Guest post by David Middleton
One of the big mysteries in the scientific world is how the ice sheets of Antarctica formed so rapidly about 34 million years ago, at the boundary between the Eocene and Oligocene epochs.
There are 2 competing theories:
The first explanation is based on global climate change: Scientists have shown that atmospheric carbon dioxide levels declined steadily since the beginning of the Cenozoic Era, 66 million years ago. Once CO2 dropped below a critical threshold, cooler global temperatures allowed the ice sheets of Antarctica to form.
The second theory focuses on dramatic changes in the patterns of ocean circulation. The theory is that when the Drake Passage (which lies between the southern tip of South America and Antarctica) deepened dramatically about 35 million years ago, it triggered a complete reorganization in ocean circulation. The argument is that the increased separation of the Antarctic land mass from South America led to the creation of the powerful Antarctic Circumpolar Current which acted as a kind of water barrier and effectively blocked the warmer, less salty waters from the North Atlantic and Central Pacific from moving southwards towards the Antarctic land mass leading to the isolation of the Antarctic land mass and lowered temperatures which allowed the ice sheets to form.
No one has thought to link these two competing explanations before
A group of researchers, led by scientists in McGill University’s Dept. of Earth and Planetary Sciences now suggest that the best way to understand the creation of this phenomenon is, in fact, by linking the two explanations.
In a paper published on the subject in Nature Geoscience earlier this week they argue that:
- The deepening of the Drake Passage resulted in a change in ocean circulation that resulted in warm waters being directed northwards in circulation patterns like those found in the Gulf Stream that currently warms northwestern Europe.
- That this shift in ocean currents, as the warmer waters were forced northward, lead to an increase in rainfall, which resulted, beginning about 35 million years ago to reduced carbon dioxide levels in the atmosphere. Eventually, as the levels of carbon dioxide in the atmosphere dropped, as a result of a process known as silicate weathering (whereby silica-bearing rocks are slowly worn away by rainfall leading the carbon dioxide from the atmosphere to eventually becomes trapped in limestone) there was such a significant drop in CO2 in the atmosphere that it reached a threshold where ice sheets could form rapidly in Antarctica.
To read “Enhanced weathering and CO2 drawdown caused by latest Eocene strengthening of the Atlantic meridional overturning circulation,” by Geneviève Elsworth, et al in Nature Geoscience: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2888.html
Meet the new theory
Same as the old theory
My apologies to Pete Townsend for borrowing his lyrics… But this doesn’t sound “new” to me. The only thing that appears to be “new” is the competing “theory” that a drop in atmospheric CO2 triggered the rapid formation of the Antarctic ice sheet. The tectonics-driven disruption of oceanic circulation has been the generally accepted theory for as long as I can remember.
The Earth’s climate rapidly cooled about 34 million years ago. This cooling marks the transition from the Eocene to the Oligocene…
Whereas atmospheric CO2 appears to have remained elevated for about 2 million years after the sudden drop in temperature…
My apologies for the different x-axes. I made these graphs several years ago and don’t recall where I saved the Excel files.
Otherwise, the press release is very well-written. The paper, which is behind a paywall, also looks to be very well done…
Enhanced weathering and CO2 drawdown caused by latest Eocene strengthening of the Atlantic meridional overturning circulation
Geneviève Elsworth, Eric Galbraith, Galen Halverson & Simon Yang
Nature Geoscience (2017) doi:10.1038/ngeo2888
Received 14 October 2016 | Accepted 03 January 2017 | Published online 30 January 2017
On timescales significantly greater than 105 years, atmospheric pCO2 is controlled by the rate of mantle outgassing relative to the set-point of the silicate weathering feedback. The weathering set-point has been shown to depend on the distribution and characteristics of rocks exposed at the Earth’s surface, vegetation types and topography. Here we argue that large-scale climate impacts caused by changes in ocean circulation can also modify the weathering set-point and show evidence suggesting that this played a role in the establishment of the Antarctic ice sheet at the Eocene–Oligocene boundary. In our simulations, tectonic deepening of the Drake Passage causes freshening and stratification of the Southern Ocean, strengthening the Atlantic meridional overturning circulation and consequently raising temperatures and intensifying rainfall over land. These simulated changes are consistent with late Eocene tectonic reconstructions that show Drake Passage deepening, and with sediment records that reveal Southern Ocean stratification, the emergence of North Atlantic Deep Water, and a hemispherically asymmetric temperature change. These factors would have driven intensified silicate weathering and can thereby explain the drawdown of carbon dioxide that has been linked with Antarctic ice sheet growth. We suggest that this mechanism illustrates another way in which ocean–atmosphere climate dynamics can introduce nonlinear threshold behaviour through interaction with the geologic carbon cycle.