Holocene
The Holocene (/ˈhɒl.əsiːn, -oʊ-, ˈhoʊ.lə-, -loʊ-/)[2][3] is the current geological epoch, beginning approximately 11,700 years ago.[4] It follows the Last Glacial Period, which concluded with the Holocene glacial retreat.[4] The Holocene and the preceding Pleistocene[5] together form the Quaternary period. The Holocene is an interglacial period within the ongoing glacial cycles of the Quaternary, and is equivalent to Marine Isotope Stage 1.
This article is about the geological epoch. For the Bon Iver song, see Holocene (song). For the album by German band The Ocean, see Holocene (album). For the music venue in Portland, Oregon, see Holocene (Portland, Oregon). For the scientific journal, see The Holocene.Holocene
The Holocene corresponds with the rapid proliferation, growth, and impacts of the human species worldwide, including all of its written history, technological revolutions, development of major civilizations, and overall significant transition towards urban living in the present. The human impact on modern-era Earth and its ecosystems may be considered of global significance for the future evolution of living species, including approximately synchronous lithospheric evidence, or more recently hydrospheric and atmospheric evidence of the human impact. In July 2018, the International Union of Geological Sciences split the Holocene Epoch into three distinct ages based on the climate, Greenlandian (11,700 years ago to 8,200 years ago), Northgrippian (8,200 years ago to 4,200 years ago) and Meghalayan (4,200 years ago to the present), as proposed by International Commission on Stratigraphy.[6] The oldest age, the Greenlandian, was characterized by a warming following the preceding ice age. The Northgrippian Age is known for vast cooling due to a disruption in ocean circulations that was caused by the melting of glaciers. The most recent age of the Holocene is the present Meghalayan, which began with extreme drought that lasted around 200 years.[6]
Etymology[edit]
The word Holocene was formed from two Ancient Greek words. Hólos (ὅλος) is the Greek word for "whole". "Cene" comes from the Greek word kainós (καινός), meaning "new". The concept is that this epoch is "entirely new".[7][8][9] The suffix '-cene' is used for all the seven epochs of the Cenozoic Era.
The International Commission on Stratigraphy has defined the Holocene as starting approximately 11,700 years before 2000 CE (11,650 cal years BP, or 9,700 BCE).[4] The Subcommission on Quaternary Stratigraphy (SQS) regards the term 'recent' as an incorrect way of referring to the Holocene, preferring the term 'modern' instead to describe current processes. It also observes that the term 'Flandrian' may be used as a synonym for Holocene, although it is becoming outdated.[10] The International Commission on Stratigraphy, however, considers the Holocene to be an epoch following the Pleistocene and specifically following the last glacial period. Local names for the last glacial period include the Wisconsinan in North America,[11] the Weichselian in Europe,[12] the Devensian in Britain,[13] the Llanquihue in Chile[14] and the Otiran in New Zealand.[15]
The Holocene can be subdivided into five time intervals, or chronozones, based on climatic fluctuations:[16]
Geologists working in different regions are studying sea levels, peat bogs, and ice-core samples, using a variety of methods, with a view toward further verifying and refining the Blytt–Sernander sequence. This is a classification of climatic periods initially defined by plant remains in peat mosses.[17] Though the method was once thought to be of little interest, based on 14C dating of peats that was inconsistent with the claimed chronozones,[18] investigators have found a general correspondence across Eurasia and North America. 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.[19]
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.[20]
According to some scholars, a third epoch of the Quaternary, the Anthropocene, has now begun.[21] 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' (a term coined by Paul J. Crutzen and Eugene Stoermer in 2000) is not a formally defined geological unit. The Subcommission on Quaternary Stratigraphy of the International Commission on Stratigraphy has a working group to determine whether it should be.
In May 2019, members of the working group voted in favour of recognizing the Anthropocene as formal chrono-stratigraphic unit, with stratigraphic signals around the mid-twentieth century CE as its base. The exact criteria have still to be determined, after which the recommendation also has to be approved by the working group's parent bodies (ultimately the International Union of Geological Sciences).[22]
Geology[edit]
The Holocene is a geologic epoch that follows directly after the Pleistocene. 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 and another 30 m in the later 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.[23]
The sea-level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. For example, marine fossils from the Holocene epoch have been found in locations such as 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 a shrinking Baltic Sea. The region continues to rise, still causing weak earthquakes across Northern Europe. An equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to its present boundaries.[24]
Ecological developments[edit]
Animal and plant life have not evolved much during the relatively short Holocene, but there have been major shifts in the richness and abundance of plants and animals. A number of large animals including mammoths and mastodons, saber-toothed cats like Smilodon and Homotherium, and giant sloths went extinct in the late Pleistocene and early Holocene. These extinctions can be mostly attributed to people.[81] In America, it coincided with the arrival of the Clovis people; this culture was known for "Clovis points" which were fashioned on spears for hunting animals. Shrubs, herbs, and mosses had also changed in relative abundance from the Pleistocene to Holocene, identified by permafrost core samples.[82]
Throughout the world, ecosystems in cooler climates that were previously regional have been isolated in higher altitude ecological "islands".[83]
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.[84] This disruption was the result of an ice dam over Hudson Bay collapsing sending cold lake Agassiz water into the North Atlantic ocean.[85] Furthermore, studies show that the melting of Lake Agassiz led to sea-level rise which flooded the North American coastal landscape. The basal peat plant was then used to determine the resulting local sea-level rise of 0.20-0.56m in the Mississippi Delta.[85] 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.[86]