Investigating the influence of climate and volcanic surface aging on erosion: a case study of Réunion Island, Indian Ocean
Résumé
La Reunion, a tropical volcanic island in the southwestern Indian Ocean, represents an ideal natural laboratory to analyze the effects of volcanic surface age and climate on erosion. The island has uniform lithology, geologically-dated volcanic surfaces from its construction over the past 2 Ma, and experiences no tectonic influence. Furthermore, the island has significant spatial and temporal rainfall gradients. Rainfall varies over several orders of magnitude between the leeward (~0.01 m/yr) and windward (~10 m/yr) flanks of the island, spatially-overlapping with surface age gradients. However, yearly cyclones and large storms create highly-variable and extreme rainfall events over the entire island. We investigate the relationships between catchment aging, rainfall, erosion, and morphology. To do so, we first collect basin morphometrics across the island, including Hack’s Law exponent, drainage density, hypsometry, roughness, and channel concavity. We then combine these with mean rainfall, rainfall variability, and channel discharge variability from records over the last 60 years, as well as channel grain size distributions collected before and after the 2024 cyclone Belal. Afterwards, we reconstruct pre-eroded edifice flanks, and pair these with radiometric ages of the paleosurfaces, which we consider as representing the time since the onset of erosion, to estimate basin-averaged erosion rates. We conduct a series of correlation analyses between all datasets. Our results highlight the intrinsic nature of precipitation and age on erosion, while also demonstrating the varying influences these signals have on morphology. We find that basin eroded volumes are most-correlated with basin age, followed by rainfall variability and mean rainfall. We also find that separating cyclone and non-cyclone rainfall has little influence on correlations compared to the combined record. Furthermore, we show that basin morphologies have varying degrees of correlation, or no correlation, between age, mean rainfall, and rainfall variability. Finally, we discuss the effect of cyclonic rainfall on grain size distributions and its implication for the influence of extreme events on the long-term erosion record.
