HR: 1340h
AN: PP43B-0683    [Abstracts]
TI: The Island Arcs as a Major Source of Mantellic Sr to the Ocean: Tectonic Control over Seawater Chemistry and Climate
AU: * Louvat, P
EM: louvat@ipgp.jussieu.fr
AF: Laboratoire de Géochimie et Cosmochimie (UMR 7154 CNRS) Institut de Physique du Globe de Paris & Université Denis Diderot (Paris7), 4 place Jussieu, Paris Cedex 05, 75252 France
AU: Allegre, C J
EM: allègre@ipgp.jussieu.fr
AF: Laboratoire de Géochimie et Cosmochimie (UMR 7154 CNRS) Institut de Physique du Globe de Paris & Université Denis Diderot (Paris7), 4 place Jussieu, Paris Cedex 05, 75252 France
AU: Meynadier, L
EM: meynad@ipgp.jussieu.fr
AF: Laboratoire de Géochimie et Cosmochimie (UMR 7154 CNRS) Institut de Physique du Globe de Paris & Université Denis Diderot (Paris7), 4 place Jussieu, Paris Cedex 05, 75252 France
AB: The evolution of ⁸⁷Sr/⁸⁶Sr in the Cenozoic ocean has been the subject of famous and vivid controversies between the BLAG model¹ and Raymo's one². No clear winner! Recently the question has been worsened because recent estimates of the hydrothermal flux at ridge crest^{3, 4, 5} and of the low-temperature oceanic crust weathering flux⁶ have shown that these fluxes are not enough to balance the continental radiogenic input to give 0.70917 (present-day seawater ⁸⁷Sr/⁸⁶Sr). We have re-examined this problem using both Sr and Nd isotopic budgets. Seawater ¹⁴³Nd/¹⁴⁴Nd isotopic ratio varies from one ocean to another as a consequence of its short residence time. For each ocean we can calculate the Nd contributions from continental (rivers) and mantellic sources. Since ridge crests cannot be the mantle-like source for Nd, this source is identified as the island arc and OIB weathering, in agreement with the observation by Goldstein and Hemming⁷. This approach leads us to examine the possibility of the same island arc origin for the missing mantle-like seawater Sr. The classical approach to the budget of water entering the ocean is to consider the river water fluxes as established by hydrological survey statistics. But these fluxes are too small, as they do not include the underground water flows, which are particularly important for volcanic terrains⁸. A budget calculation based on mean surface area, mean water fluxes and mean Sr concentrations in rivers and springs demonstrates island arc and OIB weathering is a sufficient source of mantellic Sr to the ocean to match the seawater ⁸⁷Sr/⁸⁶Sr budget. This result has a fundamental consequence on the explanation of the seawater ⁸⁷Sr/⁸⁶Sr evolution during the Cenozoic. Indeed, when a continental collision occurs a large portion of island arcs is eliminated. Thus the increase in the contribution of radiogenic ⁸⁷Sr/⁸⁶Sr from continental weathering and the decrease of the mantle contribution via island arc weathering are tectonically and mandatory coupled. We can then explain quite easily the Cenozoic ⁸⁷Sr/⁸⁶Sr curve but also the Phanerozoic one. This model has also important consequences for the climate models of the Cenozoic. References ¹ Berner R.A., Lasaga A.C., Garrels R.M., Am. J. Science 283 (1983) 641-683.
² Raymo M., Ruddiman W.F., Nature 359 (1992) 117-122.
³ Fehn U., Green K.E., Von Herzen R.P., Cathles L.M., J. Geophys. Res. 88 (1983) 1033-1048.
⁴ Sleep N.H., J. Geophys. Res. 96 (1991) 2375-2387.
⁵ Bowers T.S., Taylor H.P., J. Geophys. Res. 90 (1985) 12583-12606.
⁶ Davis A., Brickle M.J., Teagle D., Earth Planet. Sci. Lett. 112 (2003) 173-187.
⁷ Golsdtein S., Hemming S.R., in Treatise on Geochemistry, Vol. 6, The Oceans and Marine Geochemistry (2003) editor H. Elderfield, Elsevier, 453p.
⁸ Rad S., Louvat P., Allègre C.J., AGU Fall Meeting (2005) session PP14.
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1815 Erosion
DE: 8177 Tectonics and climatic interactions
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005