Mathematics – Logic
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.3645l&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 17, p. 3645-3669.
Mathematics
Logic
28
Scientific paper
Dissolved and suspended loads of the main streams on the island of Réunion have been analysed for their major and trace element contents in order to characterise both chemical and mechanical erosion products. The chemical composition of Réunion surface waters is controlled by partial dissolution of the basaltic rocks they interact with, by atmospheric input and, in some cases, by thermal spring inflow. The elemental contributions arising from these three processes have been calculated using typical concentration ratios for each endmember. The contribution from oceanic type rains is important for Cl and Na and minor for Mg, Ca, K, and Sr. Some thermal springs join their nearest rivers in Piton des Neiges calderas and influence the sulphate, Li, and B concentrations, and to a lesser extent Ca, Sr, and Rb. For all the rivers that are not affected by the thermal springs, the contribution from basalt weathering is higher than 70% of the total dissolved solids. The chemical composition of the suspended load in Réunion rivers reflects a low weathering state for basalt in each drainage basin, implying that mechanical erosion is more important than chemical erosion. Assuming a steady-state between these two complementary erosion processes, we propose a mass budget between the dissolved and suspended erosion products and the local unweathered basalt for each of the main river catchments. Given the lack of reliable long-term measurements of river suspended load at Réunion, this mass budget enables us to calculate the suspended load concentration that is required to counterbalance the concentration of total dissolved solids due to chemical weathering. The calculated suspended load concentration is much higher than that measured which implies that equilibrium between mechanical and chemical erosion was not attained at the time of sampling. However, we suggest that in such a tropical climate, most of the suspended load will be carried during cyclonic events, these short periods of time providing about 50% of the annual amount of river water. Thus, erosional steady-state must be achieved on the scale of the annual hydrologic cycle. From the total dissolved solid concentrations induced by basalt weathering, and given the annual discharge from Réunion rivers, we infer specific chemical erosion rates of 63 170 t/km2/yr and specific atmospheric CO2 consumption rates, during basalt erosion, of 1.3 4.4 × 106 mol/km2/yr. Mechanical erosion rates deduced from the calculated suspended load concentrations range from 1200 to 9100 t/ km2/yr. Total erosion (chemical plus mechanical) give basalt denudation rates of 470 3430 mm/kyr. All these erosion rates (chemical, mechanical, total, and atmospheric CO2 consumption) are among the highest global estimates and are due to high runoff, steep slopes, active volcanics, and related active tectonics, and young basaltic lithologies, which characterise Réunion.
Jean Allègre Claude
Louvat Pascale
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