Pleistocene

The Pleistocene is a geological epoch that lasted from about 2.58 million to 11,700 years before present, encompassing the most recent period of repeated global glaciations. Colloquially referred to as the Ice Age, the Pleistocene is bounded below by the base of the Matuyama C2r magnetic polarity chronozone and marked by significant biological and climatic transitions, including the extinction of calcareous nannofossils such as Discoaster. Its end coincides with the close of the last glacial period and the terminus of the Paleolithic era in human archaeology.

Etymology and Definition

The term Pleistocene was introduced by Charles Lyell in 1839 to describe geological strata in Sicily containing a majority of modern molluscan species. Derived from the Greek pleistos (“most”) and kainos (“new”), the name indicates deposits containing the greatest proportion of species still extant today. It contrasts with the preceding Pliocene (“more new”) and the succeeding Holocene (“entirely new”).
Originally defined as beginning around 1.806 million years BP, the start of the Pleistocene was redefined in 2009 by the International Union of Geological Sciences, shifting the base to 2.58 million years BP and establishing the Gelasian Stage as the lowest unit of the epoch. The upper boundary, marking the onset of the Holocene at approximately 11,700 years BP, is closely aligned with the end of the Younger Dryas cold event.

Chronology and Subdivisions

The Pleistocene is divided into four internationally recognised ages:

• Gelasian (2.58–1.80 Ma)
• Calabrian (1.80–0.77 Ma)
• Chibanian (0.77–0.13 Ma), previously the informal Middle Pleistocene
• Late Pleistocene (0.13 Ma–11,700 BP), informally called the Tarantian

Marine isotope stages (MIS), based on oxygen isotope data, provide a detailed framework for orbital-scale climatic variations and offer dates for glacial–interglacial cycles beyond the effective limit of radiocarbon dating (c. 50,000 years).

Climate and Environmental Change

The Pleistocene climate was characterised by dramatic oscillations between glacial and interglacial phases. During peak glaciation, global sea levels fell by as much as 120 metres, exposing continental shelves and enabling land bridges such as Beringia, which intermittently connected Asia and North America. Approximately 30% of the Earth’s surface was covered by ice sheets during maximum glacial expansions.
Major ice masses included:

• The Laurentide Ice Sheet across much of northern North America
• The Cordilleran Ice Sheet along the western mountains of North America
• The Fennoscandian Ice Sheet covering northern Europe
• The Patagonian Ice Cap in South America
• Extensive glaciation in mountain ranges of Africa, New Zealand and Tasmania

Permafrost zones extended hundreds of kilometres beyond the ice margins. In some interpretations, the Pleistocene climate approximated long-lasting El Niño–like conditions, with altered trade wind patterns and shifts in ocean–atmosphere circulation.

Tectonics, Geography and Sea Levels

By the Pleistocene, continental positions were essentially those of the modern world. However, falling sea levels during glacial maxima repeatedly exposed continental shelves and reshaped coastal environments. The closure of the Isthmus of Panama at the end of the Pliocene had profound consequences for ocean circulation and facilitated the Great American Interchange, altering faunal diversity in both the Americas and contributing to Northern Hemisphere glaciation after about 2.7 Ma.

Sediments and Depositional Environments

Pleistocene sediments record diverse environments:

• Fluvial deposits, lake beds and loess sequences on land
• Glacial tills, moraines and outwash plains
• Cave infillings and travertine deposits
• Volcanic ashes and lava flows associated with contemporaneous eruptions
• Shallow marine deposits in coastal basins, sometimes uplifted to significant elevations in tectonically active regions

Marine deposits contain species indicative of cold-water conditions, providing insight into glacial oceanography.

Evolution and Biogeography

The Early Pleistocene witnessed the emergence of Homo species in Africa, with archaic humans dispersing across Afro-Eurasia. Around the Mid-Pleistocene Transition (c. 1.2–0.7 Ma), glacial cycles shifted from 41,000-year periodicity to stronger, asymmetric 100,000-year cycles, increasing climatic extremes. During the Late Pleistocene, Homo sapiens expanded out of Africa, eventually populating Australasia and the Americas.
This period also saw the extinction of all other human species, including Neanderthals and Denisovans, alongside widespread Late Pleistocene megafaunal extinctions. Causes remain debated, with most explanations centring on human expansion, rapid climatic change or their combined effects.

Glaciation and Global Impacts

The effects of Pleistocene glaciation were widespread:

• Isostatic depression and rebound altered continental topography.
• Meltwater pulses reshaped river systems and created extensive proglacial lakes.
• Shifts in vegetation zones transformed ecological communities worldwide.
• Dust fluxes increased dramatically, contributing to thick loess deposits.

Antarctica remained ice-covered throughout the epoch, reinforcing global cooling and contributing to the rhythmic glacial cycles.

Human Cultural Context

The end of the Pleistocene marks the close of the Paleolithic, during which early humans developed stone tool technologies, symbolic culture and long-distance migrations. The transition to the Holocene corresponds with warming climates, retreating glaciers and the gradual emergence of agriculture in some regions.