4.8 Article

Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2

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NATURE GEOSCIENCE
卷 14, 期 9, 页码 659-+

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NATURE PORTFOLIO
DOI: 10.1038/s41561-021-00788-z

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  1. NERC [CC010, NE/L002434/1]
  2. NEIF
  3. European Research Council under the European Union
  4. European Research Council [340923]
  5. Royal Society as part of a Tata University Research Fellowship

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The study presents a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene, using bacterial molecular fossils preserved in lignites from southeast Australia. Results show a gradual decline in mean annual temperatures in southeast Australia, with a step cooling across the Eocene/Oligocene boundary, likely driven by changes in pCO2. Climate model simulations support these findings, indicating the importance of pCO2 decline in influencing temperatures in southeast Australia during this time period.
The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (similar to 41-33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from similar to 27 degrees C (+/- 4.7 degrees C) during the middle Eocene to similar to 22-24 degrees C (+/- 4.7 degrees C) during the late Eocene, followed by a similar to 2.4 degrees C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely pCO(2). We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in pCO(2) lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea-land interaction and climatic forcings at the onset of a major Antarctic glaciation.

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