A scanning electron microscope image of a foraminiferan microfossil: tiny, but packed with information.
For several million years, the Earth cycled through ice ages at a regular pace, but then, 1.25 million to 700,000 years ago, something changed: ice ages went from lasting 40,000 years to 100,000.
Now, scientists doing research in the Southern Ocean think they have a clue about why the cold snaps more than doubled in length.
By looking at the microscopic shells of microorganisms called foraminifera, Adam Hasenfratz of the Geological Institute in Zürich, Switzerland, and colleagues, find evidence of a reduction in deep water circulation, causing less carbon dioxide to be released into the air.
Atmospheric CO2, as a mountain of current research tells us, contributes to the global greenhouse effect and would have allowed the earth to come out of an ice age more quickly.
Oceanic changes in the Antarctic Zone could have ensured “that glacial conditions persisted despite orbital changes to the contrary”, the study says.
Prior to the transition, scientists say that ice ages generally mirrored variations in solar radiation from the Earth’s uneven orbital path.
The new research, presented in the journal Science, suggests a solution to the mystery surrounding what is known as the mid-Pleistocene transition (MPT).{%recommended 1335%}
“In this view, the reconstructed change in glacial Antarctic Zone conditions at the end of the MPT may have allowed Northern Hemisphere ice sheets to survive periods of orbitally paced summer insolation maxima on a more regular basis, thereby increasing the longevity of ice ages,” the researchers write
To determine the prehistoric ocean circulation, Hasenfratz and his colleagues looked at sediment samples accumulated over 1.5 million years and analysed trace elements and oxygen isotopes from preserved foraminifera shells.
The results indicated that during glacial periods after the MPT the upper regions of the Southern Ocean became fresher and cooler, while its depths grew more saline.
Laurie Menviel from Australia’s University of New South Wales writes in a related Perspective in the journal: “As the understanding of glacial-interglacial cycles and the mid-Pleistocene transition has improved, it has become clear that there is a tight coupling between continental ice sheets, oceanic circulation, and the global carbon cycle, with positive feedbacks that amplify the response of the climate system.”