Enhanced weathering by multiscale destabilization of volcanic islands

Réunion Island, a volcanic basaltic island situated in the Indian Ocean, is a well-known hotspot of weathering and erosion with particularly intense bedrock landslides. We estimated the contribution of large landslides to the dissolved load in the main rivers across Réunion Island using a mixing model, suggesting a significant enhancement of chemical weathering associated with landslides. Landslides can contribute to a decadal chemical weathering flux of up to 69 t/km²/yr, accounting for 82% of the weathering flux in one of Réunion's basins. Overall, chemical weathering in Réunion Island is predominantly driven by landslides, accounting for approximately 60% of the weathering flux. Notably, landslide-associated weathering can be promoted by intense precipitation events. We propose that large landslides on Réunion Island favor the exposure of both fresh and hydrothermal minerals to water, thereby promoting chemical weathering. Since these hydrothermal minerals were formed by fluid-rock reactions at an earlier stage of the volcano's evolution, our results highlight the cannibalistic nature of weathering in Réunion Island. As a consequence, river chemistry tends to overestimate present-day chemical weathering rates and associated ${\mathrm{CO}}_2$ consumption in Réunion Island. We expect this conclusion to hold true for other volcanic settings as well. Further work on the weathering of basaltic rocks based on river chemistry should therefore take into account the geomorphological evolution of volcanic edifices, the only truly integrated approach to assess the climatic impact of the emplacement of volcanic rocks on Earth.