River incision on hotspot volcanoes: insights from paleotopographic reconstructions and numerical modelling

The influence of climate on landscape evolution in natural settings remains debated. Here, we focus on tropical hotspot volcanic islands because they exhibit relatively uniform lithology and experience significant precipitation and climate gradients. Furthermore, intermittent volcanic flows effectively “reset” the landscape that begins to evolve post-eruption. Thus, we can constrain initial conditions by reconstructing the initial geometry of radiometrically dated volcanic flows. We constrain landscape evolution through time in several volcanic islands with strong climate gradients to assess the role of climate on incision. We perform topographic reconstructions to calculate long-term basin-averaged erosion rates in two islands of the Réunion hotspot (Réunion, Mauritius) and compile published erosion rates on Réunion and Kaua’i (Hawaii hotspot). We define the time since incision started as the age of the surface incised lava flow. To calibrate incision parameters on all three islands, we use the stream power model and apply a data-driven Bayesian approach to obtain the erodibility, K, the drainage area exponent, m, and the slope exponent, n. We also calculate a normalized erodibility index, K_n, using n = 1 to directly compare results among the different study sites. Erosion rates of Réunion Island range from 9.9 ± 0.5 mm/yr to 5.2 x 10^-3 ± 2 x 10^-4 mm/yr and erosion rates in Mauritius Island range from 6.5 x 10^-2 ± 8 x 10^-3 mm/yr to 5.1 x 10^-3 ± 4 x 10^-4 mm/yr. Incision efficiency seems to decrease with time since incision started from 63 ka to ~300 ka and then does not vary significantly with time since incision started from ~300 ka to 4300 ka. This is likely due to the covariation between the age of volcanism repaving and precipitation rates on Réunion, which is related to the configuration of the island’s two volcanoes – the active Piton de la Fournaise located on the windward side and the dormant Piton des Neiges on the center and leeward side. Our empirical calibration of the stream power law shows high dispersion in n and K_n on each individual island. m ranges from 0.2 to 2.9, and K_n ranges from 2.3 x 10^-7 to 9.8 x 10^-4 m^1-2m/yr. For Réunion, we identify a positive trend between mean annual precipitation and erosion rates, and between mean annual precipitation and K_n, for low to moderate erosion rates (<1 mm/yr). For all basins of Réunion, there is also a positive trend between mean annual cyclonic precipitation rates and erosion rates, and between mean annual cyclonic precipitation rates and n. In Kaua’i, there is a positive trend between erosion rates and mean annual precipitation, consistent with previous studies. In Réunion, the proportionality coefficient between erosion and mean annual precipitation is three times greater than in Kaua’i. In addition, considering all three islands, a nonlinear relationship exists between channel slope and incision rate: best-fit n values range from 0.5 to 6, with n generally lower than one on Kaua’i. Our results highlight different sensitivities of fluvial relief to incision, and of incision to climate, between Kaua’i and Réunion islands.