Holocene

See also: Timeline of the Holocene

The **Holocene** is the current geological epoch. It began approximately 11,650 cal years before present, after the last glacial period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene has been identified with the current warm period, known as MIS 1. It is considered by some to be an interglacial period within the Pleistocene Epoch.

The Holocene has seen the growth and impacts of the human species worldwide, including all of its written history, development of major civilizations, and overall significant transition towards urban living in the present. Human impacts on modern-era Earth and its ecosystems may be considered of global significance for future evolution of living species, including approximately synchronous lithospheric evidence, or more recently hydrospheric and atmospheric evidence of human impacts. In July 2018, the International Union of Geological Sciences split the Holocene epoch into three distinct subsections, Greenlandian (11,700 years ago to 8,326 years ago), Northgrippian (8,326 years ago to 4,200 years ago) and Meghalayan (4,200 years ago to the present), as proposed by International Commission on Stratigraphy. The boundary stratotype of Meghalayan is a speleothem in Mawmluh cave in India, and the global auxiliary stratotype is an ice core from Mount Logan in Canada.

The name *Holocene* comes from the Ancient Greek words ???? (*holos*, whole or entire) and ?a???? (*kainos*, new), meaning "entirely recent".

## Overview

It is accepted by the International Commission on Stratigraphy that the Holocene started approximately 11,650 cal years BP. The Subcommission on Quaternary Stratigraphy quotes Gibbard and van Kolfschoten in Gradstein Ogg and Smith in stating the term 'Recent' as an alternative to Holocene is invalid and should not be used and also observe that the term Flandrian, derived from marine transgression sediments on the Flanders coast of Belgium has been used as a synonym for Holocene by authors who consider the last 10,000 years should have the same stage-status as previous interglacial events and thus be included in the Pleistocene. The International Commission on Stratigraphy, however, considers the Holocene an epoch following the Pleistocene and specifically the last glacial period. Local names for the last glacial period include the Wisconsinan in North America, the Weichselian in Europe, the Devensian in Britain, the Llanquihue in Chile and the Otiran in New Zealand.

The Holocene can be subdivided into five time intervals, or chronozones, based on climatic fluctuations:

* Preboreal (10 ka–9 ka BP),
* Boreal (9 ka–8 ka BP),
* Atlantic (8 ka–5 ka BP),
* Subboreal (5 ka–2.5 ka BP) and
* Subatlantic (2.5 ka BP–present).

*Note: "ka" means "kilo-annum" Before Present, i.e. 1,000 years before 1950 (non-calibrated C14 dates)*

The Blytt–Sernander classification of climatic periods initially defined by plant remains in peat mosses, is currently being explored. Geologists working in different regions are studying sea levels, peat bogs and ice core samples by a variety of methods, with a view toward further verifying and refining the Blytt–Sernander sequence. They find a general correspondence across Eurasia and North America, though the method was once thought to be of no interest. The scheme was defined for Northern Europe, but the climate changes were claimed to occur more widely. The periods of the scheme include a few of the final pre-Holocene oscillations of the last glacial period and then classify climates of more recent prehistory.

Paleontologists have not defined any faunal stages for the Holocene. If subdivision is necessary, periods of human technological development, such as the Mesolithic, Neolithic, and Bronze Age, are usually used. However, the time periods referenced by these terms vary with the emergence of those technologies in different parts of the world.

Climatically, the Holocene may be divided evenly into the Hypsithermal and Neoglacial periods; the boundary coincides with the start of the Bronze Age in Europe. According to some scholars, a third division, the Anthropocene, has now begun. The International Commission on Stratigraphy Subcommission on Quaternary Stratigraphy's working group on the 'Anthropocene' (a term coined by Paul Crutzen and Eugene Stoermer in 2000) note this term is used to denote the present time interval in which many geologically significant conditions and processes have been profoundly altered by human activities. The 'Anthropocene' is not a formally defined geological unit.

## Geology

Continental motions due to plate tectonics are less than a kilometre over a span of only 10,000 years. However, ice melt caused world sea levels to rise about 35 m (115 ft) in the early part of the Holocene. In addition, many areas above about 40 degrees north latitude had been depressed by the weight of the Pleistocene glaciers and rose as much as 180 m (590 ft) due to post-glacial rebound over the late Pleistocene and Holocene, and are still rising today.

The sea level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. Holocene marine fossils are known, for example, from Vermont and Michigan. Other than higher-latitude temporary marine incursions associated with glacial depression, Holocene fossils are found primarily in lakebed, floodplain, and cave deposits. Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any likely tectonic uplift of non-glacial origin.

Post-glacial rebound in the Scandinavia region resulted in the formation of the Baltic Sea. Earthquakes are a leading cause of sediment deformation, leading to the creation and destruction of bodies of water. The region continues to rise, still causing weak earthquakes across Northern Europe. The equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to near its present boundaries.

## Climate

Climate has been fairly stable over the Holocene. Ice core records show that before the Holocene there was global warming after the end of the last ice age and cooling periods, but climate changes became more regional at the start of the Younger Dryas. During the transition from the last glacial to the Holocene, the Huelmo–Mascardi Cold Reversal in the Southern Hemisphere began before the Younger Dryas, and the maximum warmth flowed south to north from 11,000 to 7,000 years ago. It appears that this was influenced by the residual glacial ice remaining in the Northern Hemisphere until the later date.

The Holocene climatic optimum (HCO) was a period of warming in which the global climate became warmer. However, the warming was probably not uniform across the world. This period of warmth ended about 5,500 years ago with the descent into the Neoglacial and concomitant Neopluvial. At that time, the climate was not unlike today's, but there was a slightly warmer period from the 10th–14th centuries known as the Medieval Warm Period. This was followed by the Little Ice Age, from the 13th or 14th century to the mid-19th century.

Compared to glacial conditions, habitable zones have expanded northwards, reaching their northernmost point during the HCO. Greater moisture in the polar regions has caused the disappearance of steppe-tundra.

The temporal and spatial extent of Holocene climate change is an area of considerable uncertainty, with radiative forcing recently proposed to be the origin of cycles identified in the North Atlantic region. Climate cyclicity through the Holocene (Bond events) has been observed in or near marine settings and is strongly controlled by glacial input to the North Atlantic. Periodicities of ˜2500, ˜1500, and ˜1000 years are generally observed in the North Atlantic. At the same time spectral analyses of the continental record, which is remote from oceanic influence, reveal persistent periodicities of 1,000 and 500 years that may correspond to solar activity variations during the Holocene epoch. A 1,500-year cycle corresponding to the North Atlantic oceanic circulation may have had widespread global distribution in the Late Holocene.

## Ecological developments

Animal and plant life have not evolved much during the relatively short Holocene, but there have been major shifts in the distributions of plants and animals. A number of large animals including mammoths and mastodons, saber-toothed cats like *Smilodon* and *Homotherium*, and giant sloths disappeared in the late Pleistocene and early Holocene—especially in North America, where animals that survived elsewhere (including horses and camels) became extinct. This extinction of American megafauna has been explained as caused by the arrival of the ancestors of Amerindians; though most scientists assert that climatic change also contributed. In addition, a controversial bolide impact over North America has been hypothesized to have triggered the Younger Dryas.

Throughout the world, ecosystems in cooler climates that were previously regional have been isolated in higher altitude ecological "islands".

The *8.2 ka event*, an abrupt cold spell recorded as a negative excursion in the δ18O record lasting 400 years, is the most prominent climatic event occurring in the Holocene epoch, and may have marked a resurgence of ice cover. It has been suggested that this event was caused by the final drainage of Lake Agassiz, which had been confined by the glaciers, disrupting the thermohaline circulation of the Atlantic. Subsequent research, however, suggested that the discharge was probably superimposed upon a longer episode of cooler climate lasting up to 600 years and observed that the extent of the area affected was unclear.