HR: 1340h
AN: V23A-1241    [Abstracts]
TI: Tropical volcanic islands: best first cycle erosion laboratories.
AU: * RAD, S
EM: rad@ipgp.jussieu.fr
AF: IPGP-Laboratoire de Géochimie et Cosmochimie, 4, place Jussieu, Paris, 75252, France
AU: RIVE, K
EM: rive@ipgp.jussieu.fr
AF: IPGP-Laboratoire des isotopes stables, 4, place Jussieu, Paris, 75252, France
AU: GAILLARDET, J
EM: gaillardet@ipgp.jussieu.fr
AF: IPGP-Laboratoire de Géochimie et Cosmochimie, 4, place Jussieu, Paris, 75252, France
AU: ALLEGRE, C J
EM: allegre@ipgp.jussieu.fr
AF: IPGP-Laboratoire de Géochimie et Cosmochimie, 4, place Jussieu, Paris, 75252, France
AB: Lesser Antilles islands are located in a tropical climate with high temperatures (24 to 28 ° C), high precipitation (can reach 12 m/yr), very dense vegetation, sharp relief with a NS gradient of age. Antilles rivers have a torrential hydrologic regime with extreme erosion conditions. Chemical weathering rates are among highest world values 140-200 t/km²/yr on surface and 2 to 5 time higher when subsurface water circulations are taken account (Rad et al., 2007). Timescales and physical rates of erosion are calculated from U-Th isotopic compositions. Our results, with 3 different methods (all based on mass budgets between the river bedrock and its erosion products) show a good agreement for physical erosion rates with a maximum value of 2500 t/km²/yr. We show that solid loads of these volcanic rivers are dominated by sand, which represent more than 80% of the soil profile. Erosion rates are directly correlated to the age of basins. Indeed, among all parameters (climat, runoff, slopes, vegetation…) the age basins is a key control parameter. The younger the basin is the higher the weathering rate is. This correlation asserts that younger volcanic rocks are more easily weathered than old ones: young fresh material is easily mobilized by erosion, while for older rocks with thick soil covers, chemical and physical erosion rates are much lower, they reach a threshold with low chemical and physical rates once soils are constituted. It seems that rivers draining young lava flows are also the ones, which are not at steady state with the lowest mechanical denudation rates. Erosion processes in Lesser Antilles are typical of a first cycle erosion with simple relationship between rates, timescale of erosion and basins ages. It appears that first stage of erosion are characterized by high chemical denudation rates and low physical denudation rates, the erosion products are then close to the bedrock. It is then followed in a second stage by constant chemical weathering rates with higher mechanical denudation rates. \small Setareh Denise Rad, Claude Jean Allegre and Pascale Louvat, 2007, Hidden erosion on volcanic islands, Earth and Planetary Science Letters. In Press.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1815 Erosion
DE: 1838 Infiltration
DE: 1865 Soils (0486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting