Enhanced CO2 consumption rate from silicate weathering by landslide erosion in the volcanic island

Abstract

Silicate weathering plays an important role in sequestering CO2 over geological time scales. Physical erosion is an important process of mineral surface production, significantly promoting efficient chemical weathering. Landslides, in particular, contribute to physical erosion by generating debris avalanches, thereby accelerating the chemical weathering rate. On the one hand, this enhanced silicate weathering contributes to CO2 drawdown. On the other hand, the oxidation of sulfide minerals exposed by landslides produces H2SO4. H2SO4 weathers carbonate minerals and releases CO2 to the atmosphere, a faster weathering process than silicate dissolution. It is a consensus that landslide erosion favors chemical weathering, however, it still remains unclear to what extent it impacts chemical weathering fluxes, and resultant CO2 consumption rate or emission rate. Réunion Island, characterized by volcanic basalt composition, is a well-known hotspot of physical and chemical erosion (Louvat and Allègre, 1997) with particularly intense bedrock landslides and river incision (Garcin et al., 2005; Rault et al., 2022). It is a very high standing volcanic island with erosion rates exceeding most active mountain ranges due to the strong interaction between volcanic rocks and climate (Gayer et al., 2019). These characteristics of Réunion Island make it an excellent natural lab to study the relationships between erosion and weathering. In this study, we use stream water chemistry and discharge time series to calculate the decennial chemical weathering rates of the main catchments across Réunion Island. Our analysis unveils a substantial contribution not only from basalt dissolution in the stable area but also from hydrothermal activity and landslides to the chemical weathering flux, which results in high CO2 consumption rates. Notably, streams impacted by thermal springs and landslides show different relationships between runoff and chemical weathering rates. In addition, extreme precipitation events promote landslide weathering, instead of high average rainfall. We were able to quantify the effect of landslides on chemical weathering. For the Salazie basin, we find that the landslides contribute to chemical weathering rates up to 62 t/km²/a, accounting for 73% of the chemical weathering in the basin although landslides only affect about 1/5 of its total surface area. This corresponds to an annual CO2 consumption rate of 2.9 × 106 mol/km²/a, approximately 4 times higher than the CO2 consumption attributed to basalt weathering in landslide-free nearby areas, establishing landslide-enhanced-weathering as a significant carbon sink. Our study illuminates some of the mechanisms coupling physical and chemical weathering processes at the Earth’s surface and the impact on the climate.