New evidence for abrupt climate change in the Cretaceous and Paleogene: An Ocean Drilling Program expedition to Shatsky Rise, northwest Pacific Timothy J. Bralower, Department of dence of the response of biogeochemical Geological Sciences, University of North cycling and the recovery of oceanic Carolina at Chapel Hill, Chapel Hill, North plankton in the wake of this catastrophe. Carolina 27599-3315, USA, A new biotic event of major evolutionary [email protected] significance was found in the early late Paleocene (ca. 58.4 Ma) associated with a Isabella Premoli Silva, Dipartimento di change in deep-water circulation, possi- Scienze della Terra, Università degli Studi bly as a result of a brief pulse of warming. di Milano, Via Mangiagalli 34, 20133 Abundant evidence of the Paleocene- Milano, Italy Eocene thermal maximum (PETM; ca. 55 Mitchell J. Malone, Ocean Drilling Ma), an abrupt warming event associated Program, Texas A&M University, 1000 with major reorganization of benthic and Discovery Drive, College Station, Texas planktonic communities, was recovered 77845-9547, USA in cores from five sites along a depth tran- Scientific Participants of Leg 198* sect. PETM warming is thought to have been induced by methane derived from dissociation of methane hydrates. The ABSTRACT Shatsky Rise depth transect shows evi- Sediments recovered during an Ocean dence of the predicted response of such Drilling Program Leg on Shatsky Rise in methane input: pronounced, short-term the northwest Pacific hold clues to a num- shoaling of the lysocline and calcite com- ber of abrupt climate events that took pensation depth (CCD). place during the Cretaceous and early Shatsky Rise cores record the response Paleogene. These transient events caused of the tropical Pacific to a rapid cooling major upheaval in marine communities event near the Eocene-Oligocene bound- and profoundly altered biogeochemical ary (ca. 33.5 Ma) marking the transition to cycling. Shatsky Rise cores contain or- glacial climates that characterized the re- Figure 1. Generalized climate curve for the ganic carbon-rich strata deposited during mainder of the Cenozoic. This event is re- Cretaceous and Paleogene derived from deep- a brief interval of open ocean dysoxia or flected by a marked increase in carbonate sea benthic oxygen isotope data (from Zachos anoxia in the early Aptian (120 Ma). content of the sediment preserved on et al., 1993, and unpublished). Also shown are Analyses of exceptionally preserved or- Shatsky Rise, which signifies a profound locations of events discussed including: Eocene- ganic compounds suggest that bacterial drop in the CCD and markedly changed Oligocene (E-O) transition, PETM—Paleocene- activity helped sequester organic carbon Eocene thermal maximum, late Paleocene deep-sea circulation patterns. in these strata. Graphic evidence exists in biotic event, K-T—Cretaceous-Tertiary Shatsky cores for the mid-Maastrichtian boundary, MME—mid-Maastrichtian event, INTRODUCTION OAE1a—early Aptian oceanic anoxic event. (ca. 69 Ma) extinction of the inoceramids, Predictions for modern global warming a long-ranging, widespread group of bot- resulting from increased CO2 levels have Paleocene-Eocene thermal maximum tom-dwelling clams. This extinction is a caused a heightened interest in the me- (e.g., Kennett and Stott, 1991) and global event that was likely related to a chanics of ancient warm climates and es- Cretaceous oceanic anoxic events (e.g., profound change in deep ocean circula- pecially of geologically abrupt warming Jenkyns, 1980). tion. Stratigraphically expanded records events. The mid-Cretaceous (ca. 80–120 Among the largest obstacles facing our of the Cretaceous-Tertiary boundary ex- Ma) and early Paleogene (ca. 45–60 Ma) understanding of the climate of the tinction event (65 Ma) were recovered at were characterized by some of the most Cretaceous and Paleogene is that many four different sites. The cores contain evi- equable climates of the Phanerozoic (Fig. good stratigraphic sections on land and in 1). In addition, these “greenhouse” inter- the oceans have been buried at depths ————— vals contain significant abrupt and transient * Michel A. Arthur, Kristen Averyt, Simon C. Brassell, Paul R. where diagenetic alteration has obscured Bown, James E.T. Channell, Leon J. Clarke, Andrea Dutton, warming events that led to major changes interpretations of stable isotope and other Jason W. Eleson, Tracy D. Frank, Susanne Gylesjö, Haidi in oceanic environments, profound Hancock, Harumasa Kano, R. Mark Leckie, Kathleen M. climate proxies. In many oceanic se- Marsaglia, Jennifer McGuire, Kyaw Thu Moe, Maria Rose turnover in marine communities, includ- quences, spot-coring, coring gaps, drilling Petrizzo, Stuart Robinson, Ursula Röhl, William W. Sager, ing extinction, and perturbations to global Kotaro Takeda, Deborah Thomas, Trevor Williams, and James disturbance, and hiatuses hinder detailed C. Zachos. chemical cycles. Examples include the studies of ancient climate. Site coverage is 4 NOVEMBER 2002, GSA TODAY Figure 2. Bathymetric map of Shatsky Rise showing location of Leg 198 sites. Site 1207 is located on the Northern High, Site 1208 is on the Central High, and Sites 1209 to 1214 are on the Southern High. uneven and almost nonexistent in some plan was to locate sites along depth and chemical cycling and marine ecosystems, regions, especially the tropics and the latitudinal transects to provide additional resulting in geographically extensive or Pacific Ocean. The aerial extent and im- dimensions to reconstructions of the global oxygen-deficient water masses. portance of the Pacific in global circula- paleoenvironment through time (Fig. 2). Corg-rich sediments are known to occur tion, however, make this a critical target Intermediate- and deep-water chemistry primarily in specific stratigraphic intervals for investigation of warm climatic intervals. (i.e., carbonate solubility, oxygenation) that have been termed oceanic anoxic One of the most promising locations in and circulation are sensitive to changes in events (OAEs: Schlanger and Jenkyns, the Pacific for recovering Cretaceous and climate and can be reconstructed using 1976). The ultimate trigger(s) of OAEs, Paleogene sediments at relatively shallow depth transects. One site each was drilled however, remain elusive. burial depths is Shatsky Rise (Fig. 2). This on the North and Central Highs of Shatsky Corg-rich sedimentary rocks at Sites feature, a medium-sized large igneous Rise (Sites 1207 and 1208, respectively) 1207 and 1213 (Fig. 4) are evidence for province in the west-central Pacific was and six were drilled on the Southern High OAE1a during the early Aptian (120 Ma) formed in the Late Jurassic and Early (Sites 1209–1214) (Bralower et al., 2002). (Arthur et al., 1990), an event that is well Cretaceous between 147 and 135 Ma An impressive 140 m.y. package of documented in Tethyan sections (Nakanishi et al., 1989). Shatsky Rise was pelagic sediment was recovered at depths (Coccioni et al., 1992). At Site 1207, the target of three Deep Sea Drilling between 170 and 623 m below the sea OAE1a is found within 45 cm of finely Project (DSDP) expeditions: Legs 6, 32, floor (Fig. 3). The Cretaceous and laminated, dark brown radiolarian clay- and 86. The latter leg drilled one site on Paleogene section recovered at sites stone. The Site 1213 Corg-rich units in- Shatsky Rise (Site 577), which was limited across the depth transect provides a clude clayey porcellanites and radiolarian to the Paleogene and uppermost unique opportunity to understand long- porcellanites with associated minor tuff. Maastrichtian. Some sites in the older legs term climate change on a warm Earth. At Site 1214, a black laminated claystone were spot-cored, and chert lowered re- However, the key success of the drilling unit contains a distinctive radiolarian as- covery in others, especially in the was the abundant evidence for short-lived semblage that suggests that the recovered Cretaceous. Yet even with an extremely (<1 m.y.) warming events, and other ma- sediments correlate to the OAE1a interval patchy record, analyses of Shatsky Rise jor intervals of rapid climate and environ- (e.g., Erbacher and Thurow, 1997), but sediments have provided key data in our mental change. low-Corg contents indicate the peak of the understanding of Cretaceous and Paleo- event was not recovered. gene climate; these data are especially A CLASSIC RECORD OF THE EARLY In Tethys early Aptian OAE1a corre- significant given that the rise was located APTIAN OCEANIC ANOXIC EVENT sponds to prominent C -rich horizons in tropical latitudes during this time period. The beginning of greenhouse climate org that were deposited in open ocean envi- Ocean Drilling Program (ODP) Leg 198 conditions in the mid-Cretaceous was as- ronments; for example, the original Selli in August–October, 2001 was designed to sociated with widespread deposition of level in Italy is in a truly pelagic section understand the causes, nature, and me- organic-carbon (Corg)-rich sediments, in- (Coccioni et al., 1992). However, the chanics of the long-term Cretaceous and formally known as black shales, in the same interval in the North Atlantic is not Paleogene “greenhouse,” as well as of oceans. These C -rich deposits were the org C rich (Bralower et al., 1994). C -rich transient but critical climate events during result of fundamental oceanographic org org this period. A key aspect of the drilling changes that drastically affected biogeo- horizons of OAE1a age have been found GSA TODAY, NOVEMBER 2002 5 tional conditions that led to enhanced se- questration of organic matter. Exceptional preservation of organic compounds, com- bined with lamination in sediments at Site 1207, indicate that conditions were highly dysoxic or anoxic at the time of deposi- tion. Rock-Eval analyses and gas chro- matography–mass spectrometry (GC-MS) of extractable hydrocarbons and ketones indicate that the organic matter is almost exclusively algal and bacterial in origin. GC-MS data show biomarkers associated with cyanobacteria. The prevalence and character of bacterial biomarkers suggest the existence of microbial mats at the time of deposition.