Earth’s mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere1, and influences vertical ocean structure, deep-water production2 and the global distribution of nutrients and chemical tracers3. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth’s present and past climate system1, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields4,5,6,7) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent–ocean boundaries8, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian–Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation9 and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time10.
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H.D.S. received support from an UTAS IMAS Visiting Scholarship and an EES travel grant while this manuscript was prepared, and was supported by the US National Science Foundation (NSF) award OCE 1155630. J.M.W. was supported by ARC grant DE140100376. M.L.D. was supported by NSF award OCE-0647876. This research used data and samples from the Ocean Drilling Program (ODP), which was sponsored by the NSF and participating countries under the management of Joint Oceanographic Institutions. GPlates is free software that is licensed for distribution under the GNU General Public License v2 (http://www.gplates.org). Researchers in the EarthByte group have made GPlates compatible data available, some of which were used in this study. We thank C. Moy, C. Riesselman, R. Rykaczewski and A. Winguth for discussions. M. Seton and W. Sijp provided feedback on an early version of the manuscript. J. Aggarwal, E. Bair, M. Frank and S. Sampson provided analytical support.
The authors declare no competing financial interests.
Extended data figures and tables
Present-day coastlines (black), continental shelves (white), and range of the innermost and outermost continent–ocean boundaries (COBs) for the conjugate margins of the Tasmanian Gateway (grey) are shown for time slices from the late Eocene (a, b) to the early Oligocene (c–h). The palaeobathymetry grid is based on ref. 53. Palaeolatitudes of the innermost and outermost COB locations in Fig. 2 are based on their position along a meridional section drawn through the narrowest portion of the gateway (red arrow in each reconstruction). Palaeocurrent direction (indicated by black arrows) is based on the Nd isotope data from sites 1168 and 1172 (red circles), which indicate westward or eastward flow, depending on the εNd values at the sites. The εNd values listed on the diagrams are the average values for the time slices shown. Numbers in parentheses show the standard error (see Source Data).
The relative position of the northern conjugate margin of the Tasmanian Gateway relative to the Oligocene polar front was tested using three reference frames: a, ref. 54; b, ref. 55; and c, ref. 18. Although the absolute palaeolatitudes of these features differ between these reference frames, the timing of the northern margin transit across the Oligocene polar front is not affected by the choice of reference frame because the reconstruction of the Oligocene polar front is reconstructed using the same rotation data as the margins. In each reference frame, the northern margin of the gateway migrates across the Oligocene polar front between 29 and 30 Ma. Light blue bands correspond to the maximum gateway width based on geophysically determined positions of the innermost COBs on the South Tasman Rise and Antarctica8, selected along the narrowest meridional transect of the gateway (Extended Data Fig. 1). Dark blue bands show the minimum gateway widths based on the outermost geophysically determined COBs. The Oligocene position of the polar front (yellow band) was derived from microfossil assemblage data in deep-sea sediment cores19; these data were incorporated into the plate reconstruction used in this study. The grey band centred at 30 Ma highlights the age range of the South Tasman Rise crossing the Oligocene polar front.
Nd isotope records used to demarcate Pacific (red) and Indian–Atlantic (purple) waters over the study interval. a, Individual Nd isotope records (see Extended Data Table 3 for reference list). b, Range of the endmember waters for the Pacific and Indian–Atlantic. Envelope around the Pacific Nd isotope stack is instrumental uncertainty (2σ).
Cascade Guyot subsided at least 1,000 m since the late Eocene, judging by the sediments dredged high on the flank. Shallow marine sediments are currently ∼2,900 m below sea level at ODP site 1172, suggesting that the East Tasman Plateau subsided three times more than the Guyot over the same interval.
a, Detrital Al/Ti ratios from ODP site 1172 (red triangles). b, Fossil fish tooth Nd isotope records from the Western Tasmanian Margin (WTM; site 1168; blue), the East Tasman Plateau (ETP; site 1172; grey), and the Hikurangi Plateau (HP; site 1124; red). Error envelopes represent instrumental uncertainty (2σ) based on replicate Nd isotope analyses of the JNdi-1 Nd isotope standard. Error bars (standard errors) are shown for samples where the standard error of the measurement exceeds 2σ instrumental uncertainty. The Pacific and Indian–Atlantic εNd envelopes are based on fossil fish tooth Nd isotope records from the equatorial Pacific, southern Kerguelen Plateau, Ninetyeast ridge, and Maud Rise (Extended Data Fig. 3). The range of εNd values of weathering products in the Murray River is shown on the right axis. c, The palaeolatitudes of the conjugate margins of the Tasmanian Gateway outline the width and position of the gateway during its progressive opening. Light blue bands correspond to the maximum gateway width based on geophysically determined positions of the innermost COBs on the South Tasman Rise and Antarctica8, selected along the narrowest meridional transect of the gateway (Extended Data Fig. 1). The dark blue band shows the minimum gateway widths based on the outermost geophysically determined COBs. Error bars (standard errors) on the northern and southern boundaries of the gateway reflect uncertainty in the positions of the conjugate margins based on the 95% confidence intervals from the rotations17. The Oligocene position of the polar front (yellow band) was derived from microfossil assemblage data in deep-sea sediment cores19, which were incorporated into the plate reconstruction used in this study. The grey band centred at 30 Ma highlights the age range of the South Tasman Rise crossing the Oligocene polar front.
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Scher, H., Whittaker, J., Williams, S. et al. Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies. Nature 523, 580–583 (2015). https://doi.org/10.1038/nature14598
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