HR: 15:20h
AN: U13C-07    [Abstracts]
TI: Chemical Erosion in the Himalayas for the Past 4 Million Years Studied by Pb and Nd Isotopic Stratigraphies
AU: * Meynadier, L
EM: meynadier@ipgp.jussieu.fr
AF: IPGP, Geochimie et Cosmochimie, 4 Place Jussieu, Paris, 75252, France
AU: Gourlan, A T
EM: gourlan@ipgp.jussieu.fr
AF: IPGP, Geochimie et Cosmochimie, 4 Place Jussieu, Paris, 75252, France
AU: Louvat, P
EM: louvat@ipgp.jussieu.fr
AF: IPGP, Geochimie et Cosmochimie, 4 Place Jussieu, Paris, 75252, France
AU: Allègre, C
EM: allegre@ipgp.jussieu.fr
AF: IPGP, Geochimie et Cosmochimie, 4 Place Jussieu, Paris, 75252, France
AB: A record of the 143Nd/144Nd ratio of Northern Indian Ocean seawater and associated detrital sediments has been obtained for the past 4 Ma using a new method of differential dissolution on marine sediments. We studied ODP Sites 758 and 757 both located on the Ninetyeast Ridge. At Site 758, the εNd and δ18O curves fluctuate in conjunction during the glacial-interglacial periods. The largest variation occurred during the last 20 kyrs where εNd varies from -7.5 at Last Glacial Maximum to -10.5 during the Holocene, whereas the amplitude of the variation of the seawater signal is less than 1 εunit 3 Ma ago. The correlation between maxima and minima of δ18O and εNd is excellent (r=0.92). However, a detailed comparison of the two signals show that εNd and δ18O vary simultaneously during warming while εNd is delayed with respect to δ18O during cooling. The southern ODP Site 757 shows little variations in Nd isotopic ratio. We thus interpret the εNd fluctuations at Site 758 as being linked to the erosion regime in the Himalayas-Tibet rather than to variations in the intensity of the deep ocean conveyor belt which flows from the South to this area. Such variations were probably governed by storage of more ice in the Himalaya-Tibet highlands during glacial periods and by ice melting and the enhancement of monsoon rainfall during interglacials. A simple quantitative model assuming that seawater Nd is a mixture of Nd that was chemically eroded in the Himalaya-Tibet with Nd coming from Indonesian island arcs indicates that chemical erosion in the Himalaya-Tibet was 2 to 4 times more intense during interglacial than during glacial periods. We have shown by chemical tests that the same technique of partial dissolution can be also applied for lead isotopes. At Site 758, the lead isotopes ratios fluctuate following glacial interglacial alternance, but within a very small range. Indeed, 206Pb/204Pb, 207 Pb/204Pb , 208Pb/204Pb varies from 18.9 to 19.0, 17.735 to 17.770, 39.23 to 39.42 respectively. These variations are extremely well correlated suggesting a binary mixture. Using the lead isotopic ratio from the various parts of Himalaya-Tibet (Gariépi et al., 1985), we suggest that the two component are the Trans-Himalaya-Belt (THB) and the High-Himalaya-Chain (HHC), the HHC signature being transported by the Ganges and the THB one by the Bramaputra. During Glacial chemical erosion on the HHC was reduced by a factor of 6 to 7 while the chemical erosion on the THB was reduced by a factor of 2 only. This suggests that very large permanent glaciers were present on the Himalayas during Glacials, there were probably not as large in the Southern Tibet.
DE: 1038 Mantle processes (3621)
SC: Union [U]
MN: 2007 Fall Meeting