The Ca isotope variation of 11 Miocene and Pleistocene, authigenic,
marine phosphates is rather small compared to the corresponding
variation in delta(18)O values. The delta(44)Ca values are not
correlated with the delta(18)O values and, therefore, they are not
temperature controlled. It is likely that the delta(44)Ca values of the
phosphorites reflect the variation in the delta(44)Ca 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
delta(44)Ca 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 Ca-40 compared to
carbonates. This shift has been related to a mineral-dependent kinetic
mass fractionation during precipitation from seawater. The rather stable
delta(44)Ca value for the 19 to 9 Ma old phosphorites points to a
constant delta(44)Ca 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).
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