The formation of the Antarctic ice sheet during a major climate shift 34 million years ago was the result of decreased carbon dioxide (CO2) levels, and not continental breakup like a widely held theory suggested, according to new research.
The 40-year-old "Southern Ocean gateway opening" theory indicated that massive rearrangements of Earth's continents caused global cooling and the abrupt formation of the Antarctic ice sheet. But now researchers from the University of New Hampshire (UNH) have provided evidence refuting this popular notion, which refers to the Eocene-Oligocene transition when Earth's polar regions were ice-free.
"The Eocene-Oligocene transition was a major event in the history of the planet and our results really flip the whole story on its head," Matthew Huber of the UNH Institute for the Study of Earth, Oceans, and Space and department of Earth sciences, said in a statement.
"The textbook version has been that gateway opening, in which Australia pulled away from Antarctica, isolated the polar continent from warm tropical currents, and changed temperature gradients and circulation patterns in the ocean around Antarctica, which in turn began to generate the ice sheet. We've shown that, instead, CO2-driven cooling initiated the ice sheet and that this altered ocean circulation," he added.
Scientists supported the gateway theory because of previous evidence in the form of a "fingerprint" collected from oxygen isotope records derived from deep-sea sediments.
The declining atmospheric CO2 explanation is not one that hasn't been suggested before, but scientists have never been able to match it up to the isotopic fingerprint. Huber and his team proposed not only that the fingerprint may not be as unique as previously thought, but that the drop in CO2 may have indirectly caused it.
To get to the bottom of this, Huber and colleagues simply modeled the Eocene-Oligocene world as if it contained an Antarctic ice sheet of near-modern size and shape. They compared their results to projected future climate change and a range of CO2 levels that are likely to occur in the next 100 years (560 to 1200 parts per million).
"It should be clear that resolving these two very different conceptual models for what caused this huge transformation of the Earth's surface is really important because today as a global society we are, as I refer to it, dialing up the big red knob of carbon dioxide but we're not moving continents around," Huber explained.
The researchers are still not sure what exactly caused this sharp CO2 drawdown, but now that the gateway theory has been debunked, further study can be done to focus on that detail.
The study's findings were published in the journal Nature.