HR: 17:00h
AN: B14B-05    [Abstracts]
TI: The Coral Data Time Series Need To Be Revisited
AU: * Juillet-Leclerc, A
EM: Anne.Juillet@lsce.cnrs-gif.fr
AF: LSCE, Domaine du CNRS, Gif sur Yvette, 91198 France
AB: Coral skeleton is formed under organism control and its geochemical properties are strongly influenced by biological effects embedding environmental signal. Geochemists have been puzzled by the diversity of geochemical responses showed by colonies grown in a same area. By revisiting the Weber and Woodhead data series (1972), gathering data from enough colonies developed in similar conditions to provide a statistical isotopic value representative of one site, we demonstrate that for Porites and Acropora, the expected isotopic thermometer is revealed when the "vital effect" is removed. On the other hand, by using Acropora cultured in controlled condition, with changing temperature on a range comprised between 23 and 29øC, the comparison of oxygen and carbon isotopic values revealed the role played by kinetic fractionation. This apparent paradox of two co-existing fractionations is explained by the isotopic analyzes of wild and cultured corals operated at micrometer size scale taking into account of microstructures of the skeleton. Two different crystals appear to be the growth units of the skeleton, each crystal corresponding to a specific deposition mechanism. Thus, the measurement performed with a conventional method is a "bulk" measurement, which depends upon two isotopic fractionations. Some investigations underlined the discrepancy of the meaning of the inter-annual and seasonal isotopic records, which could be illustrated by different isotopic calibrations assessed from seasonal or annual data. It has been also explained by micrometer analyses of Porites aragonite. A smoothing at around 400microns of isotopic measurements as well as Sr/Ca indicates that at seasonal time scale the growth unit is the month. This is in agreement with extensive studies conducted by biologists describing the mechanism governing the formation of Porites skeleton: every month is deposited a framework which is progressively filled in. By combining biologists and geochemists knowledge, we are able to improve the interpretation of the coral records in term of paleoclimatic reconstruction (cf PP15 session).
DE: 4808 Chemical tracers
DE: 4870 Stable isotopes
DE: 3620 Crystal chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting