Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2

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.

Automated summary

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.