Pronounced Zonal Heterogeneity in Eocene Southern High-Latitude Sea Surface Temperatures
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Pronounced zonal heterogeneity in Eocene southern high-latitude sea surface temperatures Peter M. J. Douglasa,1,2, Hagit P. Affeka, Linda C. Ivanyb, Alexander J. P. Houbenc,3, Willem P. Sijpd, Appy Sluijsc, Stefan Schoutenc,e, and Mark Pagania aDepartment of Geology and Geophysics, Yale University, New Haven, CT 06520; bDepartment of Earth Sciences, Syracuse University, Syracuse, NY 13244; cDepartment of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584 CD, Utrecht, The Netherlands; dClimate Change Research Centre, University of New South Wales, Sydney, NSW 2052, Australia; and eDepartment of Marine Organic Biogeochemistry, Royal Netherlands Institute for Sea Research, 1790 AB Den Burg, Texel, The Netherlands Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved March 21, 2014 (received for review November 15, 2013) Paleoclimate studies suggest that increased global warmth during (10). This evidence for a tropical-to-subtropical climate in the the Eocene epoch was greatly amplified at high latitudes, a state southern high latitudes sharply contrasts with much cooler SST that climate models cannot fully reproduce. However, proxy es- estimates (10–15 °C in the early Eocene; 5–15 °C in the middle timates of Eocene near-Antarctic sea surface temperatures (SSTs) Eocene) derived from bivalve oxygen isotope values (δ18O), from have produced widely divergent results at similar latitudes, with Seymour Island (∼67°S) (11, 12), offshore of the Antarctic Penin- SSTs above 20 °C in the southwest Pacific contrasting with SSTs sula in the South Atlantic (Fig. 1). These temperature differences between 5 and 15 °C in the South Atlantic. Validation of this zonal could indicate the presence of large zonal SST gradients in the temperature difference has been impeded by uncertainties inher- Eocene southern high latitudes, with potential implications for heat ent to the individual paleotemperature proxies applied at these transport to high latitudes in greenhouse climates. sites. Here, we present multiproxy data from Seymour Island, near However, there are important uncertainties in TEX86 tem- the Antarctic Peninsula, that provides well-constrained evidence perature estimates due to multiple temperature calibrations (9) for annual SSTs of 10–17 °C (1σ SD) during the middle and late and unresolved questions regarding the provenance of sedimentary Eocene. Comparison of the same paleotemperature proxy at Sey- archaeal lipids (13, 14). Furthermore, bivalve δ18O paleotemper- mour Island and at the East Tasman Plateau indicate the presence of atures depend on assumptions for the oxygen isotopic composition a large and consistent middle-to-late Eocene SST gradient of ∼7°C of seawater (11). Uncertainty in the age of Seymour Island sedi- between these two sites located at similar paleolatitudes. Interme- ments also complicates the comparison with other paleoclimate diate-complexity climate model simulations suggest that enhanced datasets (11), although this age uncertainty is unlikely to account oceanic heat transport in the South Pacific, driven by deep-water for the observed temperature differences. These uncertainties in formation in the Ross Sea, was largely responsible for the observed the proxy datasets have made it difficult to determine whether SST gradient. These results indicate that very warm SSTs, in excess of apparent SST differences between the southwest Pacific and South 18 °C, did not extend uniformly across the Eocene southern high Atlantic are due to true zonal SST variability. latitudes, and suggest that thermohaline circulation may partially We performed clumped isotope analyses of the bivalve shells control the distribution of high-latitude ocean temperatures in green- previously analyzed for δ18O to better constrain SST estimates house climates. The pronounced zonal SST heterogeneity evident in the Eocene cautions against inferring past meridional temperature Significance gradients using spatially limited data within given latitudinal bands. Reconstructions of ancient high-latitude climates can help to paleooceanography | clumped isotopes | organic geochemistry | constrain the amplification of global warming in polar envi- climate modeling | high-latitude climate ronments. Climate models cannot reproduce the elevated high- latitude temperature estimates in the Eocene epoch, possibly he greenhouse climates of the Eocene provide an important indicating problems in simulating polar climate change. Widely Tanalog for understanding climate responses to elevated divergent near-Antarctic Eocene sea surface temperature (SST) greenhouse gas concentrations (1). Current efforts to simulate estimates, however, question the evidence for extreme warmth. Eocene climates, however, cannot reproduce geochemical evi- Our analysis of multiple temperature proxies near the Antarctic dence for very warm high-latitude temperatures and greatly re- Peninsula improves intersite comparisons and indicates a sub- duced meridional temperature gradients (2–4). This data–model stantial zonal SST gradient between the southwest Pacific and discrepancy could imply that current climate models do not ade- South Atlantic. Simulations of Eocene ocean temperatures imply quately incorporate important high-latitude climate feedbacks (5, that the formation of deep water in the southwest Pacific partly 6), limiting their ability to simulate polar amplification of green- accounts for this SST gradient, suggesting that climate models house warming. It is also possible, however, that the available underestimate Eocene SSTs in regions where the thermohaline high-latitude paleotemperature proxy records are inaccurate (3), circulation leads to relatively high temperatures. or are not representative of high-latitude temperatures generally. Widely differing paleotemperature estimates from the southern Author contributions: P.M.J.D., H.P.A., and L.C.I. designed research; P.M.J.D., A.J.P.H., and W.P.S. performed research; P.M.J.D., H.P.A., L.C.I., A.J.P.H., W.P.S., A.S., S.S., and M.P. high latitudes have complicated comparisons of paleotemperature analyzed data; and P.M.J.D. wrote the paper. data to models of Eocene climate. Paleotemperature estimates The authors declare no conflict of interest. from the East Tasman Plateau [Ocean Drilling Program (ODP) This article is a PNAS Direct Submission. site 1172; 67°S paleolatitude] (7) and New Zealand (55°S) (8) in 1Present address: Division of Geological and Planetary Sciences, California Institute of the southwest Pacific (Fig. 1), derived primarily from the archaeal- Technology, Pasadena, CA 91125. – lipid based paleotemperature proxy TEX86 (9), yield elevated sea 2 – To whom correspondence should be addressed. E-mail: [email protected]. surface temperature (SST) estimates of 26 28 °C in the early Eo- 3 – Present address: Netherlands Organization for Applied Scientific Research (TNO), Prince- cene and 17 24 °C in the middle Eocene (4) that suggest extreme tonlaan 6, 3584 CB, Utrecht, The Netherlands. polar amplification of greenhouse warmth. For comparison, mod- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ern mean annual SST at an equivalent latitude and longitude is 0 °C 1073/pnas.1321441111/-/DCSupplemental. 6582–6587 | PNAS | May 6, 2014 | vol. 111 | no. 18 www.pnas.org/cgi/doi/10.1073/pnas.1321441111 Downloaded by guest on September 28, 2021 Test of the Δ47–Temperature Relationship in Bivalves. Analyses of modern coastal bivalve shells (Table S2) agree with a synthetic calcite Δ47–temperature calibration that was developed using identical methods and equipment (Fig. 2) (23), supporting previous findings (24, 25) that the relationship between molluscan carbonate growth temperature and Δ47 agrees with inorganic calcite calibra- tion data. Some recently published clumped isotope analyses of modern bivalve shells have suggested the possibility of significant deviations from the synthetic-calcite calibration (26–28). The reason for these deviations is unknown at present, but is likely to be at least Latitude SST (º C) in part due to interlaboratory methodological differences, possibly related to the temperature of acid digestion (28). The modern bi- valve Δ47 data presented here (Fig. 2) are more relevant to our fossil samples than the datasets of refs. 27 and 28, because (i) those W studies were conducted with a 90 °C common acid bath, while we R use the same methodology as the synthetic-calcite calibration (15, 23); and (ii) in addition to shallow marine bivalves, the datasets of refs. 27 and 28 include cultured bivalves, deep-water bivalves, and Longitude other mollusk taxa that may exhibit different isotopic behavior. Seymour Island ODP 1172 New Zealand (East Tasman Plateau) Diagenetic Alteration of La Meseta Fm. Bivalve Shells. La Meseta DSDP 277 DSDP 511 Fm. bivalve fossils have been previously tested extensively for diagenetic recrystallization (11, 12, 31), including analyses of min- Fig. 1. The Eocene locations of SST proxy sites discussed in the text, shown eralogy, microstructure, minor and trace elements, and cath- with the UVic model Eocene paleogeography and sea surface temperatures odoluminescence. Furthermore, growth layers with interannual and from a model simulation with 1,600 ppm pCO2. The modeled region of δ18 δ13 deep-water formation in the Ross Sea is outlined in red. The modeled seasonal variations in Oand C values are found in the shells of depth-integrated stream function is also shown; solid and dashed lines