We have estimated the erosion rates in Riviere Langevin and Riviere de l'Est, and in the three cirques, Cilaos, Mafate and Salazie on Reunion Island. This island represents a highly carved volcanic environment due to caldeira formation and extense erosional processes caused by high rainfalls (up to 10,000 mm/year and more). Erosion rates have been estimated using two completely different methods.
A) Surface exposure ages were determined by measuring cosmogenic 3He in olivines separates from river sand (e.g. Staudacher and Allègre, 1993). B) Mass balance between dissolved and particulate loads of river water samples was used to infer erosional fluxes (e.g. Gaillardet et al., 1995).
Olivine bearing sands from the outlet of the three
cirques of Piton des Neiges ancient volcano, and from Riviere Langevin, best represent the average of the eroded surfaces of the cirques and the river valley. Olivine separates were prepared and their noble gases analysed mass
spectrometrically by ARESOBO II. All samples show low noble gas concentrations. 3He vary between 0.24 and 0.68x10-12 cm3STP/g. 4He/3He isotopic ratios range between 36,270 and 54,240. Assuming a normal 4He/3He isotopic ratio for Reunion Island basalts of 56,010 ± 800 (Staudacher and Allègre, 1993; Graham et al., 1990), we calculate cosmogenic 3He concentrations of 1.2 to 10x10-14 cm3/g. As the 3He production rate depends on altitude, we estimated for all sites mean altitudes of the olivine basalts based on a geological map. Assuming that erosion is in steady state, we get e=1/(mTex) (Lal, 1991), where e is the erosion rate, 1/µ
is the absorption coefficient of cosmic rays neutrons of
165 g/cm2 in rocks, and Tex is the 3He exposure age. Exposure ages range from 4000 to 9780 years and lead us to erosion rates of 67 to 460 mm/ky.
Waters from Riviere du Mat, Bras de Cilaos and Riviere des Galets rivers, draining the three cirques of Reunion Island, and Riviere de l'Est river have been sampled in February 1995. The samples were filtered and the particulate and dissolved loads analysed for major and trace element concentrations by HPLC and ICP-MS. The unprecise and unrepresentative field measurement of river suspended sediment concentrations constrained us to recalculate the suspended loads required to balance the dissolved ones (corrected from atmospheric and hydrothermal inputs (Louvat and Allègre, 1996), assuming that the erosion equilibrium is attained. For the most soluble elements (Na, Ca, Mg, K, Sr, Rb, Ba and U) concentrations are higher in the soluble phase and lower in the particulate one, comparing the normalized extended trace elements patterns of those two phases with those of typical basalts of Reunion Island. Making a mass balance between the unaltered basaltic rock and the river erosion products (particulate and dissolved loads), we calculate suspended sediments concentrations of 100 to 500 mg/l. Given the well known hydrologic and geographic properties of the different watersheds, we infer erosion rates of 150 to 500 mm/ky.
For the first time, it was possible to compare estimated erosion rates based on two completely different methods and we observe globally a quite good agreement between the two results. The rates estimated by rivers geochemistry are 2 to 3 times those determined by cosmogenic 3He in olivines. As for both methods accumulated errors are important, some analyses have to be precised and hypothesis discussed. Furthermore, it is worth pointing out that the watersheds with the highest erosion rates also have the highest runoffs. The summit caldeira formation of Piton des Neiges volcano started around 150 ky ago and erosion rates estimated here, although high, cannot explain the large depressions of more than 1000 m. This underlines the importance of tectonic processes such as caldeira formation in the shaping of Reunion landscape.
Gaillardet, J., Dupré, B. & Allegre, C. J., Geochim. Cosmochim. Acta 59, 3469-3485 (1995).
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Lal, D., Earth Planet. Sci. Lett. 104, 424-439 (1991).
Louvat, P. & Allegre, C.J., J. Conf. Abs. 1, 372 (1996).
Staudacher, T. & Allegre, C. J., Earth Planet. Sci. Lett. 119, 395-404 (1993).