HR: 0800h
AN: B21B-0877    [Abstracts]
TI: Mg and 18O Variations in the Shell of the Chilean Gastropod Concholepas concholepas Reflect SST and Growth Rate variations
AU: Guzman, N
EM: Nury.Guzman@bondy.ird.fr
AF: Paleontology Unit, Univ. Paris XI, Bat. 504 Fac. Sciences, Orsay, 91405 France
AU: Guzman, N
EM: Nury.Guzman@bondy.ird.fr
AF: UR Paleotropique, IRD, 32 avenue Henri Varagnat, Bondy, 93143 France
AU: * Lazareth, C E
EM: Claire.Lazareth@bondy.ird.fr
AF: UR Paleotropique, IRD, 32 avenue Henri Varagnat, Bondy, 93143 France
AU: Poitrasson, F
EM: Franck.Poitrasson@cict.fr
AF: LMTG-CNRS, 38 rue des 36 Ponts, Toulouse, 31400 France
AU: Cuif, J
EM: cuif@geol.u-psud.fr
AF: Paleontology Unit, Univ. Paris XI, Bat. 504 Fac. Sciences, Orsay, 91405 France
AU: Ortlieb, L
EM: Luc.Ortlieb@bondy.ird.fr
AF: UR Paleotropique, IRD, 32 avenue Henri Varagnat, Bondy, 93143 France
AB: To validate the use of fossil mollusc shells as recorders of environmental conditions, a primary calibration study was carried out on modern shells of the Chilean gastropod Concholepas concholepas, the so-called southern hemisphere abalone which is particularly abundant in Holocene archaeological sites. Organisms were maintained in culture tanks and feed with live mytilids. The sea water temperature in the tank was recorded every half-an-hour by an automatic device. The experiment lasted several months. Periodical marking with calcein provided a precise chronological control of the shell growth. Thus, well-dated high resolution chemical profiles could be directly compared with temperatures during shell formation. Geochemical analyses of the calcite layers include Mg, Sr and 16O/18O composition. Trace elements were analysed using Laser Ablation ICP-MS and Electron Microprobe while stable isotopes were measured on a Secondary Ion Mass spectrometry (SIMS). The shell growth rate during two months of formation varied between 30 and 140 æm/day which allows us to reach a temporal resolution for chemical profiles between a few hours and three days. The growth rate variations do not seem to be related to temperature fluctuations. Only Mg content was analytically reproducible and showed significant variations across the shells. The Mg high-resolution profiles display a grossly sinusoidal shape. Shells from different sites along the coasts of Chile showed mean Mg contents of 300 ppm and 500 ppm for mean temperatures of 17 and $20\deg$C, respectively. This suggests a gross correlation between Mg and temperature. However, high resolution Mg results do not show an exact fitting neither with temperature nor with growth rates. Other parameters, like shell ageing as suggested by an amplitude increase observed near the edge of one of the shells, or other complex biological factors, may influence Mg incorporation into the shell. \delta 18O values of the calcite vary between -1,5 and 2,0 \permil for a temperature range between 17 and $22\deg$C. Growth rate variations seem to be an important factor affecting the oxygen isotopic ratio within shells. When growth rate variations are limited, \delta 18O and temperature are well correlated. The study confirms that, like for all biogenic carbonates, elemental and isotopic composition of the calcite layer of this gastropod, should not be used in paleoenvironmental reconstructions without detailed calibration experiments, and must systematically include precise growth rate analyses. The growth rhythms, which vary under the double influence of environmental and biological factors, are of paramount importance in the relationship between environmental parameters and geochemical composition of the growth layers of the shells. Work supported by "CONCHAS" Project (PNEDC).
DE: 3670 Minor and trace element composition
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting