The Eocene‐Oligocene Transition in the South‐Western Neo‐Tethys (Tunisia): Astronomical Calibration and Paleoenvironmental Changes. Issue 8 (20th August 2020)

• Record Type: Journal Article
• Authors: Messaoud, Jihede Haj; Thibault, Nicolas; Yaich, Chokri; Monkenbusch, Johannes; Omar, Hamdi; Jemai, Hela Fakhfakh Ben; Watkins, David K.
• Is Part Of: Paleoceanography and paleoclimatology. Volume 35:Issue 8(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-08-20
• Subjects: paleoclimate modeling -- calcareous nannofossils -- orbital forcing (eccentricity, obliquity) -- X‐ray fluorescence -- greenhouse‐icehouse transition; Paleoceanography -- Periodicals; Paleoclimatology -- Periodicals; 551.46
• DOI: 10.1029/2020PA003887
• Languages: English
• ISSNs: 2572-4517

Abstract

Abstract: A detailed biostratigraphic, chemostratigraphic, and chronostratigraphic study was carried out in the South‐Western margin of the Neo‐Tethys ocean (Tunisia) covering the Eocene‐Oligocene interval to report potential paleoenvironmental and paleoclimatic changes associated with the growth of the Antarctic ice sheet (AIS). The studied section consists of marls and limestones extending from the Helicosphaera compacta nannofossil Zone (CNE21) to the Reticulofenestra umbilicus Zone (CNO2). The Bc of Clausicoccus subdistichus coincides in northeastern Tunisia with the extinction of hantkeninids and is thus used here to define the Eocene/Oligocene boundary (EOB) at 34.03 Ma. Calcareous nannofossil assemblages suggest a significant change from oligotrophic/warm surface waters during the late Eocene to eutrophic/cool waters during the early Oligocene. The observed changes in trophic conditions are synchronous with a shallowing trend testified by an increase in nearshore species and enhanced terrigenous input. Major environmental changes occurred at the EOB and across glaciation event Oi‐1a whereas we record a poor imprint of glaciation event Oi‐1. Three orbital configurations have been distinguished: (1) The late Eocene is characterized by a prominent 405‐kyr cycle and a weak obliquity component; (2) the onset of cooling at 34.35 Ma coincides with the rosette‐shaped discoaster extinction and is reflected by a shift toward enhanced obliquity and short‐eccentricity cycles. The installation of a fully developed AIS (event Oi‐1a, 32.75 Ma) shows the most pronounced obliquity imprint reflecting an increased influence of high‐latitude climate processes; (3) after 31.75 Ma, the obliquity decreases in coincidence with the return to a dominant long‐eccentricity cycle. Plain Language Summary: The Eocene/Oligocene transition corresponds to one of the most important climatic shifts in the Cenozoic. This paleoclimatic shift from the greenhouse of the late Eocene to the icehouse regime in the early Oligocene triggered changes in the paleoenvironmental conditions of the Neo‐Tethyan area. We used calcareous nannofossil assemblages, carbon isotopes, elemental data (X‐ray fluorescence), and spectral analysis applied on magnetic susceptibility to report the possible effects of ice volume changes associated with the Antarctic glaciation on the Southern Neo‐Tethyan region and on the expression of Milankovitch cycles at low latitudes. We observe a change in the calcareous nannofossil assemblages that suggests a transition from low‐nutrient/warm‐water conditions to more heterotrophic/cooler conditions across the Eocene/Oligocene interval associated with the shift from a dominant 405‐kyr long‐eccentricity cycles to a prevalent obliquity and short‐eccentricity forcing. However, the situation shifted again at 31.75 Myr, with the return of dominant long‐eccentricity cycles. Key Points: High phytoplankton productivity across the EOT of the Southern Neo‐Tethys (Tunisia) Astronomical calibration of the Tunisian record and power variations in the expression of distinct orbital components across the EOT Major changes in trophic conditions and sediment composition at the EOB and across Oi‐1a but minor changes across the main Oi‐1 glaciation