Paleoenvironmental conditions and strontium isotop

Authors: László Kocsis Anouar Ounis Fredj Chaabani Neili Mohamed Salah

Publish Date: 2012/12/13

Volume: 102, Issue: 4, Pages: 1111-1129

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Abstract

Fossil shark teeth and coprolites from three major phosphorite occurrences in the Gafsa Basin southwestern Tunisia were investigated for their geochemical compositions to improve local stratigraphy and to better assess paleoenvironmental conditions 87Sr/86Sr isotope ratios of shark teeth from the Early Maastrichtian El Haria Formation and from the Early Eocene Métlaoui ss Formation yielded Sr isotope ages of 68 ± 1 and 479 ± 13 Ma respectively which accord with the expected stratigraphic positions of these sediments Conversely shark teeth from the Paleocene–Eocene Chouabine Formation have large variation in Sr isotope ratios even within individual layers After statistical treatment and then elimination of certain outlier samples three agemodels are proposed and discussed The most reasonable solution includes three subsequent Sr ages of 618 ± 22 Ma 572 ± 18 and 546 ± 16 for layer IX layers VIII–V and layers IV–0 respectively Three scenarios are discussed for explanation of the presence of the outliers 1 diagenesis 2 reworking and 3 locally controlled seawater Sr isotope ratio The most plausible account for the higher 87Sr/86Sr ratios relative to the global ocean in some fossils is enhanced intrabasinal reworking due to low sea level Conversely the sample with lower 87Sr/86Sr than the global seawater may link to diagenesis or to seawater influenced by weathering of Late Cretaceous marine carbonates which latter is supported by model calculation as well The εNd values of these fossils are very similar to those reported for Paleogene and Late Cretaceous Tethyan seawater and are compatible with the above interpretations The relatively low oxygen isotope values in shark teeth from the topmost phosphate bed of the Chouabine Formation together with the Sr isotope results point toward recovering better connections with the open sea These δ18O data reflect elevated ambient temperature which may link to the Early Eocene Climatic OptimumThe authors are grateful for all the support and help they received from the Compagnie des Phosphates de Gafsa Tunisia for conducting this research with productive fieldwork We thank M Cooper for his assistance with TIMS analyses L K was generously funded by the Swiss National Science Foundation SNF PBLA2119669 and SNF PZ00P2 126407 and NERC NE/C00390X/1 projects Constructive comments by Dr J A Chamberlain and two anonymous reviewers are very much appreciated

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