Danian (lower Paleocene) sequences in north-central Tunisia are dominantly composed of marls and shales but a conspicuous, indurated glauconite-bearing marker bed is associated with the P3a–P3b transition. This glauconite bed is considered to correlate with the Latest Danian Event (LDE) described from the Nile Basin in Egypt, with the ‘top Chron C27n event’ (Atlantic and Pacific Oceans) and with the ‘CIE-DS1’ (Zumaia, Spain). The LDE is thought to reflect a short period of global warming, similar to the Paleocene Eocene Thermal Maximum, but of lesser magnitude. The presence of a glauconitic bed at the P3a/P3b transition in Tunisia suggests that the sequence is condensed, which is confirmed by planktic foraminifer and nannofossil biostratigraphy, and by the absence of the typical LDE beds found in most Egyptian sections. Benthic foraminiferal assemblages were quantitatively analyzed in three sections in north-central Tunisia (Sidi Nasseur, Garn Halfaya, Elles) in order to characterize the paleoenvironmental evolution during the late Danian and compare this with sections in Egypt. The benthic foraminiferal assemblages indicate that the Tunisian sections were located along a depth- and a paleoenvironmental gradient on the shelf north of Kasserine Island. The reconstructed depth range is comparable with sections in Egypt, and encompasses middle neritic (Sidi Nasseur section) to shallow upper bathyal depths (Elles section,with up to 16% Gavelinella beccariiformis below the P3a/P3b). Like in Egypt, assemblage shifts across the P3a/P3b subzonal transition indicate shallowing and a transition to amore eutrophic paleoenvironment, characterized by relatively high abundances of buliminids and Stainforthia sp. The δ13C and δ18O records generated on well-preserved specimens of the ostracode species Bairdia failed to demonstrate the presence of the carbon isotope excursion (CIE) associated with the LDE at the base of Subzone P3b. A condensed section or a hiatus associated with the glauconite bed would explain why the CIE is not recorded in the Tunisian sections.
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The Global Boundary Stratotype Section and Point for the base of the Selandian Stage is defined in the Zumaia section (Spain) at an abrupt change in lithology (base of Itzurun Formation), which coincides with the onset of a negative carbonate carbon isotope shift. However, this lithological change is not always very well expressed in other sections. In order to document the stratigraphic position of the Danian/Selandian boundary (DSB) on a more global scale, we have investigated three sections across the DSB, the Zumaia reference section (GSSP), the Loubieng section (auxiliary DSB reference section, France) and the Sidi Nasseur section (Tunisia). The Danian/Selandian boundary interval is subdivided and correlated throughout low latitudes, from the Altlantic Bay of Biscay to the Southern Tethys, on the basis of seven calcareous nannofossil and planktonic foraminiferal events (E-events). The base of the Selandian is proved to coincide with the end of the Braarudosphaera acme, which correlates with the lowest consistent occurrence (LCsO) of Lithoptychius aff. bitectus (=Fasciculithus janii sensu Steurbaut and Sztrákos, 2008) (event E4), but which is slightly posterior to the second radiation of the fasciculiths, up to now considered to represent the primary correlation tool of the DSB. A short-term δ13Corg negative excursion, associated with an increase in pCO2 is recorded at the very base of the Selandian. It is interpreted as a short period of global warming (hyperthermal), the duration of which is estimated at ~30 kyr. It is followed in all the three studied sections by a long-term decoupled carbon isotope event, marked by increasing δ13Corg and decreasing δ13Ccarb values. It may reflect a period of climatic cooling of a few 100 kyr, interpreted as a possible precursor of the global cooling event, marking the late Paleocene in the North Atlantic realm. The integration of the biostratigraphic and the isotope data indicates major differences in sedimentation rates during the early Selandian in the studied sections, but there is no evidence of substantial breaks in sedimentation in any of the sections during this interval. The lithological shift at the base of the Selandian points to an abrupt palaeoenvironmental reorganisation, although our integrated bio-chemostratigraphical investigation does not allow for estimating its duration nor the presence of a hiatus at that time.
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