Diagenèse et reconstruction de variables environnementales à partir de la géochimie du corail Porites sp. (Nouvelle-Calédonie, Pacifique Sud-Ouest )

Abstract : This work is part of an approach aimed at quantifying the impact of diagenesis upon the Sr/Ca, Mg/Ca and U/Ca elemental ratios that typify the exoskeleton (biogenic aragonite) of Porites sp corals. These elemental ratios are indeed routinely used as paleothermometers in tropical paleoclimatology in order to reconstruct environmental paleovariables such as the Sea Surface Temperature (SST). In a first step, we analyze a modern coral collected in vivo and a fossil coral dated by 14C to mid-Holocene (5545 year BP). Both corals are in pristine state. The Sr/Ca, Mg/Ca and U/Ca ratios are measured by inductively coupled plasma mass spectrometry (ICP-MS) and with a Castaing microprobe for Sr/Ca. The SSTs at the time when the corals were alive are reconstructed from the geochemistry of the samples and validated against in situ measurements / previous work in the same area. Next, the approach is extended to Pleistocene fossil corals (~125000 year BP) that have been altered by diagenesis e.g., calcitization of the samples to the detriment of the original aragonite due to the fresh water that percolates through the coral reefs. Calcitization alters the original Sr/Ca and Mg/Ca. The reconstructed SSTs can therefore include nontrivial biases whose magnitude must be evaluated. Relying on Raman spectrometry, we confirm published qualitative trends on the impact of calcite upon the climate proxies : calcite lowers (resp. increases) the Sr/Ca (resp. Mg/Ca) ratio leading therefore to warm artifacts in the reconstructed SSTs.
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Clement Lelabousse. Diagenèse et reconstruction de variables environnementales à partir de la géochimie du corail Porites sp. (Nouvelle-Calédonie, Pacifique Sud-Ouest ). Autre. Ecole nationale supérieure d'arts et métiers - ENSAM, 2012. Français. ⟨NNT : 2012ENAM0035⟩. ⟨tel-00787482v2⟩

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