The middle Miocene climate transition (~14 million years ago) was characterized by a dramatic increase in the volume of the Antarctic ice sheet. The driving mechanism of this transition remains under discussion, with hypotheses including circulation changes, declining carbon dioxide in the atmosphere and orbital forcing. Southern Ocean records of planktic foraminiferal Mg/Ca have previously been interpreted to indicate a cooling of 6–7 °C and a decrease in salinity that preceded Antarctic cryosphere expansion by up to ~300,000 years. This interpretation has led to the hypothesis that changes in meridional heat and vapour transport along with an early thermal isolation of Antarctica from extrapolar climates played a fundamental role in triggering ice growth. Here we revisit the middle Miocene Southern Ocean temperature evolution using clumped isotope and lipid biomarker temperature proxies. Our records indicate that the Southern Ocean cooling and the associated salinity decrease occurred in phase with the expansion of the Antarctic ice sheet. We demonstrate that the timing and magnitude of the Southern Ocean temperature change seen in previous reconstructions can be explained if we consider pH as an additional, non-thermal, control on foraminiferal Mg/Ca ratios. Therefore, our new dataset challenges the view of a thermal isolation of Antarctica preceding ice sheet expansion, and suggests a strong coupling between Southern Ocean conditions and Antarctic ice volume in times of declining atmospheric carbon dioxide.
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The clumped isotope and TEX86 temperature data that support the findings of this study are available in the Supplementary Information and at Pangaea (https://doi.org/10.1594/PANGAEA.919353, https://doi.org/10.1594/PANGAEA.919351). The full raw isotope data is published on the EarthChem Database (https://doi.org/10.26022/IEDA/111547).
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We thank A. Shevenell, D. Evans, G. Foster and A. Fernandez Bremer for insightful discussions, and E. Alagoz, I. Heggstad and M. Schmitt for laboratory assistance. Furthermore, we thank all authors who shared their published data. This research used data and samples provided by the International Ocean Discovery Program (IODP) and its predecessor, the Ocean Drilling Program (ODP). The work was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 638467), the Trond Mohn Foundation and the Max Planck Society.
The authors declare no competing interests.
Peer review information Primary Handling Editor: James Super.
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This file contains Supplementary Methods 1 and 2, Discussions 1–4, Figs. 1–14 and Tables 1–3. In addition to providing additional information on site locality and hydrography, we present details on clumped isotope and lipid biomarker analyses as well as age models. Furthermore, we discuss potential non-thermal and diagenetic effects on the applied temperature proxies.
This file contains Supplementary Tables 4–9. Supplementary Table 4 contains the calculated standard reproducibilities for our stable isotope measurements. Supplementary Tables 5 and 6 show all the stable isotope standard and sample data, respectively. Supplementary Table 7 contains the calculated clumped isotope temperatures. Supplementary Table 8 contains the fractional abundances of GDGTs, calculated temperatures and indices, and Supplementary Table 9 lists the revised ages for boron isotope-based estimates of pH and CO2.
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Leutert, T.J., Auderset, A., Martínez-García, A. et al. Coupled Southern Ocean cooling and Antarctic ice sheet expansion during the middle Miocene. Nat. Geosci. 13, 634–639 (2020). https://doi.org/10.1038/s41561-020-0623-0