4 NOVEMBER 2002, GSA TODAY Timothy J. Bralower, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA, bralower@email.unc.edu Isabella Premoli Silva, Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy Mitchell J. Malone, Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station, Texas 77845-9547, USA Scientific Participants of Leg 198* ABSTRACT Sediments recovered during an Ocean Drilling Program Leg on Shatsky Rise in the northwest Pacific hold clues to a num- ber of abrupt climate events that took place during the Cretaceous and early Paleogene. These transient events caused major upheaval in marine communities and profoundly altered biogeochemical cycling. Shatsky Rise cores contain or- ganic carbon-rich strata deposited during a brief interval of open ocean dysoxia or anoxia in the early Aptian (120 Ma). Analyses of exceptionally preserved or- ganic compounds suggest that bacterial activity helped sequester organic carbon in these strata. Graphic evidence exists in Shatsky cores for the mid-Maastrichtian (ca. 69 Ma) extinction of the inoceramids, a long-ranging, widespread group of bot- tom-dwelling clams. This extinction is a global event that was likely related to a profound change in deep ocean circula- tion. Stratigraphically expanded records of the Cretaceous-Tertiary boundary ex- tinction event (65 Ma) were recovered at four different sites. The cores contain evi- dence of the response of biogeochemical cycling and the recovery of oceanic plankton in the wake of this catastrophe. A new biotic event of major evolutionary significance was found in the early late Paleocene (ca. 58.4 Ma) associated with a change in deep-water circulation, possi- bly as a result of a brief pulse of warming. Abundant evidence of the Paleocene- Eocene thermal maximum (PETM; ca. 55 Ma), an abrupt warming event associated with major reorganization of benthic and planktonic communities, was recovered in cores from five sites along a depth tran- sect. PETM warming is thought to have been induced by methane derived from dissociation of methane hydrates. The Shatsky Rise depth transect shows evi- dence of the predicted response of such methane input: pronounced, short-term shoaling of the lysocline and calcite com- pensation depth (CCD). Shatsky Rise cores record the response of the tropical Pacific to a rapid cooling event near the Eocene-Oligocene bound- ary (ca. 33.5 Ma) marking the transition to glacial climates that characterized the re- mainder of the Cenozoic. This event is re- flected by a marked increase in carbonate content of the sediment preserved on Shatsky Rise, which signifies a profound drop in the CCD and markedly changed deep-sea circulation patterns. INTRODUCTION Predictions for modern global warming resulting from increased CO 2 levels have caused a heightened interest in the me- chanics of ancient warm climates and es- pecially of geologically abrupt warming events. The mid-Cretaceous (ca. 80–120 Ma) and early Paleogene (ca. 45–60 Ma) were characterized by some of the most equable climates of the Phanerozoic (Fig. 1). In addition, these “greenhouse” inter- vals contain significant abrupt and transient warming events that led to major changes in oceanic environments, profound turnover in marine communities, includ- ing extinction, and perturbations to global chemical cycles. Examples include the Paleocene-Eocene thermal maximum (e.g., Kennett and Stott, 1991) and Cretaceous oceanic anoxic events (e.g., Jenkyns, 1980). Among the largest obstacles facing our understanding of the climate of the Cretaceous and Paleogene is that many good stratigraphic sections on land and in the oceans have been buried at depths where diagenetic alteration has obscured interpretations of stable isotope and other climate proxies. In many oceanic se- quences, spot-coring, coring gaps, drilling disturbance, and hiatuses hinder detailed studies of ancient climate. Site coverage is ————— * Michel A. Arthur, Kristen Averyt, Simon C. Brassell, Paul R. Bown, James E.T. Channell, Leon J. Clarke, Andrea Dutton, Jason W. Eleson, Tracy D. Frank, Susanne Gylesjö, Haidi Hancock, Harumasa Kano, R. Mark Leckie, Kathleen M. Marsaglia, Jennifer McGuire, Kyaw Thu Moe, Maria Rose Petrizzo, Stuart Robinson, Ursula Röhl, William W. Sager, Kotaro Takeda, Deborah Thomas, Trevor Williams, and James C. Zachos. New evidence for abrupt climate change in the Cretaceous and Paleogene: An Ocean Drilling Program expedition to Shatsky Rise, northwest Pacific Figure 1. Generalized climate curve for the Cretaceous and Paleogene derived from deep- sea benthic oxygen isotope data (from Zachos et al., 1993, and unpublished). Also shown are locations of events discussed including: Eocene- Oligocene (E-O) transition, PETM—Paleocene- Eocene thermal maximum, late Paleocene biotic event, K-T—Cretaceous-Tertiary boundary, MME—mid-Maastrichtian event, OAE1a—early Aptian oceanic anoxic event.