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Eocene Cooling Linked to Early Flow Across the Tasmanian Gateway

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Eocene Cooling Linked to Early Flow Across the Tasmanian Gateway Eocene cooling linked to early flow across the Tasmanian Gateway Peter K. Bijla,1, James A. P. Bendleb,2, Steven M. Bohatyc, Jörg Prossd,e, Stefan Schoutenf, Lisa Tauxeg, Catherine E. Stickleyh, Robert M. McKayi, Ursula Röhlj, Matthew Olneyk, Appy Sluijsa, Carlota Escutial, Henk Brinkhuisa,f, and Expedition 318 Scientists3 aDepartment of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584 CD, Utrecht, The Netherlands; bGeographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom; cOcean and Earth Science, University of Southampton, Southampton SO14 3ZH, United Kingdom; dPaleoenvironmental Dynamics Group, Institute of Geosciences, University of Frankfurt, 60438 Frankfurt, Germany; eBiodiversity and Climate Research Centre, 60325 Frankfurt, Germany; fNIOZ Royal Netherlands Institute for Sea Research, 1790 AB, Den Burg, Texel, The Netherlands; gGeosciences Research Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0220; hDepartment of Geology, University of Troms, N-9037 Troms, Norway; iAntarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand; jMARUM–Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany; kDepartment of Geology, University of South Florida, Tampa, FL 33620; and lInstituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientificas (Spain)–Universite de Granada, 18002 Granada, Spain Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved April 23, 2013 (received for review November 30, 2012) The warmest global temperatures of the past 85 million years a marked, gradual cooling starting in the latest early Eocene occurred during a prolonged greenhouse episode known as the (∼49.5 Ma) (2–4), which culminated in the onset of large-scale Early Eocene Climatic Optimum (52–50 Ma). The Early Eocene Cli- glaciation of Antarctica at 34 Ma (7, 8). The timing of the opening matic Optimum terminated with a long-term cooling trend that cul- of Southern Ocean gateways seemed roughly time equivalent with minated in continental-scale glaciation of Antarctica from 34 Ma major climatic transitions in the southern high latitudes (9). onward. Whereas early studies attributed the Eocene transition Opposition to the gateway hypothesis emerged in the 1990s, from greenhouse to icehouse climates to the tectonic opening of when sophisticated computer simulations [general circulation Southern Ocean gateways, more recent investigations invoked models (GCMs)] indicated that the Eocene surface current con- a dominant role of declining atmospheric greenhouse gas concen- figuration in the Southern Ocean featured circulating gyres (10, fi trations (e.g., CO2). However, the scarcity of eld data has pre- 11). Despite the closed gateways, gyral Southern Ocean circulation vented empirical evaluation of these hypotheses. We present prevented low latitude-derived currents from reaching and marine microfossil and organic geochemical records spanning the warming the coasts of Antarctica (10, 11). Additional evidence early-to-middle Eocene transition from the Wilkes Land Margin, East against the gateway hypothesis was obtained from drill cores in the Antarctica. Dinoflagellate biogeography and sea surface tempera- Tasmanian Gateway region (Fig. S1), which showed that the ac- ture paleothermometry reveal that the earliest throughflow of celerated deepening and widening of the Tasmanian Gateway ∼ – a westbound Antarctic Counter Current began 49 50 Ma through occurred 2 million y before the onset of major Antarctic glaciation a southern opening of the Tasmanian Gateway. This early opening (12)—therefore, contradicting a direct mechanistic link between occurs in conjunction with the simultaneous onset of regional sur- gateway opening, thermal isolation, and Antarctic glaciation. The – face water and continental cooling (2 4 °C), evidenced by bio- circum-Antarctic biogeographical patterns of marine phyto- marker- and pollen-based paleothermometry. We interpret that the plankton (13) lend additional support to the ocean current regime fl fl westbound owing current ow across the Tasmanian Gateway indicated by GCM experiments (Figs. S2 and S3) (11, 14). This resulted in cooling of Antarctic surface waters and coasts, which biogeographical information is primarily based on organic-walled was conveyed to global intermediate waters through invigorated dinoflagellate cysts (dinocysts; fossil remains of dinoflagellates), deep convection in southern high latitudes. Although atmo- which are highly sensitive indicators of surface ocean conditions spheric CO forcing alone would provide a more uniform middle 2 (Table S1) (15). Before the opening of the Tasmanian Gateway, Eocene cooling, the opening of the Tasmanian Gateway better fl EARTH, ATMOSPHERIC, the Antarctic margin-derived Tasman Current in uenced surface AND PLANETARY SCIENCES explains Southern Ocean surface water and global deep ocean waters in the Pacific sector of the Tasmanian region, which is cooling in the apparent absence of (sub-) equatorial cooling. consistent with high abundances of Antarctic-endemic dino- cysts (11). In contrast, surface waters in the Australo-Antarctic climate cooling | dinoflagellate cysts | organic palaeothermometry | Gulf (AAG), west of Tasmania, were sourced by low latitude- paleoceanography derived Indian Ocean waters by the Proto-Leeuwin Current (PLC) (11). Indeed, dinocyst assemblages in the AAG were dominated by lant microfossils reveal the presence of paratropical rainforest cosmopolitan and low-latitude taxa (13) (SI Text and Figs. S2 and Pbiomes on the Antarctic continent during the early Eocene (1). S3). The early GCM experiments, strongly supported by the Organic biomarker paleotemperature reconstructions also reveal extremely warm Southern Ocean sea surface temperatures (SSTs) – at that time (2 4). Both lines of evidence indicate remarkably Author contributions: P.K.B., J.A.P.B., J.P., S.S., A.S., and H.B. designed research; P.K.B., J.A.P.B., warm climate conditions for the highest southern latitudes during J.P., S.S., and C.E.S. performed research; Exp. 318 sci. gathered samples; P.K.B., J.A.P.B., S.M.B., peak Cenozoic greenhouse conditions. Early hypotheses explain- J.P., S.S., L.T., C.E.S., R.M.M., U.R., M.O., A.S., C.E., and H.B. analyzed data; and P.K.B., J.A.P.B., ing the Eocene warmth at high latitudes invoked a pivotal role of S.M.B., J.P., S.S., L.T., C.E.S., R.M.M., U.R., A.S., C.E., and H.B. wrote the paper. fl ocean surface currents that, subject to a continental configuration The authors declare no con ict of interest. fundamentally different from today, facilitated an increased heat This article is a PNAS Direct Submission. transport to the Antarctic coastline (5, 6). Closed Southern Ocean 1To whom correspondence should be addressed. E-mail: [email protected]. gateways (i.e., Drake Passage and the Tasmanian Gateway) 2Present address: Birmingham Molecular Climatology Laboratory (BMC), School of Geog- raphy, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 obstructed the formation of an isolating circumpolar surface cur- 2TT, United Kingdom. rent: the Antarctic Circumpolar Current (ACC) and associated 3A complete list of the Expedition 318 Scientists and their affiliations can be found in coastal currents. In the absence of the isolating ACC, warm, low Supporting Information. latitude-derived currents were thought to have bathed the Ant- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. arctic continent (5). Southern Ocean SSTs were characterized by 1073/pnas.1220872110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1220872110 PNAS | June 11, 2013 | vol. 110 | no. 24 | 9645–9650 Downloaded by guest on September 30, 2021 biogeographical patterns in microfossils (11, 16), suggested that Eocene time interval (Fig. 1 and Fig. S1). We generated dinocyst poleward ocean heat transport was not much larger than today’s assemblage and organic biomarker data from Site U1356 (Fig. S4), transport with closed oceanic gateways and cannot be the sole which was on the Antarctic margin within the AAG. Dinocyst as- explanation for the warmth on Antarctica (11, 16). semblage analyses were also generated from Ocean Drilling Pro- As an alternative explanation, it was proposed that elevated gram (ODP) Leg 182, Site 1128 in the Australian Bight (23) (Fig. 1 atmospheric greenhouse gas concentrations (e.g., CO2)drove and Fig. S1). Dinocyst assemblage and organic biomarker records Eocene climates into hothouse conditions. Accordingly, a decline from ODP Leg 189, Site 1171 on the South Tasman Rise (STR) in the concentration of these greenhouse gases was considered the and Site 1172 on the East Tasman Plateau (13) were revisited primary forcing agent for the cooling of the middle Eocene, ulti- for the purpose of this study (sediments from Site 1171 lack mately leading to the development of the Oligocene and Neogene biomarkers for SST reconstructions) (Fig. 1 and Fig. S1). icehouse world (17). However, the apparent absence of cooling through the middle and late Eocene in subequatorial regions Dinocyst assemblage data from the Otway Basin and RV Sonne (2, 18, 19) is not compatible with the greenhouse gas hypothesis. and RV Rig Seismic gravity cores on the west coast of Tasmania In the absence of ice albedo feedbacks, which would amplify as well as dredge sample surveys were obtained from the lit- high-latitude cooling
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