<<

New evidence for abrupt climate change in the and : An 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 (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, thermal maximum (PETM; ca. 55 Mitchell J. Malone, Ocean Drilling Ma), an abrupt warming event associated Program, 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 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 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- 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 . This event is re- Cretaceous and Paleogene derived from deep- a brief interval of open ocean dysoxia or flected by a marked increase in sea benthic oxygen isotope data (from Zachos anoxia in the early (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 content of the sediment preserved on et al., 1993, and unpublished).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- Shatsky cores for the mid- boundary, MME—mid-Maastrichtian event, INTRODUCTION OAE1a—early Aptian oceanic . (ca. 69 Ma) 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 (Fig. good stratigraphic sections on land and in 1). In addition, these “greenhouse” inter- the 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, , 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 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 , regions, especially the 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 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 - 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 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 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 , in the same interval in the North 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 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. -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. Compounds identified in Leg 198 sediments also include the old- est known alkenones, a signature of hap- tophyte algae (S.C. Brassell, 2002, per- sonal commun.). Thus, biomarker data indicate that profound changes in prokaryote and protistan populations were intimately associated with processes that led to sequestration of Corg during OAE1a. Further studies of the well-pre- served organic compounds are planned to elucidate these processes. At Sites 1207 (~1.3 km paleodepth dur- ing OAE1a) and 1213 (~2.8 km pale- odepth), the Corg-rich units lack carbon- ate, but calcareous sediments occur directly underneath the Corg-rich sedi- ments at Site 1213, indicating that the cal- cite compensation depth (CCD) shoaled by at least 1.5 km during the event. The magnitude of the change in the CCD dur- ing OAE1a was at least partially a result of increased rates of CO2 outgassing that may also be directly responsible for global warmth at this time (e.g., Arthur et al., 1985; Larson, 1991).

EXTINCTION EVENTS IN THE MID- MAASTRICHTIAN AND AT THE Figure 3. Summary of and lithologic succession from Sites 1207 to 1214. Lithology is plotted against time to show duration of periods of deposition and location of unconformities. CRETACEOUS-TERTIARY BOUNDARY Southern High Sites 1209–1214 are ordered by water depth. Arrows show stratigraphic position Stable isotope evidence indicates that of transient events discussed (see Fig. 1 for abbreviations). cooling in the was inter- rupted by a significant event in the mid- in a number of other locations in the Leg 198 Corg-rich units represent the most Maastrichtian at 69 Ma when the source Pacific Ocean, but only DSDP Site 463 pelagic records of OAE1a outside of of deep waters changed abruptly from (Mid Pacific Mountains) and ODP Site 866 Tethys, and provide important informa- low to high latitudes (e.g., MacLeod and (Resolution ) have good recovery tion about the nature of environmental Huber, 1996). This event appears to have (Sliter, 1989; Jenkyns, 1995). Both of these change during the event. coincided with the extinction of the in- sites have a shallow-water influence: Site The Corg contents of lower Aptian inter- oceramid bivalves (MacLeod et al., 1996). 866 is located in shallow-water carbon- vals from Sites 1207 and 1213 (Fig. 4) are Growing evidence, however, suggests ates and Site 463 has a considerable frac- among the highest ever recorded in that the Inoceramus extinction is di- tion of material derived from shallow- pelagic Cretaceous sequences. They attest achronous. Moreover, the magnitude and water carbonate environments. Thus the to the extraordinary nature of the deposi- direction of stable isotope changes are quite variable at different sites (Frank and

6 NOVEMBER 2002, GSA TODAY tems is still not completely constrained. A remarkable set of cores was taken across the K-T boundary on the Southern High at Sites 1209, 1210, 1211, and 1212 (Fig. 5). The lithologic sequence in the K-T boundary interval is similar at all of these sites (Fig. 5). The boundary succession in- cludes uppermost Maastrichtian (nanno- Zone CC26) white to very pale or- ange, slightly indurated, nannofossil ooze overlain by an 8–12-cm-thick layer of Paleocene (foraminiferal Zone Pα) grayish orange foraminiferal ooze. This layer grades into 19–23-cm-thick white foraminiferal nannofossil , then into grayish orange nannofossil ooze. The boundary between the uppermost Maastrichtian and the lowermost Paleocene is clearly bioturbated as shown by the irregular nature of the contact and the pale orange burrows that extend as much as 10 cm down into the white Maastrichtian ooze (Fig. 5). Sampling of the deepest sections of the burrows of Paleocene ooze within the uppermost Maastrichtian yields highly abundant, minute planktonic foraminiferal assem- blages that are dominated by Guembelitria with rare Hedbergella holmdelensis, suggesting a possible Zone P0 age (Smit, 1982). Burrows also contain common light brown to spherules up to 100–150 µm in diameter with textures similar to the spherules composed of and Figure 4. Cores of early Aptian OAE1a interval recovered on Leg 198. Core photo, % carbonate, from the K-T boundary in other locations % C , and hydrogen indices for lower Aptian sedimentary rocks recovered at Sites 1207, 1213, org (Smit and Romein, 1985). and 1214. Note that Sites 1207 and 1213 recovered C -rich intervals that represent OAE1a. org The substantial thickness of the upper- most Maastrichtian M. prinsii (CC26) Arthur, 1999), possibly as a result of un- open ocean setting is not currently under- Zone and the lowermost P. eugu- certainties in stratigraphic correlation or of stood. However, such occurrences have bina (Pα) Zone indicates that the K-T true differences in deep-water properties. previously been noted in the Pacific boundary is expanded compared to the Thus the relationship between the extinc- (MacLeod et al., 1996) and the strati- majority of deep-sea sites (the Pα Zone is tion event and changing deep-water graphic position suggests they are related either unrecovered or poorly preserved at properties is not firmly established. to the Inoceramus extinction and deep- most other deep-sea sites). Moreover, the An unusual record of the mid- water changes in the mid-Maastrichtian Zone Pα interval in Shatsky cores Maastrichtian event was observed in the determined at other deep-sea locations similarities to other sites such as ODP Site sedimentary record at two sites on the (e.g., Barrera et al., 1997; Frank and 1049 (western North Atlantic), where the Southern High of Shatsky Rise. At Sites Arthur, 1999). Benthonic and planktonic correlative interval corresponds to a dark, 1209 and 1210, large Inoceramus shell foraminiferal isotope and assemblage data burrow-mottled clay underneath 5–15- fragments are common for several meters, from Shatsky Rise will help characterize cm-thick white foraminiferal nannofossil but disappear abruptly. This disappear- changes in deep- and surface-water prop- ooze (Norris et al., 1998). A similar white ance is in the same stratigraphic position erties as well as constrain the timing and unit is found directly above the boundary at both sites. Furthermore, isolated origin of the extinction. at DSDP Site 536 (; Buffler Inoceramus prisms were recovered in The origin of at the K-T et al., 1984), and ODP Sites 999 and 1001 foraminiferal separates at correlative lev- boundary (65 Ma) is well understood, (Caribbean; Sigurdsson et al., 1997). The els at Site 1211. The significance of the however, the effect of the event on bio- ultrafine micrite in this oceanwide white short range of visible specimens in this geochemical cycling and marine ecosys- layer may be related to the collapse of the

GSA TODAY, NOVEMBER 2002 7 Figure 5. The Cretaceous-Tertiary boundary on Shatsky Rise. Arrows show level of paleontological boundary as recognized by planktonic foraminiferal biostratigraphy (see text for details).

Figure 6. The Paleocene-Eocene thermal maximum (PETM) on Shatsky Rise. Arrows show onset of event as recognized by major changes in carbonate content and preservation (confirmed by the presence of bulk sediment carbon isotope excursion). Flags show top of clay-rich ooze horizon. PETM at Site 1208 is recognized by preservational change and confirmed by bulk sediment carbon isotope stratigraphy. Sites are organized by present (and paleo) water depth.

8 NOVEMBER 2002, GSA TODAY marine biosphere and inorganic produc- ered at Site 1208 on the Central High. At would generate CO2, which would lower tion of carbonate in the surface ocean the Southern High sites, the PETM corre- the saturation state of with re- (e.g., Kump, 1991), a hypothesis that re- sponds to an 8–23-cm-thick layer of yel- spect to calcite and cause a dramatic quires further testing. The Leg 198 sections lowish brown clayey nannofossil ooze shoaling in the depth of the lysocline and represent some of the best-preserved and with a sharp base and a gradational up- CCD. This response should be recorded least-disrupted deep-sea records of the per contact. The clay-rich layer is often in changes in carbonate content and K-T and the subsequent bioturbated into the underlying sediment. preservation in sections below the mid- biotic radiation. A thin (1 mm) dark brown clay seam lies slope. Shallower sections should show at the base of the PETM in several holes. less change in dissolution and carbonate KEY EVIDENCE FOR ABRUPT, Preliminary biostratigraphy and stable- content than deeper sections. The range TRANSIENT WARMING EVENTS IN isotope stratigraphy suggest that the of present water depths (PETM paleo- THE PALEOGENE PETM is complete. This biostratigraphy depths were broadly similar), from 2387 An abrupt warming event is well also shows that the PETM interval at the m at Site 1209 to 3346 m at Site 1208, documented at the Paleocene-Eocene Southern High sites is condensed com- provides a significant transect to observe boundary (the Paleocene-Eocene thermal pared to continental-margin records from changes in dissolution at the PETM as a maximum [PETM; 55 Ma]). However, a the Atlantic and Tethys (e.g., Kennett and function of depth. number of other intervals of rapid tem- Stott, 1991), but somewhat expanded Nannofossil preservation is moderate to perature increase, or hyperthermals, akin compared to other deep-sea sites. At the good below the PETM at all of the to the PETM although smaller in magni- relatively deep Site 1208, biostratigraphic Southern High sites, indicating that they tude, may also exist in the midst of the and bulk stable isotopic data confirm were located in the upper part of the warm early Paleogene (Thomas et al., that the recovered PETM is a highly con- lysocline. All sites show a short-lived de- 2000). Leg 198 discovered a new transient densed (~3 cm) record. terioration in nannofossil preservation at climate event of evolutionary significance The PETM interval at all of the sites the onset of the event. Carbonate con- in the early late Paleocene at ca. 58.4 Ma. contains a clear record of nannofossil and tents have been measured in detail across A prominent clay-rich ooze found at Sites planktonic foraminiferal assemblage the PETM at Site 1210. These data record 1209, 1210, 1211, and 1212 coincides with transformation at this time of environ- a decrease from ~96 to ~86 wt% CaCO3 the evolutionary first occurrences of mental upheaval. One of the dominant at the base of the event, a change that Heliolithus kleinpellii and primitive dis- nannolith genera, Fasciculithus, is re- would involve a substantial increase in coasters, both of which are important, placed by Zygrhablithus bijugatus, a dissolution, indicating a shoaling of the and often dominant, components of late holococcolith that is often a lysocline. Shallower sites (Sites 1209, Paleocene and younger nannoplankton highly abundant component of Eocene 1210, 1212) show less lithologic and fossil assemblages. Planktonic foraminifers in assemblages. The genus is preservational change at the base of the the clay-rich layer are characterized by a highly abundant, likely as a result of PETM than deeper sites (Sites 1208, 1211) low diversity, largely dissolved assem- warming or increased oligotrophy (Fig. 6); changes in carbonate solubility at blage dominated by representatives of the (Bralower, 2002). Also found are abun- the onset and the termination of the event genus Igorina (mainly I. pusilla and I. dant calcispheres, which are possibly are more marked at the deep sites, sug- tadjikistanensis). The clay-rich layer con- calcareous resting cysts produced by gesting that they were close to the CCD tains common crystals of phillipsite, dinoflagellates at times of environmental as it shoaled. The Shatsky Rise depth tran- teeth, and phosphatic micronodules. The perturbation. Planktonic foraminiferal sect shows clear evidence for an abrupt abundance of phillipsite and fish teeth assemblages contain an ephemeral group rise in the level of the lysocline and CCD suggests either very slow sedimentation of ecophenotypes or short-lived species during the PETM, and thus supports the or intervals of seafloor exposure, possibly of the genera Acarinina and Morozovella predicted ocean response to massive resulting from pervasive dissolution of (Kelly et al., 1996). methane input. carbonate. Even though microfossil assem- The depth transect strategy of Leg 198 blages are clearly altered by dissolution, was specifically designed to address the THE END OF THE GREENHOUSE: they appear to record a significant environ- response of the ocean to the greenhouse EOCENE-OLIGOCENE BOUNDARY mental perturbation in surface waters as forcing mechanism proposed for the COOLING IN THE TROPICAL PACIFIC the underlying cause of the biotic event. PETM. This warming is generally thought OCEAN We speculate that the event was a hyper- to have resulted from a massive release of The Eocene-Oligocene (E-O) boundary thermal, an abrupt warming that possibly methane from clathrates into the ocean- interval recovered on Shatsky Rise caused a brief switch in the source of deep atmosphere (e.g., Dickens et al., records the response of the tropical waters bathing Shatsky Rise. 1997). Methane can explain the magni- Pacific Ocean to a major global cooling Sediments cored on Shatsky Rise show tude of the warming and the rate of car- event when ice sheets developed on evidence of a strong deep-ocean re- bon isotopic change at the onset of the and cold water circulated sponse to warming in the PETM. The event. The oceanic response to this throughout the deep ocean (e.g., PETM interval was cored in nine holes at methane input has been predicted but is Shackleton and Kennett, 1975). This cool- Sites 1209, 1210, 1211, and 1212 on the currently untested (e.g., Dickens, 2000). ing that signaled the end of the warm Southern High (Fig. 6). The Paleocene- Regardless of how the transfer to the Paleogene occurred largely in a rapid step Eocene boundary interval was also recov- ocean took place, oxidation of methane in the earliest Oligocene at ca. 33.5 Ma

GSA TODAY, NOVEMBER 2002 9 (Zachos et al., 1996; Fig. 1). The bound- outstanding support. We are grateful to in the tropical Pacific (ODP Site 865) during the late Paleocene thermal maximum: Geology, v. 24, ary interval was identified at four sites Hope Jahren, Ken MacLeod, Woody Wise, p. 423–426. across a large depth range. At the and an anonymous reviewer Kennett, J.P., and Stott, L.D., 1991, Abrupt deep-sea warming, Southern High sites (Sites 1209, 1210, and for helpful comments on an earlier palaeoceanographic changes, and benthic extinctions at the 1211), a gradual change from light brown manuscript. This research used samples end of the Palaeocene: Nature, v. 353, p. 225–229. Kump, L.R., 1991, Interpreting carbon-isotope excursions: to tan nannofossil ooze with clay to a and data provided by the Ocean Drilling Strangelove oceans: Geology, v. 19, p. 299–302. light gray to white nannofossil ooze, is Program, funded by the National Science Larson, R.L., 1991, Geological consequences of superplumes: observed over a 4–7.5 m interval of the Foundation. Funding for this research Geology, v. 19, p. 963–966. uppermost Eocene to lowermost was provided by the U.S. Science Support MacLeod, K.G., and Huber, B.T., 1996, Reorganization of deep-sea circulation accompanying a Late Cretaceous extinc- Oligocene. The E-O boundary interval at Program, administered by Joint Ocean- tion event: Nature, v. 380, p. 422–425. Site 1208 on the Central High is much ographic Institutions. MacLeod, K.G., Huber, B.T., and Ward, P.D., 1996, The bio- more abrupt, corresponding to a 1–2 cm stratigraphy and paleobiogeography of Maastrichtian inoce- REFERENCES CITED ramids, in Ryder, G., et al., eds., The Cretaceous-Tertiary transition from a dark brown zeolitic clay- event and other catastrophes in Earth history: Boulder, stone with extremely low carbonate con- Arthur, M.A., Brumsack, H.-J., Jenkyns, H.C., and Schlanger, Colorado, Geological Society of America Special Paper 307, S.O., 1990, Stratigraphy, geochemistry, and paleoceanogra- p. 361–373. tent to a gray-orange nannofossil ooze. phy of organic carbon-rich Cretaceous sequences, in The distinctive color change across the Ginsburg, R.N., and Beaudoin, B., eds., Cretaceous resources, Nakanishi, M., Tamaki, K., and Kobayashi, K., 1989, events, and rhythms: Kluwer Academic Publishers, p. 75–119. magnetic anomaly lineations and seafloor spread- E-O boundary in all of the Leg 198 ing history of the northwestern Pacific: Journal of Geophysical Arthur, M.A., Dean, W.E., and Schlanger, S.O., 1985, Research, v. 94, p. 15,437–15,462. records reflects an increase in carbonate Variations in the global during the Cretaceous related to climate, volcanism, and changes in atmospheric Norris, R.D., Kroon, D., Klaus, A., et al., 1998, Proceedings content as a result of a deepening of the of the Ocean Drilling Program, Initial Reports, Volume 171B: CO2, in Sundquist, E.T., and Broecker, W.S., eds., The carbon College Station, Texas, Ocean Drilling Program, 749 p. lysocline and CCD. This interpretation is cycle and atmospheric CO2: Natural variations to consistent with the observation that the present: Washington, D.C., American Geophysical Union Schlanger, S.O., and Jenkyns, H.C., 1976, Cretaceous oceanic Monograph 32, p. 504–529. lithologic change is more pronounced at anoxic events: Causes and consequences: Geologie en Barrera, E., Savin, S.M., Thomas, E., and Jones, C.E., 1997, Mijnbouw, v. 55, p. 179–184. the deepest site, Site 1208. Microfossil Evidence for thermohaline-circulation reversals controlled Shackleton, N.J., and Kennett, J.P. 1975, Paleotemperature preservation in the interval above and be- by change in the latest Cretaceous: Geology, v. 25, history of the Cenozoic and the initiation of Antarctic glacia- p. 715–718. tion: Oxygen and carbon isotope analyses in DSDP Sites 277, low the transition suggests that the CCD Bralower, T.J., 2002, Evidence for surface water oligotrophy 279 and 281: Initial Reports of the Deep Sea Drilling Project, dropped from just below the depth of Site during the Paleocene Eocene Thermal Maximum: Nannofossil v. 29: Washington, D.C., U.S. Government Printing Office, p. 743–755. 1211 to well below the depth of Site 1208, assemblage data from Ocean Drilling Program Site 690, Maud Rise, Weddell Sea: Paleoceanography, v. 17 (in press). Sigurdsson, H., Leckie, R.M., Acton, G., et al., 1997, thus by at least 450 m. This significant Bralower, T.J., Premoli Silva, I., Malone, M.J., et al., 2002. Proceedings of the Ocean Drilling Program, Initial Reports, change is observed in other ocean basins Proceedings of the Ocean Drilling Program, Initial Reports, Volume 165: College Station, Texas, Ocean Drilling Program, 724 p. and possibly reflects an increase in me- Volume 198 [online]: College Station, Texas, Ocean Drilling Program (www odp.tamu.edu/publications/198_IR/ Sliter, W.V., 1989, Aptian anoxia in the Pacific Basin: chanical and chemical rates 198ir.htm). Geology, v. 17, p. 909–912. on associated with cooling Bralower, T.J., Arthur, M.A., Leckie, R.M., Sliter, W.V., Allard, Smit, J., 1982, Extinction and evolution of planktonic (e.g., Zachos et al., 1996). D.J., and Schlanger, S.O., 1994, Timing and paleoceanogra- foraminifera after a major impact at the Cretaceous-Tertiary phy of oceanic dysoxia-anoxia in the Late to early boundary, in Silver, L.T., and Schultz, H., eds., Geological Aptian: Palaios, v. 9, p. 335–369. implications of impacts of large and comets on the CONCLUSIONS Buffler, R.T., Schlager, W., et al., 1984, Proceedings of the Earth: Boulder, Colorado, Geological Society of America Drilling on Leg 198 recovered diverse Deep Sea Drilling Project, Initial Reports, Volume 77: Special Paper 190, p. 329–352. Washington, D.C., U.S. Government Printing Office, 747 p. evidence for abrupt environmental Smit, J., and Romein, A.J.T., 1985, A sequence of events Coccioni, R., Erba, E., and Premoli Silva, I., 1992, Barremian- across the Cretaceous-Tertiary boundary: Earth and Planetary changes in the Cretaceous and Paleogene Aptian calcareous plankton biostratigraphy from the Gorgo Science Letters, v. 74, p. 155–170. warm climate interval. These changes in- Cerbara section, Marche, central Italy, and implications for Thomas, E., Zachos, J.C., and Bralower, T.J., 2000, Ice-free to plankton evolution: Cretaceous Research, v. 13, p. 517–537. glacial world transition as recorded by benthic foraminifera, clude a short period of anoxia in the early Dickens, G.R., 2000, Methane oxidation during the late in Huber, B.T., et al., eds., Warm climates in Earth history: Aptian (ca. 120 Ma) that led to deposition Palaeocene thermal maximum: Bulletin de la Société Cambridge, UK, University of Cambridge, p. 132–160. of highly carbonaceous sediments; an Geologíque de , v. 171, p. 37–49. Zachos, J.C., Lohmann, K.C, Walker, J.C.G., and Wise, S.W., Dickens, G.R., Castillo, M.M., and Walker, J.G.C., 1997, A Jr., 1993, Abrupt climate change and transient climates during abrupt reorganization of oceanic circula- blast of gas in the latest Paleocene: Simulating first-order ef- the Paleogene: A marine perspective: Journal of Geology, tion in the Maastrichtian (ca. 69 Ma) that fects of massive dissociation of oceanic methane hydrate: v. 101, p. 191–213. caused extinction of a group of deep-sea Geology, v. 25, p. 259–262. Zachos, J.C., Quinn, T.M., and Salamy, K.A., 1996, High- Erbacher, J., and Thurow, J., 1997, Influence of oceanic resolution deep-sea foraminiferal stable isotope records of the mollusks; the extinction event at the K-T anoxic events on the evolution of mid-Cretaceous radiolaria Eocene-Oligocene climate transition: Paleoceanography, boundary (65 Ma); a prominent biotic in the North Atlantic and western Tethys: Marine v. 11, p. 251–266. event in the late Paleocene (ca. 58.4 Ma); , v. 30, p. 139–158. Manuscript received June 24, 2002; Frank, T.D., and Arthur, M.A., 1999, Tectonic forcings of ◆ the PETM (ca. 55 Ma) that shows litho- Maastrichtian ocean-climate evolution: Paleoceanography, accepted September 9, 2002. logic and geochemical evidence consis- v. 14, p. 103–117. tent with methane outgassing; and Jenkyns, H.C., 1980, Cretaceous anoxic events: From conti- nents to oceans: Journal of the Geological Society of London, changes in circulation and rapid cooling v. 137, p. 171–188. near the E-O boundary (ca. 33.5 Ma) that Jenkyns, H.C., 1995, Carbon-isotope stratigraphy and paleo- correspond to a sharp lithologic change. ceanographic significance of the Lower Cretaceous shallow- water of , Mid-Pacific Mountains, in Winterer, E.L., Sager, W.W., Firth, J.V., and ACKNOWLEDGMENTS Sinton, J.M., eds., Proceedings of the Ocean Drilling Program, Scientific Results, Volume 143: College Station, Texas, Ocean We thank the highly capable drilling Drilling Program, p. 99–104. operations team, the crew, and the tech- Kelly, D.C., Bralower, T.J., Zachos, J.C., Premoli-Silva, I., nicians who sailed on Leg 198 for their and Thomas, E., 1996, Rapid diversification of planktonic

10 NOVEMBER 2002, GSA TODAY