Reflected light image of the shell of a fossil planktic foraminifera Globigerinoides ruber. The boron isotopic composition of the shells of this species was used to reconstruct atmospheric CO2, from 1 million years ago, in this study. Photo: Tom Chalk

The Mid-Pleistocene Transition (MPT) was a watershed change in Earth’s climate about 1 million years ago. What had been regular cycles of 40,000 years between massive glacial cover and gradual melting and back again transitioned to more extreme and deep ice age cycles of 100,000 years now known as Milankovitch Cycles, after the Serbian mathematician who proposed their existence.

Now a team of scientists, using frozen bubbles in Antarctic ice cores and boron isotopes trapped in ancient microscopic seashells, have determined that carbon dioxide levels dropping extremely low triggered the change, as described in the latest Proceedings of the National Academy of Sciences.

The bubbles in the ice documented the carbon dioxide levels – but only back as far as 800,000 years ago, reports the team from the University of Southampton. That time frame did not give a picture of the atmospheric conditions before the planet-altering MPT.

They looked instead to another marker: the shells of marine plankton known as foraminifera. The boron levels in those organisms living near the surface of the ancient waters also recorded the chemical markers of its environment, the authors write.

“From these boron isotope measurements we were able to recover a snapshot of the variability of atmospheric [carbon dioxide] around 1.1 million years ago,” said Gavin Foster, one of the authors, in a school statement. “During the glacials before the MPT, [carbon dioxide] did not drop as low as it did in the ice cord record after the MPT, remaining about 20-40 parts per million higher. Secondly, the climate system was also more sensitive to changing carbon dioxide after the MPT than before.”

The Milankovitch Cycles theory relies on gradual changes in the orbit of Earth, and the rotation on its axis – both of which are effected (and affected) by gravitational pulls of other planets. But the theory has had difficulties explaining the MPT sea change.

The new Southampton addition to the theory is that the chemical components of our planet itself changed the precursors for the climate tipping points over the last million years.

“Our best model fit to the available data suggests that the reduced drawdown of carbon dioxide during glacial periods prior to the MPT was due to a reduced flux of dust to the Southern Ocean at this time,” said Mathis Hain, another of the Southampton authors. “A higher dust flux during more recent glacial intervals brought much needed iron to that region, stimulating primary productivity and phytoplankton growth, locking more carbon dioxide away in the deep ocean.

“We don’t know yet exactly why the climate became dustier after MPT, but it is likely due to the ice sheets getting bigger and changing atmospheric circulation,” Hain added.

Foraminafera have been assessed for other chemical clules to the ancient climate and its change over time. For instance, a team led by Oxford and Washington University in St. Louis scientists instead gauged the oxygen isotopes to understand the pre-MPT climate, as they reported in January in the journal Nature Communications.