Author's personal copy Early Oligocene cooling reflected by the dinoflagellate cyst Svalbardella cooksoniae Katarzyna K. Śliwińska ⁎, Claus Heilmann-Clausen Department of Earth Sciences, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark abstract article info Article history: Received 13 September 2010 Received in revised form 31 January 2011 Accepted 20 February 2011 Available online 26 February 2011 Keywords: Dinoflagellate cysts Eastern North Sea Early Oligocene Oi1a cooling event Paleoclimate Sea level change Svalbardella cooksoniae Areosphaeridium diktyoplokum The early Oligocene interval in several boreholes from the eastern North Sea and Denmark has been studied for dinoflagellate cysts. The cold water dinoflagellate cyst Svalbardella cooksoniae was recorded in a narrow lowermost Oligocene interval in all offshore sections. A critical evaluation of previous records of Svalbardella cooksoniae reveals that it is present in the same narrow interval of Chron 12r, close to the NP21/NP22 boundary in many high and mid latitude Northern Hemisphere sections, ranging from the Greenland Sea in the north to Italy in the south. The Svalbardella cooksoniae interval is closely associated with the LO of the dinoflagellate cyst Areosphaeridium diktyoplokum. The age of the Svalbardella cooksoniae event indicates that it is synchronous with the Oi1a oxygen isotope maximum of Pekar and Miller (1996) and Pekar et al. (2002). In the Danish land area the Svalbardella cooksoniae interval and hence the Oi1a event is shown to coincide with an unconformity. A regional comparison shows that synchronous unconformities are present also in Belgium and New Jersey. A slightly older regional unconformity occurring on both sides of the North Atlantic coincides with the Oi1 oxygen isotope maximum of Pekar and Miller (1996). The finding of a sedimentary package separated by two successive regional unconformities is interpreted to reflect a significant, temporary eustatic sea level rise, and hence a melting phase on the East Antarctic ice sheet between the Oi1 and Oi1a glacioeustatic sea level falls. The paleogeographic distribution and lower frequency of Svalbardella during the earliest Oligocene Oi1a event, as compared with the mid Oligocene Oi2b Svalbardella spp. event, suggest that the Oi1a glacial episode was less severe than the Oi2b glaciation. This is in agreement with recent ice volume estimates for the two events. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The global benthic foraminifera isotope record reveals two strong cooling events at the beginning of the Oligocene, as indicated by the Oi1 and Oi1a oxygen isotope (δ 18 O) maxima of Pekar et al. (2002; and references therein). The early Oligocene cooling reflects the first major Antarctic glaciation and is the most distinctive cooling phase in the Paleogene (Zachos et al., 2001) The Oi1 cooling event is dated to magnetochron C13n and the slightly younger Oi1a event is dated to the early part of Chron C12r (Pekar et al., 2002). Based on data from offshore New Jersey, Pekar and Miller (1996) linked the Oi1 event with a significant global eustatic sea level fall and the TA4.3/TA4.4 third order sequence boundary of Haq et al. (1987). Pekar et al. (2001) showed that also the Oi1a oxygen maximum coincides with an unconformity. Both unconformities were therefore interpreted as effects of glacioeustatic sea-level falls. An important earliest Oligocene dinoflagellate event of the Northern Hemisphere is the LO (last occurrence) of Areosphaeridium diktyoplokum, which has been used for recognition of the Eocene– Oligocene boundary and for interregional correlation (e.g., Schiøler, 2005; Van Mourik and Brinkhuis, 2005). Van Mourik and Brinkhuis (2005) suggested that in Central Italy the Oi1 cooling event is syn- chronous with the LO of A. diktyoplokum. In the southern North Sea Basin the approximate stratigraphic position of the early Oligocene cooling was constrained on basis of oxygen isotope data by De Man et al. (2004). According to these authors, the lowermost part of the Oligocene (mid nannofossil zone NP21, below the LO of Areosphaeridium diktyoplokum) was deposited under warm conditions whereas somewhat younger sediments, above the base of NP23, were deposited under a colder climate. Furthermore, in the same area, an unconformity at the NP21/22 transition was recognized by Vandenberghe et al. (2003) as the TA4.3/TA4.4 sequence boundary. De Man et al. (2004) suggested that the cooling event coincided with the unconformity level. The arctic dinoflagellate Svalbardella is generally considered as a reliable cold water indicator (e.g., Head and Norris, 1989; Sluijs et al., 2005). Episodes of southward migration have therefore been inter- preted to indicate cooling events. The best documented southward migration event took place in the mid Oligocene (Van Simaeys et al., 2005a). The mid Oligocene Svalbardella interval was shown by Van Simaeys et al. (2005a) to be related to the Oi2b cooling event (Pekar Palaeogeography, Palaeoclimatology, Palaeoecology 305 (2011) 138–149 ⁎ Corresponding author. E-mail address: kasia.sliwinska@geo.au.dk (K.K. Śliwińska). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.02.027 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo