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. Abstract Copyright (2011) Elsevier, B.V.