Abstract:
Paleoceanographic condition in the deep-sea ocean during the Eocene-Oligocene transition is affected by change in ice volume in the Antarctic region and deep-water formation in the Southern Ocean. ODP Leg 199 in the eastern equatorial Pacific Ocean recovered continuous sections containing well-preserved microfossils during the Eocene-Oligocene interval, and provides an opportunity to study this transition. In this paper, we review recent results of the Oligocene paleoceanography and outlines of faunal change in benthic foraminifers in the eastern equatorial Pacific Ocean. Sedimentary records from ODP Leg 199 represent a deepening of calcite compensation depth (CCD) more than 1 km near the Eocene/Oligocene boundary in the eastern equatorial Pacific Ocean. Around the interval of CCD change, two-step positive sifts of oxygen isotope occurred in benthic foraminifers within approximately 300 kyrs. The glaciation events marked by the Oil isotope event coincide with the intervals of low eccentricity and low obliquity amplitude variations, due to absence of warm summer. Other three glacial events of 29.16, 27.91 and 26.76 Ma during the Oligocene are probably related to low obliquity amplitude variation fluctuating with about a 1200 kyr.-cycle. Abyssal benthic foraminifers show an increase in Antarctic-bottom water fauna (Nuttallides umbonifer) around the Early/Late Oligocene boundary in the eastern equatorial Pacific Ocean. The short-term abundance peaks of Nuttalides umbonifer are correlative with the Oi event, suggesting that this species could be useful as a proxy of the Antarctic ice-sheet expansions.