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La coquille de Comptopallium radula (Bivalvia; Pectinidae), archive eulérienne haute-fréquence de la variabilité de l'environnement côtier tropical (Océan Pacifique)

Abstract : Coastal ecosystems are exposed to many disturbances leading to the alteration of their structure and functioning. In order to assess the respective roles of natural variability and anthropogenic influences on this process, and to predict the future state of these ecosystems more accurately, we must be able to reconstruct the past variations of certain parameters (ocean temperature, biological productivity, contaminant inputs, etc.) prior to any major anthropogenic influence. Instrumental measurements of these parameters are, however, fairly recent and the reconstruction of the variability of coastal ecosystems prior to the Industrial Revolution can only be done by using proxies embedded in the available archives.
This PhD dissertation focuses on the use of the shell of Comptopallium radula, a tropical bivalve of the mollusc family Pectinidae, for the reconstruction of environmental variability in the southwest lagoon of New Caledonia, an ecosystem influenced by multiple stresses (global warming, urbanization, opencast mining). This work is based on the comparison of geochemical data archived in the scallop shell through ontogeny, with several physical, chemical, and biological variables measured in the lagoon (weekly environmental survey). The aim of this approach is to calibrate and to make use of different climatic and environmental proxies.
Calcein markings, performed in situ on juvenile scallops, demonstrated that the growth striae wrinkling the external surface of C. radula's shell are formed with a 2-day periodicity.
Isotopic analyses (δ18O and δ13C) were performed on calcite micro-samples collected along the maximal growth axis of 6 juvenile shells. Based on the periodicity of stria formation, a calendar date was accurately assigned to each sample (weekly resolution). Our results indicate that the ontogenetic variations of δ18Ocalcite allow the reconstruction of seawater temperature with a mean accuracy of 1 °C. The carbon isotope composition (δ13Ccalcite) cannot, however, be used as a proxy for the pelagic primary production as its variations could be controlled by the benthic metabolism (sediment and corals).
Following these isotopic analyses, elemental analyses were performed on the same shells using a laser ablation system coupled to an inductively coupled plasma mass spectrometer (LA-ICP-MS). Through the use of skeletal trace metal concentrations, our results show that C. radula cannot be used as a biomonitor of metal inputs in the lagoon: the ontogenetic variations of these concentrations probably reflect high-frequency temporal variations of metal fluxes at the sediment-water interface. As for barium and molybdenum, their skeletal concentrations may be used as proxies for the biomass of diatoms and diazotrophs, respectively.
This study points out the considerable potential of the shell of C. radula as a high-frequency Eulerian archive of climatic and environmental variability in the southwest lagoon of New Caledonia. It also highlights the major role of benthic processes in the incorporation of geochemical signals in the calcitic matrix of the shell. Its use as a paleoceanographic tool is now governed by the discovery of fossil beds of the species.
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https://tel.archives-ouvertes.fr/tel-00012033
Contributor : Julien Thébault <>
Submitted on : Friday, March 24, 2006 - 3:28:57 PM
Last modification on : Tuesday, March 30, 2021 - 12:26:08 PM
Long-term archiving on: : Saturday, April 3, 2010 - 8:52:30 PM

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  • HAL Id : tel-00012033, version 1

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Julien Thébault. La coquille de Comptopallium radula (Bivalvia; Pectinidae), archive eulérienne haute-fréquence de la variabilité de l'environnement côtier tropical (Océan Pacifique). Océan, Atmosphère. Université de Bretagne occidentale - Brest, 2005. Français. ⟨tel-00012033⟩

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