2. Origin of the Mid-Pleistocene Transition
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 = Rsample/Rref – 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. However, understanding the CO2–climate relationship prior to the MPT is impeded by the lack of direct observations on CO2 in the 40k world.
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. Our Allan Hills ice cores samples therefore provide a unique opportunity to test these hypotheses regarding the MPT.
In Yan et al (under revision; Nature), 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 256 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 and provided feedback to cause glacial periods to become colder. Our data are consistent with the scenarios in Martinez-Garcia et al (2011), although other hypotheses are also permitted.