Bralower, T.J., Premoli Silva, I., and Malone, M.J. (Eds.) Proceedings of the Ocean Drilling Program, Scientific Results Volume 198 1. LEG 198 SYNTHESIS: A REMARKABLE 120-M.Y. RECORD OF CLIMATE AND OCEANOGRAPHY FROM SHATSKY RISE, NORTHWEST PACIFIC OCEAN1 Timothy J. Bralower,2 Isabella Premoli Silva,3 and Mitchell J. Malone4 1Bralower, T.J., Premoli Silva, I., and ABSTRACT Malone, M.J., 2006. Leg 198 synthesis: A remarkable 120-m.y. record of climate and oceanography from Samples collected from a depth transect of eight sites during Ocean Shatsky Rise, northwest Pacific Ocean. Drilling Program Leg 198 to Shatsky Rise contain a remarkable sedimen- In Bralower, T.J., Premoli Silva, I., and tary record of surface and deepwater circulation in the tropical Pacific Malone, M.J. (Eds.), Proc. ODP, Sci. over the past 120 m.y. In addition, basement sills recovered provide Results, 198, 1–47 [Online]. Available valuable constraints on the age and origin of the volcanic foundations from World Wide Web: <http:// www-odp.tamu.edu/publications/ of the rise. 198_SR/VOLUME/SYNTH/ The sediments recovered contain evidence of the long-term transi- SYNTH.PDF>. [Cited YYYY-MM-DD] tion from greenhouse to icehouse climate state and of several abrupt 2Department of Geosciences, climate change events. Shatsky Rise cores contain an exceptional record Pennsylvania State University, of an Oceanic Anoxic Event (OAE1a) in the early Aptian (120 Ma), with University Park PA 16802, USA. [email protected] some of the highest organic carbon contents measured in pelagic sedi- 3Dipartimento di Scienze della Terra, ments. These strata contain exceptionally preserved organic com- Università degli Studi di Milano, Via pounds including the oldest known alkenones. Organic geochemistry Mangiagalli 34, 20133 Milano, Italy. suggests that bacterial activity played a significant role in sequestering 4Ocean Drilling Program, Texas A&M organic carbon. Stable isotope data from Upper Cretaceous and Paleo- University, 1000 Discovery Drive, College Station TX 77845-9547, USA. gene sediments reveal several abrupt switches in the sources of interme- Present address: Integrated Ocean diate waters bathing Shatsky Rise. Neodymium isotopes also show Drilling Program, Texas A&M evidence for these changes and help to identify source regions in the University, 1000 Discovery Drive, North Pacific, Southern Ocean, and, possibly, Tethys. College Station TX 77845-9547, USA. Strong evidence exists in Shatsky cores for the mid-Maastrichtian Initial receipt: 19 July 2005 (~69 Ma) global extinction of inoceramids, a long-ranging, widespread Acceptance: 26 January 2006 group of bottom-dwelling clams. Stable and neodymium isotopes com- Web publication: 13 April 2006 Ms 198SR-101 T.J. BRALOWER ET AL. LEG 198 SYNTHESIS: SHATSKY RISE CLIMATE AND OCEANOGRAPHY 2 bined with biotic data show changes in intermediate water sources at this time as well as significant changes in surface water oceanography. Shatsky Rise sites contain high-quality records of the Cretaceous/Ter- tiary boundary event. Detailed nannofossil assemblage studies demon- strate that the survivor taxa are those that were adapted to unstable environmental conditions of shelves, including taxa that have cyst stages. The Paleogene sedimentary record from Shatsky Rise is strongly cyclic with variations in the amount of dissolution. Superimposed on this record are “hyperthermal” episodes including the Paleocene/ Eocene Thermal Maximum (PETM; ~55.0 Ma) and events in the early late Paleocene (~58.4 Ma) and early Eocene (52.7 Ma). The PETM on Shatsky Rise contains evidence for 5°C warming of tropical sea-surface temperatures, major reorganization of benthic and planktonic commu- nities, and pronounced short-term shoaling of the lysocline. Oxygen isotope and Mg/Ca data demonstrate warming of surface and interme- diate waters (possibly combined with decreasing salinity) during the early Eocene and help constrain the timing of the acceleration of Ant- arctic glaciation during the middle Eocene. Recovery of basaltic sills provides valuable age and geochemical con- straints for interpreting the origin of Shatsky Rise. Radiometric ages confirm previous suggestions that this large igneous province was em- placed rapidly. Isotope geochemistry shows a mid-ocean-ridge basalt signature that argues against a mantle plume origin. However, alterna- tive origins are difficult to prove unequivocally. INTRODUCTION The sedimentary record recovered over the course of several decades of deep-sea drilling has yielded a wealth of information on Earth’s cli- mate system. Pelagic sedimentary sections provide some of the most faithful time series of climate states from the Middle Jurassic to the Ho- locene. Potentially more significant, however, are the clues within the sedimentary archives of the mechanisms and processes that drive cli- mate change and the effect of that change on biotic evolution and geochemical cycling (e.g., Schlanger et al., 1987; Dickens et al., 1995; Leckie et al., 2002). The early decades of deep-sea drilling, largely associated with the Deep Sea Drilling Project (DSDP), involved a great deal of exploratory coring in locations where the stratigraphy of the sedimentary section was undocumented and poorly constrained. More recently, however, knowledge from previous drilling expeditions combined with the avail- ability of high-resolution multichannel seismic reflection data have al- lowed us to target particular time slices at locations where the sedimentary section looks essentially complete. In addition, hydraulic piston coring technology developed during the Ocean Drilling Program (ODP) allows us to obtain high-quality records when these locations are revisited. F1. Bathymetric map of Shatsky Shatsky Rise, a medium-sized large igneous province (LIP) in the Rise, p. 26. west-central Pacific (Fig. F1) was the target of three DSDP expeditions, 1207 (3101 m) 2000 Legs 6, 32, and 86, and ODP Leg 132 (Fischer, Heezen et al., 1971; Lar- 4000 1208 (3346 m) Depth (m) ο 39 son, Moberly, et al., 1975; Heath, Burkle, et al., 1985; Natland, Storms, ο 38 1209 (2387 m) 1212 (2681 m) ο 1210 (2573 m) 37 ο 1211 (2907 m) ο 36 164 1214 (3402 m) ο 3 Latitu ο 1213 (3883 m) 16 et al., 1993). The quality of the records from the older legs was ham- 35 ο de 162 ο ο 34 161 ο ο 160 33 ο 159 ο ο gitude 32 on pered by rotary and spot-coring as well as low recovery in the Creta- 158 L ο ο 31 157 ο 56 ShatskyShatsky RiseRise ο 1 ceous because of the presence of persistent chert. Yet these legs revealed 30 the potential of Shatsky Rise sediments to provide high-quality records T.J. BRALOWER ET AL. LEG 198 SYNTHESIS: SHATSKY RISE CLIMATE AND OCEANOGRAPHY 3 of Cretaceous and Paleogene climate. This interval is located at shallow burial depths on the rise and thus is relatively unaltered by burial dia- genesis (e.g., Schlanger and Douglas, 1974). Leg 198 was designed to re- cover high-quality sedimentary records of this age along a depth transect (Bralower, Premoli Silva, Malone, et al., 2002). The transect has the potential to provide two-dimensional reconstructions of ocean tem- F2. Generalized climate curve for perature, chemistry (i.e., carbonate solubility and oxygenation), circula- Cretaceous and Paleogene, p. 27. tion, and biology through some of the most fundamentally interesting Benthic δ18O (‰) -1.5 -0.5 0.5 1.5 2.5 3.5 20 Mio. 25 intervals of climate change during the entire Phanerozoic. 30 Eocene/Oligocene Transition Oligocene 35 The mid-Cretaceous (~120–80 Ma) and early Paleogene (~60–45 Ma) 40 45 Eocene were characterized by equable climates including low latitudinal ther- 50 55 PETM Paleocene 60 biotic event Paleocene mal gradients, lack of extensive cryosphere, a relatively warm deep 65 K/T MME 70 Maast. ocean, and high atmospheric pCO2 levels (Fig. F2) (e.g., Barron and Age (Ma) 75 80 Campanian 85 Sant. Washington, 1985; Berner, 1994). These “greenhouse” intervals also Coniac. 90 Tur. 95 Cenom. contain significant abrupt and transient warming events that led to ma- 100 105 OAEs Albian jor changes in oceanic environments, profound turnovers in marine 110 115 Aptian 120 OAE1a communities including extinctions, and perturbations to global chemi- 125 Barrem. cal cycles. Examples include the Paleocene/Eocene Thermal Maximum Warm Cold (PETM) (e.g., Kennett and Stott, 1991; Dickens et al., 1995; Katz et al., 1999; Norris and Röhl, 1999) and Cretaceous Oceanic Anoxic Events F3. Stratigraphy and lithology, (OAEs) (e.g., Schlanger and Jenkyns, 1976; Jenkyns, 1980; Arthur et al., Sites 1207–1214, p. 28. Northern Central Southern High High High 1985, 1988). Leg 198 was designed to understand the causes, nature, 1207 1208 12091210 1212 1211 1214 1213 Water depth (m) 3101 3346 2387 2573 2681 2907 3402 3883 Age 0 Pleistocene late Plio. 5 early and mechanics of the long-term Cretaceous and Paleogene “green- late 10 middle 20 Miocene early ? house” as well as of the transient events during this period. The loca- late 30 Oligo. early E/O transition late 40 tion of Shatsky Rise in the tropical Pacific during the interval of interest middle Eocene 50 early PETM late Paleocene biotic event 60 is fundamentally significant—the aerial extent and importance of the Paleoc. early K/T Maast. 70 MME Age (Ma) Camp. Pacific in global circulation make this a critical target for investigation 80 Sant. Coniac. 90 Late Cretaceous Tur. of warm climatic intervals. Yet, this ocean basin contains far less com- Cenom. 100 Albian 110 plete records than the Atlantic, Indian, and Tethyan Oceans. During Aptian 120 OAE1a Barrem. Early Cretaceous Haut. 130 Leg 198, one site was drilled on each of the North and Central Highs of Val. Berrias. 140 L. Jur. Tith. Shatsky Rise (Sites 1207 and 1208, respectively) and six sites were Nannofossil ooze Clay Chalk Chert Porcellanite drilled on the Southern High (Sites 1209–1214) (Bralower, Premoli Silva, Malone, et al., 2002).