HR: 0800h
AN: B21B-0878    [Abstracts]
TI: Sclerochronological and Geochemical Study of {\it Protothaca thaca} (Veneridea, Bivalve) Shells of the Peru-Chile Region: Environmental vs Physiological Controls
AU: * Lazareth, C E
EM: Claire.Lazareth@bondy.ird.fr
AF: UR Pal\'{e}otropique, Ird, 32 av. Henri Varagnat, Bondy Cedex, 93143 France
AU: Li\'{e}tard, C
AF: Lodyc, Univ. P. & M. Curie, 5 pl. Jussieu, Paris, 75005 France
AU: Lasne, G
AF: UR Pal\'{e}otropique, Ird, 32 av. Henri Varagnat, Bondy Cedex, 93143 France
AU: Pierre, C
AF: Lodyc, Univ. P. & M. Curie, 5 pl. Jussieu, Paris, 75005 France
AU: Ortlieb, L
AF: UR Pal\'{e}otropique, Ird, 32 av. Henri Varagnat, Bondy Cedex, 93143 France
AB: Climatic and oceanographic variability along the South-American coasts of the Pacific Ocean, including ENSO impacts, might be recorded in fossil bivalve shells. Therefore, coupled sclerochronological and geochemical studies were undertaken on the aragonite shell of present {\it Protothaca thaca}, a fairly common Chilean-Peruvian ($8\deg$-$45\deg$S) coastal bivalve, to test it. Microscopic observations of thin sections combined with growth increment measurements allowed the identification of structural growth units which characterize the infra-daily to yearly (or longer) temporal scales. The lowest frequency unit is limited by conspicuous growth anomalies. The application of Bertalanffy growth equation and a comparison of growth increment width with SST show that growth rate is positively related to temperature. In northern Chile-southern Peru, this relationship is verified for temperature lower than $18\deg$C. When summer daily SST exceeds $18\deg$C, growth is strongly reduced, or stops, and a clear "summer check" forms in the shell, the width of which is closely related to the length of the period of SST $>$ $18\deg$C. During the 1997-98 El Ni\~{n}o event, long lasting high summer SST produced $>$ 520 $\mu$m-wide summer check. The relative width of summer check on fossil shells may thus be an indicator of paleo-ENSO events. Stable isotopes of O and C were analyzed on micro-milled aragonite samples taken along growth layers for precise correlations with the recorded daily SST. Delta O-18 values do not follow the SST record suggesting a non-equilibrium precipitation. Consistent delta C-13 profiles on different individuals display a quite good correlation with growth rate. Beside, unexpectedly, the C-13 composition shows some relationship with SST. This is interpreted as the result of a strong physiological control which also drives the growth rhythm variations. High-resolution trace elements studies, presently underway, may provide new clues to understand the complex interaction between metabolic, kinetic and environmental processes during the {\it P. thaca} shell growth. Work supported by "CONCHAS" Project (PNEDC).
DE: 9360 South America
DE: 4808 Chemical tracers
DE: 3344 Paleoclimatology
DE: 4522 El Ni¤o
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting