Universita di Milano, Italy

The occurrence of the PETM demonstrates that short-term, rapid climate change is possible during a period of greenhouse climate in the absence of climate feedbacks that depend on the presence of polar ice. An obvious question is whether such rapid climate change occurred only once, or whether it occurred more often, possibly modulated by Milankovitch forcing. The primary objective of Ocean Drilling Program Leg 198 on Shatsky Rise in the central Pacific was to understand long-term transitions into and out of the warm-climate "greenhouse", as well as transient but critical events that involved major changes in oceanic environments, geochemical cycling, and marine biota. Complete sections of the early Paleogene were recovered at four sites (Sites 1209 to 1212) spanning a modern depth range of more than 500 meters from 2387 m at Site 1209 to 2907 m at Site 1211. These sites provide an ideal opportunity to test the response of the Pacific lysocline and CCD to events like the PETM as further, unknown paleoceanographic events, like a new mid-Paleocene biological event we describe here. A prominent, 5 to 25 cm-thick, dark-brown, nannofossil ooze with clay was found in cores at four sites (Sites 1209 to 1212), this layer shows a sharp magnetic susceptibility increase. Shipboard micropaleontological investigations suggest that this interval may represent a previously unrecognized event of considerable evolutionary significance. This horizon lies within planktonic foraminiferal Zone P4 and coincides exactly with the evolutionary first occurrence of the nannolith Heliolithus kleinpellii an important component of the late Paleocene assemblages and a marker for the base of Zone CP5. Planktonic foraminifera are characterized by a low diversity, largely dissolved assemblage, dominated by representatives of Igorina (mainly I. tadjikistanensis and I. pusilla). The composition of the assemblage suggests some kind of oceanic perturbation. Biostratigraphic (foraminifera, calcareous nannoplankton) data has been combined with measurements of major chemical elements and bulk stable isotopes to understand the oceanographic conditions that gave rise to this mid-Paleocene event as well as the detailed evolutionary relationships of the key taxa.