Mid-Pleistocene Transition

The Mid-Pleistocene Transition (MPT), also known as the Mid-Pleistocene Revolution (MPR),[1] is a fundamental change in the behaviour of glacial cycles during the Quaternary glaciations.[2] The transition occurred gradually,[3] taking place approximately 1.25–0.7 million years ago, in the Pleistocene epoch.[4] 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.[2] Because of this, sheets were more dynamic during the Early Pleistocene.[5] 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.[2] This led to less dynamic ice sheets.[5] Interglacials before the MPT had lower levels of atmospheric carbon dioxide compared to interglacials after the MPT.[6] One of the MPT's effects was causing ice sheets to become higher in altitude and less slippery compared to before.[7] The cycle lengths have varied, with an average length of approximately 100,000 years.[2][4]

Five million years of glacial cycles are shown, based on oxygen isotope ratio believed to be a good proxy of global ice volume. The MPT is the transition between the periodicities shown in green.

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.[2] The reduction in CO₂ may be related to changes in volcanic outgassing, the burial of ocean sediments, carbonate weathering or iron fertilization of oceans from glacially induced dust.[8]

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.[4]

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.[9] 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.[10]

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.[11]