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Anouar Ounis, Département de Géologie, Faculté des Sciences de Tunis, University of Tunis El Manar (Tunisia)
László Kocsis, Institut de Minéralogie et Géochimie, Faculté des Géosciences et Environnement, University of Lausanne (Switzerland)
Fredj Chaabani, Département de Géologie, Faculté des Sciences de Tunis, University of Tunis El Manar (Tunisia)
Hans-Rudolf Pfeifer, Centre d'Analyse Minerale, Faculté Geosciences et Environnement, University of Lausanne (Switzerland)
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The oxygen isotope analyses of marine biogenic apatite have gained increasing recognition as climatic and oceanographic proxies and are commonly used to reconstruct past environmental, as well as thermal or climatic conditions. Phosphatic coprolites and shark teeth was sampled from diverse phosphatic layers of the Late Cretaceous and Paleocene-Eocene phosphorite in the Gafsa basin, south western Tunisia. Oxygen isotopic composition of the structural phosphate (δ 18OPO4) of these phosphatic remains indicates a stable warm, tropical climate during the studied time interval. The phosphatic coprolites have generally higher δ 18OPO4 values than the shark teeth reflecting the expected differences between the bottom condition and the neritic region.
During phosphate digenesis and growing the oxygen isotopic composition of the phosphate within the apatite depends on the oxygen isotopic composition of the ambient water (δ18OW) and the water temperature (T) described by the following equation:
T (°C) =113.3 - 4.38(δ 18OPO4 - δ 18OWater) (Kolodny et al., 1983). Assuming a value of -1‰ for seawater isotopic composition of an ice-free World, the ancient seawater temperature can be calculated. The former water temperature in the Early Maastrichtian and the Paleocene-Eocene had an average of 21.9 ± 4.2 °C and 19.3 ± 1.9 °C in the neritic region and 16.8 ± 0.8 °C and 17.4 ± 1.5 °C at the bottom, respectively.
The changes in the temperature gradient in the basin might indicate arrival of colder deep water in the region (upwelling) and/or by deepening of the basin maybe due to sea-level rise that was reported from these periods and hence also could cause the partial suboxic-anoxic condition at the bottom due to lack of vertical mixing in the basin. This coincides with the Rare Earth Element and mineralogical analyses showing reductive bottom water condition in the Late Cretaceous.
The global oxygen isotopic record deduced from benthic foraminifera shows a sudden negative shift at the Paleocene-Eocene boundary and also a longer negative trend during the Lower Eocene caused by global warming. In our homogenous record such a negative excursion was not detected. As there was not ice cap in the Polar Regions during this time therefore globally only temperature changes could have caused such oxygen isotope shift in the marine precipitates. In case of fishes as they are nekton they might have followed their preferred water temperature and this could be one of the reason the lack of negative shift in the δ 18OPO4 values of the teeth. However such explanation of the partly bottom formed coprolites it is not applicable.
To better constrain the thermal evolution of the Gafsa Basin further proxies like the salinity-independent organic paleothermometer of TEX86 or "clumped isotopes" that are depended only on the ambient temperature and not on the water isotopic composition can be the base of future studies.
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