Variations of the Ca/Ca ratio in seawater during the past 24 million years: evidence from δCa and δO values of Miocene phosphates

The Ca isotope variation of 11 Miocene and Pleistocene, authigenic, marine phosphates is rather small compared to the corresponding variation in δ¹⁸O values. The δ⁴⁴Ca values are not correlated with the δ¹⁸O values and, therefore, they are not temperature controlled. It is likely that the δ⁴⁴Ca values of the phosphorites reflect the variation in the δ⁴⁴Ca values of paleo-seawater but, in contrast to Sr, not the isotopic composition of seawater itself. Furthermore, Ca and Sr isotopic compositions are decoupled with decreasing stratigraphic age of the phosphate peloids with Sr isotopic compositions changing to more radiogenic values while Ca isotopic compositions remain rather stable. All samples have δ⁴⁴Ca values below present-day seawater values, suggesting that phosphate formation discriminates against heavy Ca isotopes.Phosphorites and carbonaceous sediments have a similar Ca isotopic variation during the Miocene. A systematic and more or less constant shift between marine carbonates and phosphates is observed: the phosphate samples are slightly less enriched in ⁴⁰Ca compared to carbonates. This shift has been related to a mineral-dependent kinetic mass fractionation during precipitation from seawater. The rather stable δ⁴⁴Ca value for the 19 to 9 Ma old phosphorites points to a constant δ⁴⁴Ca fractionation of about 1.1 between seawater and phosphorites during the past and suggests steady-state conditions for the Mid-Miocene seawater (sedimentation flux equals erosion flux).