Yuzhen Yan

Between 2.7 to 1.2 million years (Ma), Earth’s glacial cycles had a 40-kyr period (the “40k world”), revealed by the isotopic composition of oxygen (δ¹⁸O; δ = R_sample/R_ref – 1; R is the ¹⁸O/¹⁶O ratio) in benthic foraminifera shells, a proxy combining deep ocean temperature and global ice volume. Since 1.2-0.8 Ma, an interval known as the “Mid-Pleistocene Transition” (MPT), the period of glacial cycles lengthened to ~100 kyr (the “100k world”) and ice sheet grew larger during the glacial times. This interval is of particular interest to paleoclimatologists because it did not involve significant changes in solar radiation or Earth’s orbit around the Sun. Rather, internal components of the Earth system are thought to be responsible, such as ice sheet dynamics and greenhouse gas concentrations.

Hypotheses for the MPT can be categorized as (1) those invoking changes in ice sheet dynamics and (2) those involving an external forcing of the global carbon cycle. Across the MPT, ice sheet grew larger, became more stable, and thereby was harder to melt. Clark and Pollard (1998) and Raymo et al (2006) identified the Laurentide Ice Sheet and the East Antarctic Ice Sheet as where this critical ice volume expansion occurred, respectively. On the other hand, Raymo et al (1988) first put forward that the MPT could be explained by a long-term decline in atmospheric CO₂ driven by enhanced continental weathering. More recently, Martinez-Garcia et al (2011) hypothesized that the MPT is a consequence of increasing glacial dust flux to the Southern Ocean beginning shortly after 1.5 Ma. Dust would deliver iron, enhance productivity, accelerate biological CO₂ uptake, lower atmospheric CO₂, and lead to global cooling and to bigger, longer-lived polar ice sheets. To evaluate these hypotheses, it is crucial to have accurate reconstructions of past atmospheric CO₂ concentrations. Million-year-old Allan Hills blue ice samples provide a unique opportunity to test these hypotheses regarding the MPT.

In Yan et al (2019), we report a 37-ppm decline in the minimum CO₂ concentrations during glacial maxima across the MPT, from 217 to 180 ppm. Despite this glacial CO₂ decrease, maximum CO₂ did not appreciably change during the interglacials: the measured highest CO₂ concentration in the 40k world is 279 ppm. The similarity between the interglacial CO₂ argues against the notion that a long-term decline in atmospheric CO₂ caused cooling and eventually the initiations of 100 kyr glacial cycles. Instead, it appears that CO₂ responded to some initial changes around the MPT, including although not limited to the enhanced dust flux into the Southern Ocean as observed in Martinez-Garcia et al (2011). Acting as an amplifier, a decline in glacial CO₂ levels caused the cold periods to become even colder.

Clark, P.U. and Pollard, D., 1998. Origin of the middle Pleistocene transition by ice sheet erosion of regolith. Paleoceanography, 13(1), pp.1-9.

Martínez-Garcia, A., Rosell-Melé, A., Jaccard, S.L., Geibert, W., Sigman, D.M. and Haug, G.H., 2011. Southern Ocean dust–climate coupling over the past four million years. Nature, 476(7360), pp.312-315.

Raymo, M.E., Lisiecki, L.E. and Nisancioglu, K.H., 2006. Plio-Pleistocene ice volume, Antarctic climate, and the global δ¹⁸O record. Science, 313(5786), pp.492-495.

Yan, Y., Bender, M.L., Brook, E.J., Clifford, H.M., Kemeny, P.C., Kurbatov, A.V., Mackay, S., Mayewski, P.A., Ng, J., Severinghaus, J.P. and Higgins, J.A., 2019. Two-million-year-old snapshots of atmospheric gases from Antarctic ice. Nature, 574(7780), pp.663-666.