Cosmogenic Nuclides, River Geochemistry, and Lanforms Reconstruction Methods Comparison to Study the Coopling Between Chemical Weahering and Mechanical Erosion in a Steep Reunion Island Basin.

Abstract:

Understanding mechanisms that modify landscapes is essential for risk assessment in tropical islands. Because measurements of erosion rates are critical for understanding landform evolution, the use of cosmogenic isotopes in river sediments and the use of river load geochemistry have grown rapidly in recent years.
In this study we aim to estimate erosion rate of an highly eroded drainage area of the Réunion Island and we compare 3 methods of measurment : i) from cosmogenic ³He concentrations [³He_c], ii) from river geochemistry and iii) from landforms reconstruction in order to estimate chemical weathering, mechanical erosion, and to understand the message provided by each method.
The basin under invistagation is the Rivière Langevin Canyon, located on the south flank of the Piton de la Fournaise Volcano (PdF). Helium concentrations and isotopic ratios have been measured in olivine rich sands from the Langevin river. [³He_c] have been calculated using: (i) the ³He/⁴He ratio measured by crushing and (ii) the ³He and ⁴He concentrations measured by melting the resulting powder. Digital elevation model derivatives and K-Ar geochronological data have been used to reconstruct the PdF morphology and calculate the volume of material eroded over the past 65Ka. Finally, dissolved and suspended loads in Langevin river will be analysed for their major and trace elements contents in order to characterise both chemical and mechanical erosion products. A soil will also be analysed in order to estimate the chemical weathering of the bedrock and to understand soil formation in the studied basin. Initial results indicate ³He_c average erosion rates of 4.9±11.6 mm/yr and 4.7±0.1 mm/yr from PdF morphology reconstruction. Cosmogenic and landform reconstruction results show a reasonable agreement. However, the lack of olivine in the sampled soil rises the question of river sediment olivine sources. Such olivine need to be delivered either by deep-seated catastrophic events either from no-soil sources. In this context, ³He_c in olivine rich sands can be used as a tool for mechanical erosion estimates. Comparison with river geochemistry results in the Langevin river will help understand the coopling between chemical weathering and mechanical erosion and will confirm if olivine can be used as a proxy for mechanical erosion estimation.