from the fitting. Data from other study areas are reported in Bufe et al. (2024).
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The uplift-weathering hypothesis suggests that tectonic uplift increases erosion, exposes fresh silicate minerals, and accelerates reactions that consume atmospheric CO2. This process has often been invoked as a driver of long-term cooling. However, erosion can also expose sulfide minerals. When sulfides oxidize, they produce sulfuric acid. This acid can dissolve carbonate minerals and release CO2 over geological time scales. The net carbon effect of weathering therefore depends on the balance among silicate weathering, carbonate weathering, and sulfide oxidation. However, the responses of different rock weathering processes to changes in erosion rate remain poorly understood.
To address this scientific issue, Dr. Jinke Liu and Prof. Guilin Han from China University of Geosciences (Beijing) conducted a detailed study of the Lancang River basin on the southeastern margin of the Tibetan Plateau. The team integrated river water chemistry, suspended geochemistry, stable isotope data, channel steepness indices, and erosion rate records. They further applied a convex optimization-based inverse mixing model to quantify the contributions of different sources to dissolved solutes in the river.
The results show that dissolved solutes in the Lancang River mainly come from carbonate and evaporite weathering, while sulfate is mainly derived from sulfide oxidation. After the influence of runoff was removed, different weathering processes showed contrasting responses to erosion. As erosion increased, silicate weathering rates decreased, whereas carbonate weathering and sulfide oxidation rates increased.
The study also found evidence for secondary calcite precipitation in river water, based on calcium isotopes in suspended sediment and river water. Changes in the chemical composition of suspended sediment suggest that silicate minerals may continue to dissolve during downstream transport. Lithium isotopes, hydrogen and oxygen isotopes, and water chemistry further indicate that non-geothermal groundwater contributes substantial amounts of dissolved solutes to the river. Together, these processes can modify the link between erosion rate and chemical weathering at the basin scale. They may also help explain why weathering rates respond differently to erosion in different river basins.
On geological time scales, CO2 released by sulfide oxidation coupled to carbonate weathering in the Lancang River basin exceeds the CO2 consumed by silicate weathering. Downstream in the lower Mekong River plain, where erosion rates are lower and lithology changes, the carbon effect of weathering shifts from a CO2 source to a CO2 sink. Comparison of the Lancang River basin with small silicate-dominated catchments in orogenic belts shows that the Lancang River basin has a moderate erosion rate but still acts as a carbon source. The study suggests that this phenomenon is closely related to differences in basin lithology, thereby complementing and deepening previous understanding of the relationship between erosion rate and carbon effects.
The findings provide new geochemical evidence that lithology, suspended sediment weathering, and groundwater input are important controls on chemical weathering and its carbon effect in large river basins. They also refine how scientists evaluate the relationship among tectonic erosion, chemical weathering, and climate change.
Scaling between Ca/Na in suspended load and calcite saturation index (A), riverine Ca and carbonate cation concentrations (B), and Ca lost to precipitation (C). The concentration of Ca lost to precipitation is calculated from the fraction of Ca remaining in the water (β) and riverine Ca concentration.
The circles represent the Lancang River, while diamonds represent small mountain streams. The colored symbols are calculated with runoff correction θsil =0 and θsulf,carb = −0.75 while the gray symbols are calculated without runoff correction. For most data points, the application of runoff correction does not change the qualitative determination of net CO2 source or sink status. The data for the whole Mekong River basin and other mountain rivers are reported in Bufe et al. (2024) and Relph et al. (2021).
Credit
©Science China Press
©Science China Press
Journal
Science China Earth Sciences
Method of Research
Observational study
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