HR: 1340h
AN: PP43B-0682    [Abstracts]
TI: The Fundamental Importance of the ''Hidden'' Source of Chemical Erosion in Island Arcs : Guadeloupe and Martinique
AU: * RAD, S
EM: rad@ipgp.jussieu.fr
AF: IPGP, Paris 7, Laboratoire de Geochimie et Cosmochimie, 4, place jussieu, paris, 75252 France
AU: LOUVAT, P
EM: louvat@ipgp.jussieu.fr
AF: IPGP, Paris 7, Laboratoire de Geochimie et Cosmochimie, 4, place jussieu, paris, 75252 France
AU: Allegre, C
EM: allegre@ipgp.jussieu.fr
AF: IPGP, Paris 7, Laboratoire de Geochimie et Cosmochimie, 4, place jussieu, paris, 75252 France
AB: The chemical erosion of island arcs is of fundamental importance for the erosion regime of the earth and fixing the chemical and isotopic composition of the ocean. In previous work, specially Louvat et Allegre (1997, 1998) it has been shown that erosion of basalt and andesit are in the average 10 to 20 times more efficient than granitic type one. In a comprehensive study Dessert et al.(2003) concluded that basalt erosion contributes to 30 to 35 % of the atmospheric CO₂ consumed by worldwide weathering. However since this time our group has surmised that island arcs contribution as estimated by the river weathering has been greatly underestimated. The source of error is supposed to be the contribution of water that circulates in island arc underground and goes directly to the ocean. We have then studied two islands of Lesser Antilles: Guadeloupe and Martinique. Guadeloupe and Martinique are characterised by a uniform andesitic lithology. They are located in a tropical climate with high temperature (24 to 28 ° C), high precipitation (they can reach 14000 mm/year), very dense vegetation, sharp relief and very thick soils. On parts of these islands, especially in Martinique, pyroclastic formations are dominant, creating a very porous surface. This allows water transfer through infiltration in more important proportions than through runoff. The proportion of this infiltrated water is 30% to 90% of the precipitation (Folio, 2001) in Reunion, which has a similar torrential regime. From hydrological budget and the chemical composition of underground waters we calculate the chemical weathering rates of infiltrated water: 280 t/km²/year for Guadeloupe and 190 t/km²/year for Martinique. Those values are much higher than the one calculated from the runoff and corrected TDS of different rivers: 120 t/km²/year in Guadeloupe and 100 t/km²/year in Martinique. This could mean that an important part of the chemical weathering process takes place in sub-surface and is not taken into account in the present evaluation of the dissolved load transferred from land to the ocean. Dessert C., Dupre B., Gaillardet J., Fran‡ois L. and Allegre C. J., 2003. Chem. Geol. 202: 257-273. Folio J.L., 2001. These. Universite de la Reunion. Louvat P. and Allegre C.J., 1997. Geochim. Cosmochim. Acta 61: 3645-3669. Louvat P. and Allegre C.J., 1998. Chem. Geol. 148: 177-200.
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
DE: 1815 Erosion
DE: 1838 Infiltration
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005