Mid-Pleistocene Transition

The Mid-Pleistocene Transition (MPT), also known as the Mid-Pleistocene Revolution (MPR), is a fundamental change in the behaviour of glacial cycles during the Quaternary glaciations. The transition occurred gradually, taking place approximately 1.25–0.7 million years ago, in the Pleistocene epoch. Before the MPT, the glacial cycles were dominated by a 41,000-year periodicity with low-amplitude, thin ice sheets, and a linear relationship to the Milankovitch forcing from axial tilt. Because of this, sheets were more dynamic during the Early Pleistocene. After the MPT there have been strongly asymmetric cycles with long-duration cooling of the climate and build-up of thick ice sheets, followed by a fast change from extreme glacial conditions to a warm interglacial. This led to less dynamic ice sheets. Interglacials before the MPT had lower levels of atmospheric carbon dioxide compared to interglacials after the MPT. One of the MPT's effects was causing ice sheets to become higher in altitude and less slippery compared to before. The MPT greatly increased the reservoirs of hydrocarbons locked up as permafrost methane or methane clathrate during glacial intervals. This led to larger methane releases during deglaciations. The cycle lengths have varied, with an average length of approximately 100,000 years.

The Mid-Pleistocene Transition was long a problem to explain, as described in the article 100,000-year problem. The MPT can now be reproduced by numerical models that assume a decreasing level of atmospheric carbon dioxide, a high sensitivity to this decrease, and gradual removal of regoliths from northern hemisphere areas subject to glacial processes during the Quaternary. The reduction in may be related to changes in volcanic outgassing, the burial of ocean sediments, carbonate weathering or iron fertilization of oceans from glacially induced dust.

Regoliths are believed to affect glaciation because ice with its base on regolith at the pressure melting point will slide with relative ease, which limits the thickness of the ice sheet. Before the Quaternary, northern North America and northern Eurasia are believed to have been covered by thick layers of regoliths, which have been worn away over large areas by subsequent glaciations. Later glaciations were increasingly based on core areas, with thick ice sheets strongly coupled to bare bedrock.

It has also been proposed that an enlarged deep ocean carbon inventory in the Atlantic Ocean played a role in the increase in amplitude of glacial-interglacial cycles because this increase in carbon storage capacity is coincident with the transition from 41-kyr to 100-kyr glacial-interglacial cycles.

A 2023 study formulates an innovative hypothesis on the origin of the MPT (obliquity damping hypothesis). This hypothesis is based on the observational evidence of obliquity damping in climate proxies and sea-level record during the Last 1.2 Ma. Obliquity damping is linked with short eccentricity amplification which appears as a missing-link for the MPT. The study hypothesises that both the glacio-eustatic water mass component in the obliquity band may controlled the Earth's oblateness changes and the obliquity phase lag estimated to be <5.0 kyr, explain obliquity’s damping by the obliquity-oblateness feedback as latent physical mechanism at the origin of the MPT. The obliquity damping might have contributed to the strengthening of the short eccentricity response by mitigating the obliquity ‘ice killing’ during obliquity maxima (interglacials), favouring the obliquity-cycle skipping and a feedback-amplified ice growth in the short eccentricity band.

However, a 2020 study concluded that ice age terminations might have been influenced by obliquity since the Mid-Pleistocene Transition, which caused stronger summers in the Northern Hemisphere. Evidence suggests that fluctuations in the volume of the West Antarctic Ice Sheet continued to be governed dominantly by fluctuations in obliquity until about 400,000 years ago.

In Europe, the MPT was associated with the Epivillafranchian-Galerian transition and may have led to the local extinction of, among other taxa, Puma pardoides, Megantereon whitei, and Xenocyon lycaonoides.

The Bay of Bengal experienced increased stratification as a result of the strengthening of the Indiam Summer Monsoon (ISM), which resulted in increased riverine flux, inhibiting mixing and creating a shallow thermocline, with stratification being stronger during interstadials than stadials. Paradoxically, variability in Δδ18O in the Bay of Bengal between glacials and interglacials decreased as a result of the MPT.

In Australia the MPT resulted in the formation of the dunes of Fraser Island and the Cooloola Sand Mass. The increasing amplitude of sea level variations led to increased redistribution of sediments stored on the seafloor across the continental shelf. The development of Fraser Island indirectly led to the formation of the Great Barrier Reef by drastically decreasing the flow of sediment to the area of continental shelf north of Fraser Island, a necessary precondition for the growth of coral reefs on such an enormous scale as found in the Great Barrier Reef.