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Geology; May 2004; v. 32; no. 5; p. 389-392; DOI: 10.1130/G20438.1
© 2004 Geological Society of America
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44Ca/42Ca and 143Nd/144Nd isotope variations in Cretaceous–Eocene Tethyan francolites and their bearing on phosphogenesis in the southern Tethys

David Soudry1, Irena Segal1, Yaacov Nathan1, Craig R. Glenn2, Ludwik Halicz3, Zeev Lewy3 and Denys L. VonderHaar4

1 Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
2 Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA
3 Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
4 Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA

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 {delta}44Ca values are significantly lower in the Aptian–Albian samples ({delta}44Ca = –0.11{per thousand} to –0.06{per thousand}) than in the Campanian–Eocene samples ({delta}44Ca = +0.22{per thousand} to +0.42{per thousand}), whereas the {varepsilon}Nd(T) values increase from continental crust like in the Aptian–Albian [{varepsilon}Nd(T) = –10.9] to more radiogenic, Pacific-like [{varepsilon}Nd(T) = –6.6 to –6.1] in the Campanian. Both peaks of {delta}44Ca and {varepsilon}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 {delta}44Ca and {varepsilon}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.

Key Words: {delta}44Ca • {varepsilon}Nd(T) • phosphogenesis • Aptian–Eocene • Tethys • Negev






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