Stefaan Van Simaeys

Evidence for an earliest Oligocene ice sheet on the Antarctic Peninsula

There is growing consensus that development of a semipermanent ice sheet on Antarctica began at or near the Eocene-Oligocene (E-O) boundary. Beyond ice-rafted debris in oceanic settings, however, direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to East Antarctica. It is unclear where glacier ice first accumulated and how extensive it was on the Antarctic continent in the earliest Oligocene. Sediments at the top of the Eocene marine shelf section on Seymour Island, Antarctic Peninsula, include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula. Dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the E-O boundary. Glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive West Antarctica ice sheet, and thus an even more dramatic response to the forcing factors that facilitated high-latitude ice expansion in the earliest Oligocene.

Integrated stratigraphy of the Oligocene pelagic sequence in the Umbria-Marche basin (northeastern Apennines, Italy): A potential Global Stratotype Section and Point (GSSP) for the Rupelian/Chattian boundary

The Oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations. The pelagic succession of the Umbria-Marche Apennines (central Italy) includes a complete and continuous sequence of marly limestones and marls, with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period. We present here a synthesis of detailed biostratigraphic, magnetostratigraphic, and chemostratigraphic studies, along with geochronologic results from several biotite-rich volcaniclastic layers, which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. From this study, the interpolated ages for the Rupelian/Chattian stage boundary, located in the upper half of Chron 10n at meter level 188 in the Monte Cagnero section, and corresponding to the O4/O5 planktonic foraminiferal zonal boundary, are 28.36 Ma (paleomagnetic interpolation), 28.27 ± 0.1 Ma (direct radioisotopic dating), and 27.99 Ma (astrochronological interpolation). These ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations. The Monte Cagnero section is a potential candidate for defining the Chattian Global Stratotype Section and Point (GSSP) and some reliable criteria are here proposed for marking the Rupelian/Chattian boundary according to International Union of Geological Sciences (IUGS) recommendations.

DINOFLAGELLATE CYSTS FROM THE MIDDLE EOCENE TO ?LOWERMOST OLIGOCENE SUCCESSION IN THE KYSING RESEARCH BOREHOLE, CENTRAL DANISH BASIN

Abstract Rich and well-preserved assemblages of organic walled dinoflagellate cysts in 50 samples from a 154 m thick and almost complete, Middle Eocene to ?lowermost Oligocene section from the Kysing Research Borehole in eastern Jylland, Denmark are systematically described. One genus Costacysta gen. nov., and twelve species Chiropteridium eocaenicum sp. nov., Costacysta bucina gen. et sp. nov., Distatodinium pilosum sp. nov., Echinidinium? lucidum sp. nov., Hapsocysta kysingensis sp. nov., Horologinella? pentagonalis sp. nov., Operculodinium eisenackii sp. nov., Phthanoperidinium cornutum sp. nov., Selenopemphix septum sp. nov., Svalbardella partimtabulata sp. nov., Thalassiphora gracilis sp. nov. and Thalassiphora microperforata sp. nov. are formally described. The genus Hapsocysta is emended, and is considered to be a senior synonym of Piccoladinium. The gradual change of the dinoflagellate cyst assemblages in this section demonstrates that sedimentation was almost continuous during the time span of ca. 12 Ma. The biostratigraphic resolution based on dinoflagellate cyst events is generally finer than the calcareous nannofossil NP biozonation. A comparison with published dinoflagellate cyst data from the Norwegian–Greenland Sea shows that several important events are mainly synchronous throughout the region, suggesting a general similarity of the water masses. An isolated occurrence of Svalbardella suggests a short cooling event during early Bartonian times. The palynofacies in most of the section is almost purely of pelagic type. A more proximal, mid shelf palynofacies and a distinctive episode of reworking coincide with the Upper Eocene Moesgaard Clay and indicate a brief, but rather large sea level fall during the deposition of this unit. The Eocene/Oligocene boundary is tentatively identified near the base of the Viborg Formation, and approximately coincides with increased reworking of dinoflagellate cysts and a strong increase in bisaccate pollen, indicating more proximal conditions and possibly also a climatic cooling.

Arctic dinoflagellate migrations mark the strongest Oligocene glaciations

Here we report on mid-Oligocene globally synchronous Arctic dinoflagellate migration events, calibrated against chron C9n. We show that sudden appearances and marked abundance increases of the Arctic taxon Svalbardella at lower and middle latitudes coincide with the Oi-2b benthic d 18 O glacial episode, dated as ca. 27.1 Ma. These unprecedented migrations are taken to indicate anomalously strong surface-water cooling during Oi-2b time, in turn associated with strong concomitant Antarctic ice-sheet growth and sea-level lowering. We estimate the duration of these unique Svalbardella migrations and the associated episode of profound cooling as ;500 k.y. Our records suggest a close link between this distinct Oligocene glaciation episode, strong sea-level fall, and the classic lower-upper Oligocene, or Rupelian-Chattian, boundary, dating this boundary as ca. 27.1 Ma.