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Le rapport B/Ca des foraminifères : un proxy pour le cycle du carbone dans l’océan

Abstract : It has been shown recently that the B/Ca ratio of planktonic foraminifers depends on the seasurface water pH, while benthic foraminifer B/Ca depends on the carbonate ion saturation of deep water. This new tracer could allow to better constrain the variations of oceanic carbon cycle, especially during the glacial-interglacial transitions (G/IG) of the Pleistocene. The objectives of this thesis were therefore : (1) to develop the foraminiferal B/Ca analysis by mass spectrometry (ICP-QMS) at LSCE, (2) to identify some potential biases of this tracer to assess its limits and (3) to apply the B/Ca proxy to paleoceanographic reconstructions. Contaminations and the ease with which the boron may be adsorbed (memory effect) make it difficult to analyze it in foraminifer’s shells, which have low B/Ca, typically in the range ~40-120 µmol/mol for planktonic foraminifers and ~100-250 µmol/mol for benthic foraminifers. Foraminifers cleaning techniques and analysis by ICP-QMS that I used and improved now allow the foraminiferal B/Ca measure with an accuracy of about 3.5%. Based upon the study of surface sediments collected along a depth transect in the Atlantic (Sierra Leone Rise), I was able to show that dissolution of the planktonic species Globigerinoides sacculifer results in the preferential loss of boron. The increasing G. sacculifer dissolution with depth of deposition results, therefore, in a ~0.1 units decrease of the seasurface pH estimated from B/Ca, an order of magnitude similar to the expected G/IG variations. To overcome this dissolution effect, I propose a correction procedure based on the B/Ca of benthic foraminifera (bottom water ΔCO₃²⁻). However, the dissolution effect is not the only problem. When trying to reconstruct surface water paleo-pH, we also have to deal with the severe contradictions between recent studies which dealt with the calibration of the apparent boron partition coefficien (KD) in the calcite of planktonic foraminifers. To overcome the co-variance of temperature, salinity and CO₃²⁻ in Atlantic surface waters, which can mask the real important(s) factor(s) for KD calibration, I analyzed the B/Ca of Globigerinoides ruber picked from both Atlantic and Indo-Pacific core-tops. My data suggest that there is no robust relationship between KD and temperature or CO₃²⁻ water surface. To emphasize the uncertainties on the reconstruction of paleo-pH since the last glacial stage, I conducted a sensitivity test based on G. ruber B/Ca data from a Mozambic Channel core. I propose several ways to try to improve our understanding of the B/Ca proxy in planktonic foraminifers. If the use of planktonic foraminifer B/Ca is complex and still requires some dedicated work to understand the problems and improve the calibrations, the use of benthic foraminifer B/Ca is more simple, the B/Ca being directly related to the carbonate ion saturation of bottom waters. Analyses of Cibicidoides wuellestorfi B/Ca through two G/IG transitions (MIS16/15 and MIS12/11) on ODP Site 849 (equatorial Pacific) allowed me to show that changes in deep water chemistry estimated from the sedimentological dissolution proxies (e.g. %CaCO3, foraminifer shell weight and fragmentation, …) display incorrect amplitudes and temporal biases. Based on the comparison of benthic foraminifer B/Ca and δ¹³C, I was able (i) to highlight the link between oceanic CO2 degassing and deep-sea carbonate preservation in the Pacific and (ii) better unravel the dynamics of pelagic carbonate compensation.
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Romain Coadic. Le rapport B/Ca des foraminifères : un proxy pour le cycle du carbone dans l’océan. Sciences de la Terre. Université Paris Sud - Paris XI, 2012. Français. ⟨NNT : 2012PA112408⟩. ⟨tel-00920663⟩

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