On the Cause of the Mid‐Pleistocene Transition

Abstract

Uncertainties in long-term projections of sea-level rise beyond the 21st century are dominated by the rate at which the Greenland and Antarctic ice sheets will melt in a warming climate (Oppenheimer et al., 2019). Since the response of these ice sheets to any change in climate occurs extremely slowly on the human time scale, observational evidence of ice-sheet retreat cannot sufficiently reduce these uncertainties. Instead, the research within paleoclimatology and paleoglaciology must help answer the question of how the ice sheets will evolve in a changing climate on long time scales. Scientists study the Pleistocene glacial cycles because they show dramatic changes in both global climate and ice-sheet size, as reflected in the landscape and in various different proxy archives. Although this research has been going on for almost two centuries, their potential for improving our understanding of the Earth's climate system in the context of anthropogenic climate change has made this field more relevant now than ever before.The hypothesis that the Pleistocene glacial cycles were caused by small, periodic changes in the shape of Earth's orbit around the Sun, the result of gravitational perturbation by the Moon and the other planets in the solar system, was first proposed by Serbian scientist Milutin Milankovic (1941). Although the Scottish scientist James Croll proposed a very similar theory as early as 1875 (Fleming, 2006), he proposed a link to precession rather than to obliquity and/or eccentricity. The resulting prediction of 20 kyr glacial cyclicity led to his theory being rejected, and subsequently ignored for almost a century, by the scientific community. A revival occurred when Hays et al. (1976) published their isotopic analysis of a deep-sea sediment core, spanning over 400,000 years and clearly showing four distinct glacial cycles, introducing many new questions. Although spectral analysis of the isotopic signal indicated minor peaks at the precession (19/23 kyr) and obliquity (41 kyr) periodicities, the dominant peak occurred at ∼100 kyr. This seemed to correspond to one of the eccentricity terms, which however provides only a small contribution to insolation changes. Later that year, Shackleton and Opdyke (1976) published results from another core, dating back well over two million years. Their findings complicated matters even further: Although their core displayed the same

Ice core evidence for atmospheric oxygen decline since the Mid-Pleistocene transition

“…Oxidizing this entire surface reservoir would only lead to a 2‰ decline in atmospheric O 2 . We thus conclude that the initiation of the decline in atmospheric P O 2 approximately coincided with the Mid-Pleistocene transition (MPT), a period between ~1250 and 700 ka where Earth’s climate underwent a fundamental shift characterized by the emergence of high-amplitude glacial-interglacial oscillations and an increase in the duration of glacial cycles from 40 to 100 ka ( 30 – 32 ).…”

“…The blue ice δO 2 /N 2 values were systematically increased by 17.4‰, so δO 2 /N 2 reaches 0 when extrapolated from the 810- and 400-ka data to present, consistent with the treatment of the composite atmospheric δO 2 /N 2 time series ( 8 ). ( B ) A continuous ice core CO 2 record over 800 ka (solid purple line) ( 64 – 67 ), atmospheric CO 2 reconstructions based on boron isotopes (dashed purple line) ( 38 ), and the Allan Hills blue ice CO 2 data, grouped to the same age assignment as for δO 2 /N 2 (purple circles) ( 7 , 32 ). ( C ) Stacked oxygen isotope composition of benthic foraminifera (LR04) ( 30 ).…”

Characterization of Viscous Dissipative Heating in the Earth's Mantle Caused by Surface Forces

“…Here we discuss possible dissipation rate for the early Earth conditions, focusing on loading processes of different periods with reduced mantle viscosities. Long‐term climate processes such as glacial cycles are mostly driven by insolation variations that are controlled by the Earth's orbital parameters including eccentricity, obliquity, and precession rate (Berends et al., 2021; Milankovic, 1941). However, given that the energy flux of dissipation decreases with decreasing viscosity at relatively long periods (Figure 10), it is unlikely that these long‐term climate driven processes would contribute any significant dissipative heating in the mantle.…”

Vibrational resonance in 100-kyr glacial cycles

“…We focused here on the ∼100-kyr glacial cycles of the last 1 Myr, i.e., after the Mid-Pleistocene transition (MPT) 7,8 . Before the MPT, the periodicity of glacial cycles was predominantly 41 kyr; the VR mechanism did not operate at ∼100-kyr periodicity.…”