Deep Atlantic Ocean used to be 20 °C in past ‘greenhouse climate’

A new study of the temperature of ancient oceans shows that the deep Atlantic Ocean was once a balmy 20 °C - , similar to the surface of the modern Mediterranean.  Scientists say the new data, which covers the last 60 million years, shows the huge impact of higher CO2 levels in the geological past, and stresses the need to avoid a continued rise in CO2 in the future. Their research was published this week in Science.

"Our new data show that this deep ocean was surprisingly warm during past greenhouse climates," says Dr. Martin Ziegler of Utrecht University, co-author of the study, along with colleagues Prof. Lucas Lourens and Tobias Agterhuis. The very warm temperatures in the Atlantic Ocean date back to a time called the Eocene, about 15 million years after the extinction of the dinosaurs. Atmospheric CO2 was about 3 times higher than today. Previous fossil indicators have shown that such a greenhouse world looked very different from our modern planet, with palm trees and crocodiles inhabiting the Arctic. The new data provide, for the first time, accurate estimates of the temperatures of the deep oceans at that time.

Chemical fingerprints from fossils

The international team of scientists from Norway, Switzerland, the UK, the US and the Netherlands used the chemical 'fingerprints' of small fossil shells from mud cores in the deep sea to reconstruct the temperature of the ocean in ancient times. Using state-of-the-art new laboratory measurements, they were able to make the most accurate temperature estimates to date and show that temperatures were not only warmer but also more variable than previously thought. 

Prof. Nele Meckler of the University of Bergen, who led the study, explains: "We looked at how different carbon and oxygen atoms clumped together in these ancient microfossils, which turns out to be a very accurate method of calculating the temperature at the time the shells were formed. Because mud and microfossils are constantly accumulating on the sea floor, a long tube of this mud - up to 3 km long - is like a time capsule.  The deeper you get into the core, the older the microfossils get, and by measuring their chemistry we get a long continuous record of past climate changes.

Climate System

"It is believed that the change in deep-sea temperature is closely related to the change in mean surface temperature. We still need additional data to confirm that these unexpectedly high temperatures were present at depth in the deep Atlantic globally," said Dr. Ziegler. "These new observations are very important because they may ultimately imply a greater sensitivity of the Earth's climate system to increased greenhouse gas concentrations." 

Article

Authors: A.N. Meckler1*, P.F. Sexton2, A.M. Piasecki1†, T.J. Leutert1, J. Marquardt1, M. Ziegler3, T. Agterhuis3, L.J. Lourens3, J.W.B. Rae4, J. Barnet4, A. Tripati5, S.M. Bernasconi6.10.1126/science.abk0604. Cenozoic evolution of deep ocean temperature from clumped isotope thermometry.

https://www.science.org/doi/10.1126/science.abk0604