Austrian journal of earth sciences vol:105 issue:1 pages:77-87
Ostracode and other microfossil assemblages from the Tunisian Trough have previously been studied to characterize paleoenvironmental conditions during the late Danian interval. Whereas the preservation of foraminifera is generally not sufficient for stable isotope studies, well preserved ostracodes provide continuous upper Danian stable isotope records (delta C-13, delta O-18) at Sidi Nasseur, W Tunisia. The late Danian is considered to cover a hyperthermal, known as the "Latest Danian Event" (LDE; similar to 61.75 Ma) or "Top C27n Event" and has been unequivocally identified in benthic foraminiferal isotopes from shelf sediments in Egypt and deep-sea material from the Pacific Ocean. Stratigraphic changes in the isotope ratios of the genus Bairdia reveal a rather scattered record for delta C-13 lacking any pronounced negative delta C-13 excursion, probably due to the many factors influencing delta C-13 in ostracodes like metabolism, diet or dissolved inorganic carbon of sea-water. The accompanying delta O-18 record is less ambiguous showing a shift towards lower values close to the level where the LDE is to be expected, but minimum values are similar to delta O-18 values at the base and top of the studied sequence. These data suggest that the core of the negative stable isotope excursions of the LDE is lost in the stratigraphic gap at the planktic foraminiferal P3a/P3b and calcareous nannofossil NTp7A/NTp7B subzonal boundaries, so that the LDE could not be unequivocally identified in Tunisia. A cross-plot of delta C-13 and delta O-18 of ostracode valves displays distinctive clusters for smooth-shelled taxa like Bairdia and others as well as for ornamented taxa, with the latter group showing substantially lower values for both delta C-13 and delta O-18. This pattern strongly resembles those from early Eocene data from southwestern France and Recent ostracodes from the Iceland Plateau. The offsets suggest substantial differences in life style, food source or isotope fractionation during the calcification process for different shell morphologies in ostracodes.