44Ca/42Ca and 143Nd/144Nd isotope variations in Cretaceous–Eocene Tethyan francolites and their bearing on phosphogenesis in the southern Tethys

Measurements of 4 4 Ca/ 4 2 Ca and 1 4 3 Nd/ 1 4 4 Nd isotope ratios in carbonate fluorapatite (CFA) through the Aptian-Eocene section of the Negev (Israel), together with quantified rates of P and Ca accumulation and bulk sedimentation, permit an examination of the relationships of these five factors to Tethyan phosphogenesis. The data provide an Aptian-Eocene (ca. 70 Ma) record of the Ca isotope composition of 35 CFA samples (24 of which were also analyzed for Nd isotopes), representing 11 time-stratigraphic phosphate horizons within the sequence. The δ 4 4 Ca values are significantly lower in the Aptian-Albian samples (δ 4 4 Ca = -0.11‰ to -0.06‰) than in the Campanian-Eocene samples (δ 4 4 Ca = +0.22‰ to +0.42‰), whereas the e N d (Τ) values increase from continental crust like in the Aptian-Albian [e N d ( T ) = -10.9] to more radiogenic, Pacific-like [e N d ( T ) = -6.6 to -6.1] in the Campanian. Both peaks of δ 4 4 Ca and e N d ( T ) in the Campanian coincide with the peak of Tethyan phosphogenesis in the Negev, which is marked by a sharp increase of P accumulation rates (from <200 μmol.cm - 2 .k.y. - 1 in pre-Campanian time to ∼1700 μmol.cm - 2 .k.y. - 1 in the Campanian) and a decrease in rates of Ca accumulation and bulk sedimentation. The coincident increases of δ 4 4 Ca and e N d ( T ) values and P accumulation rates in the Negev area during the Campanian suggest that they are related in this time interval and were induced by the global Late Cretaceous sea-level rise and increasing circumequatorial Tethyan flow.