Early Paleogene d13C and d18O records based on marine ostracodes: implications for the upper Danian succession at Sidi Nasseur (Tunisia) and their application value in paleoceanography
Bornemann, A.; Pirkenseer, C.M.; Steurbaut, E.; Speijer, R.P. (2012). Early Paleogene d13C and d18O records based on marine ostracodes: implications for the upper Danian succession at Sidi Nasseur (Tunisia) and their application value in paleoceanography. Austrian J. Earth Sci. 105(1): 77-87
In: Austrian Journal of Earth Sciences = Mitteilungen der Österreichischen Geologischen Gesellschaft. Austrian Geological Society: Wien. ISSN 2072-7151; e-ISSN 2072-7151, more
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 (d13C, d18O) at Sidi Nasseur, W Tunisia. The late Danian is considered to cover a hyperthermal, known as the “Latest Danian Event” (LDE; ~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 d13C lacking any pronounced negative d13C excursion, probably due to the many factors influencing d13C in ostracodes like metabolism, diet or dissolved inorganic carbon of sea-water. The accompanying d18O 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 d18O 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 d13C and d18O 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 d13C and d18O. 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.
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