P. Deschamps1, N. Durand1, E. Bard1, B. Hamelin1, G. Camoin1,
A. L. Thomas2, G. M. Henderson2 and Y. Yokoyama3
1CEREGE, UMR CNRS – IRD – Aix-Marseille Université - Collège de France, Europôle Méditerranéen de l’Arbois, BP 80, F-13545 Aix-en-Provence Cedex 4
2Department of Earth Science, Parks Road, Oxford, OX1 3PR, United Kingdom
3Ocean Research Institute and Department of Earth and Planetary Sciences, University of Tokyo, 1-15-1 Minamidai, Nakanoku, Tokyo 164-8639, JAPAN
4Institute for Research on Earth Evolution, JAMSTEC, Yokosuka, Japan.
So far, the most complete and accurate sea-level record that encompassed the period between the Last Glacial Maximum and the present day is based on cores drilled offshore from the Barbados coral reef [1, 2]. That record suggests a non-monotonous sea-level rise punctuated by two dramatic accelerations, the MWP1-A and MWP1-B events, centered at ~14,000 and ~11,300 cal. yr BP respectively [3, 4]. However, the occurrence, the hemispheric origin as well as the exact relationship between those events and the global climatic evolution remain enigmatic and controversial. The recent IODP Expedition 310 "Tahiti Sea Level" offers a unique opportunity to extend the previous Tahiti coral reef record that documented the deglacial sea-level rise over the last 13.8 ka [5, 6]. Located at a considerable distance from the major former ice sheets and characterized by slow and regular subsidence rates, Tahiti provides an ideal setting to constrain MWP events that are thought to have punctuated the last deglaciation. The offshore coring operations carried out during the Expedition 310 recovered more than 400 m of post-glacial reef material, ranging from 122 to 40 m below modern sea level [7, 8].
More than 60 U-Th ages were obtained on various types of corals that are indicative of a range of modern reef environments, from the reef crest to the reef slope . Together with previous on-shore data [5, 6], this new data set extend the previous Tahiti record to the last 16 ka and allow to document the sea-level rise during the previously defined key period of the MWP-1A . Our results confirm the occurrence of an acceleration of the sea-level rise during this period. However, the timing and duration of this event are significantly different to those of the MWP-1A as defined in Barbados . These new results allow us to revisit the relationship between the MWP-1A and the climate history of the last deglaciation. We will discuss also their implications in terms of the potential sources of the ice that generated the MWP-1A.
We also take advantages of the good preservation of the post-glacial coral material collected during the IODP Expedition 310 to investigate potential variations with time of the seawater uranium-isotope ratio. The history of seawater (234U/238U)SW, as inferred from compilation of d234U values in U-series dated corals, suggested long-term to abrupt shifts in the (234U/238U)SW related to changes in the oceanic budget due to fluctuations of sea-levels or global weathering rates [9-13]. Before U-Th datings, rigorous mineralogical and isotopic screening criteria have been applied in order to preclude any post-mortem diagenetic alteration of the aragonitic coral skeleton. We checked the absence of secondary calcite content in aragonite skeleton by using X-ray diffraction precisely calibrated by using gravimetric standards . In particular, we made a significant effort to improve detection and quantification of very small amount of secondary calcite . Only coral samples with less than 1% calcite content were analyzed. U-Th analyses were performed using a VG-54 thermo-ionisation mass spectrometer equipped with a 30-cm electrostatic analyzer and a pulse-counting Daly detector. Calculated initial (234U/238U)0 values of post-glacial samples analysed in this study fall within 2-3‰ of the most recent determinations of the uranium isotopic composition of modern corals and present-day seawater [15-17].
The analytical reproducibility achieved in the course of the study for the ratio (234U/238U) is about 0.8‰ (2s). All samples fall within the isotopic range adopted by Hughen et al.  for the interval 0-17 ka ((234U/238U)0 = 1.1452±0.0048, 2s) as a strict isotopic screening criteria. However, for coeval coral samples, the (234U/238U)0 values are highly consistent within our internal reproducibility. The high consistency of the new Tahiti dataset highlights the remarkable preservation of samples recovered in the Tahiti offshore reef cores. The clustering of the (234U/238U)0 values substantially narrows the uncertainties in marine signature for the period 11 – 16 ka and allow to portray the (234U/238U) marine signature fluctuation in an unprecedented way. We will compare our results with previous dataset (Vanuatu, Papua New Guinea and Barbados) that encompass this period [3, 17, 19] in order to elucidate subtle fluctuations of the marine signature during the last deglaciation.
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