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

摘要

Measurements of 44Ca/42Ca and 143Nd/144Nd 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 44Ca values are significantly lower in the Aptian–Albian samples (44Ca = –0.11 to –0.06) than in the Campanian–Eocene samples (44Ca = +0.22 to +0.42), whereas the _Nd(T) values increase from continental crust like in the Aptian–Albian [_Nd(T) = –10.9] to more radiogenic, Pacific-like [_Nd(T) = –6.6 to –6.1] in the Campanian. Both peaks of 44Ca and _Nd(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 44Ca and _Nd(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.