Calcium isotopes: Analytical developments. Application to present and past oceanic budget

Calcium is the fifth most abundant element of the silicate Earth. Its interest results from its abundance in the Earth crust and from the existence of a huge quantity of minerals containing it. A maximal fractionation of 1.3 o/oo per a.m.u was recorded up to now. The work realised during the Ph-D deals with new studies on Ca isotopes. First, a measurement protocol was developed at the Centre de Geochimie de la Surface to avoid instrumental fractionation. After chemical separation on ion exchanging resins, the Ca isotopic compositions were measured on a thermo-ionisation mass spectrometer, using a double-spike technique. The Ca isotopic ratios were expressed in a delta notation. To do this, a reference material is needed. No international standard presently exists for Ca isotopes. For that reason, we proposed to use the SRM 915a carbonaceous standard from the NIST. Seawater, the only common sample of most of previous studies has also been used: it was calibrated against the carbonaceous standard and used in present study. This protocol was then applied to study the Ca oceanic budget. The isotope data of continental waters allowed to constrain the Ca flux to the ocean. A limited variability of δ⁴⁴Ca (0.5 o/oo) was recorded at the outlet of main rivers or for hydrothermal vents. Moreover, no relationship was observed between δ⁴⁴Ca and the lithology or the climate of the studied watersheds. This allowed to determine the mean Ca isotopic composition of the Ca flux to the ocean (-1.1 ± 0.2 o/oo) and to suggest that this value remains rather constant through time. A consequence of this value is that present-day seawater is at steady-state with regard to Ca isotopes. The δ⁴⁴Ca study of marine phosphates from Miocene and Pleistocene, which reflect the paleo-seawater variations through time, has shown that this was not necessarily the case in the past. At 22 Ma, a deviation in δ⁴⁴Ca through more positive values suggests that the intensity of the incoming and out-coming fluxes changed, but not their isotopic composition. (author)