New research shows that mean annual temperatures in southeast Australia gradually declined from 27 degrees Celsius during the Middle Eocene epoch to 22-24 degrees Celsius during the Late Eocene epoch, followed by a 2.4-degree-step cooling across the Eocene/Oligocene boundary, 33-34 million years ago.
Reconstruction of the West Antarctic mid-Cretaceous temperate rainforest. Image credit: J. McKay / Alfred-Wegener-Institut / CC-BY 4.0.
Between 40 and 34 million years ago, Earth’s climate underwent a major climatic transition.
During the Eocene, Antarctica was covered by lush forests, but by 34 million years ago, in the Oligocene, these forests had been replaced by thick continental ice sheets, as we know Antarctica today.
The main driver of this greenhouse to icehouse transition is widely debated, and little information is available about how climate changed on land.
In the current study, University of Bristol’s Dr. Vittoria Lauretano and colleagues used molecular fossils preserved in ancient coals to reconstruct land temperature across this transition.
They used a new approach based on the distribution of bacterial lipids preserved in ancient wetland deposits.
“These compounds originally comprised the cell membranes of bacteria living in ancient wetlands, with their structures changing slightly to help the bacteria adapt to changing temperature and acidity,” said Dr. Rich Pancost, also from the University of Bristol.
“Those compounds can then be preserved for tens of millions of years, allowing us to reconstruct those ancient environmental conditions.”
To reconstruct temperature change across the greenhouse to icehouse transition, the researchers applied their approach to coal deposits from the southeast Australian Gippsland Basin. These remarkable deposits span over 10 million years of Earth history.
The new data show that land temperatures cooled alongside the ocean’s and by a similar magnitude of about 3 degrees Celsius.
They then conducted climate model simulations to explore causes of the temperature decline.
Crucially, only simulations that included a decline in atmospheric carbon dioxide could reproduce a cooling consistent with the temperature data reconstructed from the coals.
These results provide further evidence that atmospheric carbon dioxide plays a crucial role in driving Earth’s climate, including the formation of the Antarctic ice sheet.
“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,” the authors said.
The study was published this week in the journal Nature Geoscience.
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V. Lauretano et al. Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2. Nat. Geosci, published online August 2, 2021; doi: 10.1038/s41561-021-00788-z
