MOLLUSCAN BIODIVERSITY AND STABLE ISOTOPIC EVIDENCE FOR THE COLD LATE OLIGOCENE ARCTIC OCEAN

The onset of an “Icehouse” conditions in the Early Oligocene was a single most important turning point in the evolution of the global Cenozoic climate. New evidence from the Arctic Alaska provides new evidence about the extent and degree of the Oligocene chill in the Arctic Ocean. The taxonomic composition of the late Oligocene biota of the Nuwok member of the Sagavanirktok Formation in the North Slope of Alaska suggests that a relatively cold marine climate prevailed there and has persisted in the Arctic Ocean to the present day. A notable and possibly unique aspect of the Nuwok molluscan fauna is that all but one of its genera, and possibly one species, are still extant in the Arctic Ocean or adjacent parts of the high-latitude North Atlantic. In contrast to the Oligocene deposits worldwide, the Nuwok beds contain only one species that is a remnant of cosmopolitan Eocene faunas. Similar patterns were observed for ostracod assemblages.

Late Oligocene sea surface temperatures were evaluated using oxygen isotope ratios of the benthic bivalves. Oxygen stable isotopic record of Thyasira alaskana and Arctica carteriana from the Nuwok type section provide an evidence for a seasonal range of temperatures from 1.04 to 9.2^oC. Late Oligocene Arctic Ocean temperatures appear to be slightly warmer than the modern Beaufort Sea, but significantly colder than most known to date Paleogene sea-temperatures. Paleotemperature data strongly imply that the Arctic Ocean may have became cold much earlier than it was previously thought. Our preliminary data suggest that the Arctic Ocean may have been a significant center of evolution for arctic taxa. As a result of dwelling in a continuously cold ocean, perhaps Arctic Ocean mollusks did not evolve as fast or as profoundly, from the late Oligocene to the Holocene, as have mollusks elsewhere in the world ocean. The presence of modern-looking Nuwok mollusks and ostracodes in a context of late Oligocene benthic foraminifers and Sr data imply that arctic heterochroneity is real, at least for shelf biotas, and possibly implies different rates of evolution between these invertebrate groups.