34 million years ago, a lush forest still grew in Antarctica. Within 200,000 years, the Earth's atmosphere cooled - Antarctica became an ice-covered continent. What contributed to climate change? Researchers report in the journal Nature Geoscience.
The forest in Antarctica around 34 million years ago was lush, with beech trees and palm ferns. Within 200,000 years - a very short time in geological terms - the Earth's atmosphere cooled drastically and Antarctica became the ice-covered continent we know today. These far-reaching climatic changes are linked to weathering processes on the Antarctic mainland. Researchers from the Max Planck Research Group Marine Isotope Geochemistry at the University of Oldenburg and the University of Florida have now reported on the causes of these processes in the journal Nature Geoscience.
Dr Chandranath Basak and Dr Ellen Martin found that the weathering of Antarctic rocks contributed to climate change during the transition from the Eocene to the Oligocene. They studied deep-sea sediments from a large-scale programme of scientific ocean drilling. Based on their results, Basak and Martin draw conclusions about weathering processes on the Antarctic continent. One consequence of this is that the concentration of the greenhouse gas carbon dioxide has decreased to such an extent that the climate has cooled and the ice sheet has subsequently built up.
When rock weathers, chemical compounds are released that change the seawater. Sooner or later, the residues end up at the bottom of the ocean. Scientists can "read" particular events in the Earth's history from these sediments, which have been deposited over many millions of years. To do this, they use certain properties in the composition of the sediment that allow them to trace processes in the past.
Basak and Martin have analysed lead isotopes and developed a new approach with which they can determine the weathering of rocks in the past. "With this method, we can tell whether sediments were formed by chemical weathering, i.e. by changes caused by chemical processes, or by physical weathering, for example by glacial erosion," says Basak. For example, they were able to prove that carbonate-containing rock was weathered when the ice sheet formed. Chemical changes in the seawater and the resulting increased deposition of carbonates were presumably the result. This process can be understood as "deacidification", in contrast to the ocean acidification that is taking place today.
"It is not easy to reconstruct the processes that led to climate change millions of years ago at the boundary between the Eocene and Oligocene. Nevertheless, we believe that our work makes an important contribution to understanding this transitional period," says Basak.