Late Paleocene to Eocene Paleoceanography of the Equatorial Pacific Ocean: Stable Isotopesrecorded at Ocean Drilling Program Site 865, Allison Guyot

PALEOCEANOGRAPHY,VOL. 10, NO. 4 , PAGES 841-865, AUGUST 1995 Late Paleocene to Eocene paleoceanography of the equatorial Pacific Ocean: Stable isotopesrecorded at Ocean Drilling Program Site 865, Allison Guyot TimothyJ. Bralower,1James C. Zachos,2 Ellen Thomas, 3,4 Matthew Parrow, 1 Charles K. Paull,• D. ClayKelly, • IsabellaPremoli Silva, 5 WilliamV. Sliter,6 and Kyger C. Lohmann 7 Abstract. An expandedand largely complete upper Paleocene to upperEocene section was recoveredfrom the pelagic cap overlying Allison Guyot, Mid-Pacific Mountains at OceanDrilling Program(ODP) Site865 (18ø26'N,179ø33'W; paleodepth 1300-1500 m). Reconstructionsshow thatthe sitewas within a few degreesof theequator during the Paleogene.Because no other Paleogenesections have been recovered in thePacific Ocean at sucha low latitude,Site 865 providesa uniquerecord of equatorialPacific paleoceanography. Detailed stable isotopic investigationswere conducted on three planktonic foraminiferal taxa (species of Acarinina, Morozovella,and Subbotina). We studiedbenthic foraminiferal isotopes at muchlower resolution on speciesof Cibicidoidesand Lenticulina, Nuttallides truernpyi and Gavelinella beccariiformis, becauseof theirexceptional rarity. The •5180 and •513C stratigraphies from Site 865 are generally similarto thosederived from otherPaleocene and Eocene sections. The planktonicforaminiferal recordsat Site 865, however,include significantly less short-term, single-sample variability than thosefrom higher-latitude sites, indicating that this tropical, oligotrophic location had a comparativelystable water column structure with a deepmixed layer and less seasonal variability. Low-amplitude(0.1-0.8%o) oscillations on timescalesof 250,000to 300,000years correlate betweenthe/513C records of all planktonictaxa and may represent fluctuations in the mixing intensityof surfacewaters. Peak sea surface temperatures of 24ø-25øCoccurred in theearliest Eocene,followed by a rapidcooling of 3-6øCin thelate early Eocene. Temperatures remained cool andstable through the middle Eocene. In thelate Eocene, surface water temperatures decreased further.Vertical temperature gradients decreased dramatically in thelate Paleocene and were relativelyconstant through much of theEocene but increased markedly in thelate Eocene. Intermediatewaters warmed through the late Paleocene,reaching a maximumtemperature of 10øC in the earlyEocene. Cooling in themiddle and late Eocene paralleled that of surfacewaters, with latestEocene temperatures below 5øC. Extinctionpatterns of benthicforaminifera in thelatest Paleocene were similar to those observed at other Pacific sites and were coeval with a short-term, veryrapid negative excursion in/513C values in planktonicand benthic taxa as at othersites. Duringthis excursion, benthic foraminiferal/5180 values decreased markedly, indicating warming of 4 to 6øCfor tropicalintermediate waters, while planktonictaxa show slight warming (1 øC) followedby 2øC of cooling.Convergence of/5180 values of planktonicand benthic foraminifera suggeststhat thermalgradients in the watercolumn in thistropical location collapsed during the excursion.These data are consistentwith the hypothesisthat equatorial Pacific surface waters were a potentialsource of warm, highersalinity waters which filled portionsof the deepocean in the latestPaleocene. Oxygen isotopic data indicate that equator to high southernlatitude sea surface thermalgradients decreased to as little as 4øC at the peak of the excursion,suggesting some fundamentalchange in globalheat transport. 1Departmentof Geology, University of NorthCarolina, Chapel Hill. 6Branchof Paleontologyand Stratigraphy, U.S. Geological Survey, 2EarthScience Board, University of California,Santa Cruz. Menlo Park, California. 3Departmentof Geology and Geophysics, Yale University, New 7Departmentof Geological Sciences, University of Michigan,Ann Haven, Connecticut. Arbor. 4Alsoat Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut. Introduction 5Dipartimentodi Scienza della Terra, Universita di Milano,Milan, Italy. Paleogenesediments have proven to be elusive targets for high-resolutionpaleoceanographic studies: they are too young Copyright1995 by the AmericanGeophysical Union. to be widely exposedin mountainbelts yet too old to be easily Papernumber 95PA01143. accessibleto shallow drilling technology. Most researchhas 0883- 8305/95/95PA- 01143510.00 thusbeen doneon materialrecovered by the Deep Sea Drilling 842 BRALOWERET AL.: PALEOGENE EQUATOR/AL PACIFIC PALEOCEANOGRAPHY Project (DSDP) and OceanDrilling Program(ODP), especially in the Paleogenesubtropical Pacific Ocean [Zachos and Arthur, sedimentsfrom high southernlatitude sites such as from Maud 1986; Miller et al., 1987; Corfield and Cartlidge, 1992; Pak Rise in the southernmostAtlantic Ocean [Barker et al., 1990] and Miller, 1992]. This section accumulated at low and KerguelenPlateau in the Indian Ocean[Wise et al., 1992]. sedimentationrates and containshighly condensedintervals or These sectionshave been analyzed at a resolutioncommonly short-termhiatuses (particularly in the lower Eocene), the seen in studieson younger sections[e.g., Kennett and Stott, middleEocene is missing,and part of the uppermostPaleocene 1990; Stott and Kennett, 1990; Barrera and Huber, 1991; lies in a core break in two holes. A rotary-drilledrecord from Zachos et al., 1993; Thomas and Shackleton, 1995], DSDP Site 47.2 also on the ShatskyRise hasbeen discussed by demonstratingthat Paleogeneoceans and climate witnesseda Stott [1992], but significant drilling disturbanceprecludes pattern of short-term(103-10 4 years) fluctuationsin high-resolution studies. temperature. These short-termexcursions are superimposedon The Paleogenepelagic cap sectionat Site 865 on Allison long-term(>106 years) climatic fluctuations [e.g., Shackleton Guyot (18ø 26' N, 179ø 33' W; 1530 m waterdepth; Figure 1) and Kennett, 1975; Savin, 1977; Kennett and Stott, 1991; lies on Cretaceousshallow water limestonestargeted on Leg Corfield and Cartlidge, 1992; Miller, 1992]. During the latest 143. Calculationsusing the Pacific polar wanderpath indicate Paleocene to earliest Eocene, high-latitude surface waters paleolatitudesof about 2 ø N in thelate Paleocene to about6 ø N warmed graduallyfrom 11ø to 15øC and deep watersfrom 9 ø to in the late Eocene (R. Larson, oral communication, 1994). 13øC [Miller et al., 1987; Stott et al., 1990; Zachos et al., Conventionalpaleodepth back-tracking techniques could not 1993], but in the middle through late Eocene, high-latitude be employeddue to the unusualthermal history of the Mid- surface and deep waters cooled to 6øC and 4øC, respectively Pacific Mountains. Depth estimates based on benthic [e.g., Zachos et al., 1994]. foraminifera are not precisefor the bathyal-abyssalrange in The latest Paleocenerapid climatic change was one of the Paleogenesediments [Berggren and Miller, 1989],but detailed most dramatic warming events in the geological record. comparisonbetween Paleocene/Eocene faunas at Site 865 and Antarctic surface and deep water temperaturesincreased from those at Sites 762 (Indian Ocean, paleodepth1000-1500 m), 14øC to 20øC and from 10øC to 18øC, respectively,over less Site 577 (Pacific Ocean, paleodepthabout 1500 m), Site 752 than 10,000 years, so that vertical thermal gradientsdecreased; (Indian Ocean, paleodepth500-1000 m), Site 689 (Weddell they remained at these levels for 50,000 to several hundred Sea, paleodepthabout 1100 m), and Site 525 (Walvis Ridge, thousand years [Stott et al., 1990; Kennett and Stott, 1991; paleodepthabout 1600 m) suggeststhat the paleodepthwas in Thomas and Shackleton, 1995]. Synchronous with the the upper half of the lower bathyal, i.e., betweenabout 1300 dramaticdecrease in •180 valueswas a largenegative excursion and 1500 m [Thomas and Shackleton, 1995; E. Thomas, in the •13C recordsof planktonicand benthic foraminifera. manuscriptin preparation,1995], suggestinglittle subsidence The event is believedto have been associatedwith a temporary since the Paleogene.We presentthe isotopicstratigraphy of change in dominant deep water sources from high to low the Paleogene section at Site 865 and its general latitudes [Kennett and Stott, 1991; Pak and Miller, 1992; paleoceanographic significance. The paleoclimatic Thomas, 1992; Eldholm and Thomas, 1993] which could have implicationsof this recordare furtherexplored by J. C. Zachos been the major cause of the associated mass extinction of et al. (manuscript in preparation, 1995); the benthic benthic foraminifera [Braga et al., 1975; Schnitker, 1979; foraminiferal faunal record will be discussedby E. Thomas Tjalsmaand Lohmann,1983] possiblyresulting from decreased (manuscriptin preparation,1995). dissolved oxygen contents of warmer deep waters [e.g., Kennett and Stott, 1990; Thomas, 1990a, 1992; Kennett and Stott, 1991; Kaiho, 1991, 1994]. Methods and Procedures The short-termchanges in Eocenesea surfacetemperatures (SST)are well documented in high-latitude •80 recordsbut not Sample Selection and Preparation in the low latitudes.Low-latitude •80 recordsare essential, Samples were analyzed from the upper Paleocenethrough however, for reconstructing the low- to high-latitude upper Eocene interval of Hole 865B. Hole 865C was sampled temperature gradient, especially since inconsistenciesexist in detail only in the interval close to Paleocene/Eocene between faunal and isotopic temperatureestimates for the boundarybecause the uppermostPaleocene part of the section tropics on longer

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