ESTIMATING EROSION RATES IN REUNION ISLAND: TIME SCALES, WEATHERING AND SEDIMENT TRANSPORT.

Abstract:

Understanding the mechanisms that modify landscapes is essential for
identifying feedbacks between climate, tectonic and
topography. Because measurements of erosion rates are critical for
quantifying landform evolution, the coupling of different techniques
has often been used. However, different methods often estimate erosion
rates over different time scales, and are sensitive to different erosion
processes.
In this study we estimate erosion rates of three highly eroding
drainage areas of Reunion Island. We compare three methods: i) from cosmogenic 3He concentrations, ii) from river
geochemistry and iii) from landforms reconstruction. Our aim is to derive
the message provided by each method in terms of chemical weathering and mechanical erosion.
Helium concentrations and isotopic ratios were measured in
olivine-rich sands from the Langevin and Remparts rivers, and from
landslide products. Digital elevation model derivatives and K-Ar
geochronological data were used to reconstruct basins initial
topographies and to calculate the volumes of material eroded over the
past ~65Ka. Finally, dissolved loads, suspended loads and river
bottom sediments were analyzed for their major and trace elements contents, and a geochemical mass balance was built in
order to quantify both chemical and mechanical erosion rates.
Results show a good agreement between long-term erosion rates derived
from initial topography reconstructions and so called short-term
erosion rates from the geochemical mass balance analyses of dissolved and
suspended load. The cosmogenic method largely underestimates erosion rates, but comparison with the geochemical mass balance shows that episodic landslides dominate erosion of the basins.
Finally a new approach of the geochemical mass balance with a systematic
study along the range of river sediment grain size allows to depict weathering vs genesis and transport of sediments.