Early–Middle Pleistocene transitions: Linking terrestrial and marine realms

Abstract

Marked by a progressive increase in the amplitude of climate oscillations, an evolving waveform, and a shift towards a quasi-100 ky frequency, the Early–Middle Pleistocene transition (EMPT), previously known as the Mid-Pleistocene Transition (or Mid-Pleistocene Revolution) (1.4–0.4 Ma), represents a fundamental transformation in the Earth's climate state. The EMPT began with a substantial change in climate dynamics and ended with the Mid-Brunhes Event, signaling the establishment of a new steady state. The reasons for the EMPT while uncertain appear to involve a non-linear response of the Earth climate system. The physical and biotic responses to this transition, amplified by the growth of Northern Hemisphere ice sheets, have been profound. Two important chronostratigraphic markers characterize the EMPT, the Jaramillo Subchron (1.070–0.988 Ma) and the Matuyama–Brunhes Chron boundary (∼773 ka). The latter has been chosen as the primary guide for the Lower–Middle Pleistocene Subseries boundary, as it lies at the approximate midpoint of the EMPT and aids in global recognition both in marine and terrestrial deposits. The Jaramillo Subchron has received less attention, but the late Early Pleistocene is important in Europe because it saw the progressive transition from the Villafranchian to Galerian mammal faunas, and expansion of hominins into western and northern Europe. The Jaramillo Subchron is represented by Marine Isotope Stages (MIS) 31 to 28, with MIS 30 already showing the asymmetrical (sawtooth) pattern characteristic of the Middle Pleistocene. Indeed, while variation in the 40-ky band (obliquity) remains strong throughout the EMPT, low frequency variability begins at around 1250–1200 ky, which coincides with a progressive increase in global ice volume. Against a backdrop of increasingly severe glacial cycles, notably during MIS 36, 34, 24–22 (the so-called “0.9 Ma event”), 16 and 12, pronounced phases of warming are also documented globally, including the “super-interglacial” MIS 31.The early phase of the EMPT is characterized by important glaciations beginning with MIS 36 and continuing to MIS 24–22, a major intensification of the East Asian monsoon system, intensification of loess deposition in northern Europe, development of open landscapes in western Siberia, increased fluvial incision, higher amplitude sea-level change, and spread of large mammals across northern Eurasia, and a strong reduction in the North Atlantic thermohaline circulation. In Europe, the loss of thermophilous plant taxa during the EMPT and indeed throughout the Quaternary is a reminder of the progressive cooling that took place here and elsewhere.