Rapport sur la mission PALEOMARQ du N.O. Alis aux îles Marquises (Polynésie Française) du 14 au 27 septembre 1997

Cette mission entre dans le cadre du programme "Paléocéan" du Grand Programme "Variabilité climatique et impacts régionaux" de l'UR 1. Paléocéan propose une approche paléocéanographique de la variabilité climatique tropicale par l'analyse des coraux du Pacifique central et sud-ouest. La mission PALEOMARQ entre dans le volet Paléocéan récent, i.e., l'identification des anomalies climatiques des siècles précédents

Microfaunes et paléoenvironnements des récifs frangeants quaternaires de Mamié et Ricaudy (Nouvelle Calédonie)

L'étude sédimentologique et paléontologique d'une série de forages implantés sur les récifs frangeants permet de proposer un modèle d'évolution paléoécologique à partir de la reconstitution des paléoenvironnements qui correspondent aux divers types de dépôts récifaux ou pararécifaux identifiés. D'autre part, l'étude micropaléontologique des foraminifères et des ostracodes précise les caractéristiques des milieux de déposition au cours du Quaternair

Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record

A geochemical proxy for surface ocean nutrient concentrations recorded in coral skeleton could provide new insight into the connections between sub-seasonal to centennial scale nutrient dynamics, ocean physics, and primary production in the past. Previous work showed that coralline P/Ca, a novel seawater phosphate proxy, varies synchronously with annual upwelling-driven cycles in surface water phosphate concentration. However, paired contemporaneous seawater phosphate time-series data, needed for rigorous calibration of the new proxy, were lacking. Here we present further development of the P/Ca proxy in Porites lutea and Montastrea sp. corals, showing that skeletal P/Ca in colonies from geographically distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO4 SW) concentration. Further, high-resolution P/Ca records in multiple colonies of Pavona gigantea and Porites lobata corals grown at the same upwelling location in the Gulf of Panama were strongly correlated to a contemporaneous time-series record of surface water PO4 SW at this site (r2 = 0.7–0.9). This study supports application of the following multi-colony calibration equations to down-core records from comparable upwelling sites, resulting in ±0.2 and ±0.1 lmol/kg uncertainties in PO4 SW reconstructions from P. lobata and P. gigantea, respectively.P/Ca Porites lobata (lmol/mol) = (21.1 ? 2.4)PO4 SW (lmol/kg) + (14.3 ? 3.8)P/Ca Pavona gigantea (lmol/mol) = (29.2 ? 1.4)PO4 SW (lmol/kg) + (33.4 ? 2.7)Inter-colony agreement in P/Ca response to PO4 SW was good (±5–12% about mean calibration slope), suggesting that species-specific calibration slopes can be applied to new coral P/Ca records to reconstruct past changes in surface ocean phosphate. However, offsets in the y-intercepts of calibration regressions among co-located individuals and taxa suggest that biologically-regulated “vital effects” and/or skeletal extension rate may also affect skeletal P incorporation. Quantification of the effect of skeletal extension rate on P/Ca could lead to corrected calibration equations and improved inter-colony P/Ca agreement. Nevertheless, the efficacy of the P/Ca proxy is thus supported by both broad scale correlation to mean surface water phosphate and regional calibration against documented local seawater phosphate variations