An interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene interglacial began at the end of the Pleistocene, about 11,700 years ago. During the 2.5 million year span of the Pleistocene, numerous glacials, or significant advances of continental ice sheets in North America and Europe, have occurred at intervals of 40,000 to 100,000 years. These long glacial periods were separated by shorter interglacials. During interglacials, such as the present one, the climate warms and the tundra recedes polewards following the ice sheets. Forests return to areas. Interglacials are identified in shallow epicontinental seas by their paleontology. Floral and faunal remains of species pointing to temperate climate and indicating a specific age are used to identify particular interglacials. Used are mammalian and molluscan species and plant macro-remains. However, many other fossil remains may be helpful: insects, foraminifera, etc.
Ice cores and ocean sediment cores provide more quantitative and dated evidence for temperatures and total ice volumes. The interglacials and glacials coincide with cyclic changes in the Earth's orbit. Three orbital variations contribute to interglacials; the first is a change in the Earth's orbit around the sun, or eccentricity. The second is a shift in the tilt of the obliquity; the third is precession, or wobbling motion of Earth's axis. Warm summers in the Southern hemisphere occur when that hemisphere is tilted toward the sun and the Earth is nearest the sun in its elliptical orbit. Cool summers occur; these effects are more pronounced. When the obliquity is large, seasonal changes are more extreme. Interglacials are a useful tool for geological mapping and for anthropologists, as they can be used as a dating method for hominid fossils. Brief periods of milder climate that occurred during the last glacial are called interstadials. Most interstadials are shorter than interglacials. Interstadial climate may have been warm but this is not so.
Because the colder periods have been dry, wetter periods have been registered in the sedimentary record as interstadials as well. The oxygen isotope ratio obtained from seabed sediment core samples, a proxy for average global temperature, is an important source of information about changes in the climate of the earth. An interglacial optimum, or climatic optimum of an interglacial, is the period within an interglacial that experienced the most'favourable' climate that occurred during that interglacial during the middle part; the climatic optimum of an interglacial follows, is followed by, phases that are within the same interglacial and that experienced a less favourable climate. During an interglacial optimum, sea levels rise to their highest values, but not exactly at the same time as the climatic optimum. In the present interglacial, the Holocene, the climatic optimum occurred during the Subboreal and Atlanticum. Our current climatic phase following this climatic optimum is still within the same interglacial.
This warm period was followed by a gradual decline until about 2,000 years ago, with another warm period until the Little Ice Age. The preceding interglacial optimum occurred during 131 -- 114 ka. During the Eemian the climatic optimum took place during pollen zone E4 in the type area. Here this zone is characterized by the expansion of Quercus, Taxus, Fraxinus and Picea. During the Eemian Stage sea level was between 5 and 9.4 meters higher than today and the water temperature of the North Sea was about 2 °C higher than at present. Greenhouse and icehouse Earth Milankovitch cycles Snowball Earth Interstadial periods Last glacial maximum Timeline of glaciation
In paleoclimatology of the Holocene, the Boreal was the first of the Blytt-Sernander sequence of north European climatic phases that were based on the study of Danish peat bogs, named for Axel Blytt and Rutger Sernander, who first established the sequence. In peat bog sediments, the Boreal is recognized by its characteristic pollen zone, it was preceded by the Younger Dryas, the last cold snap of the Pleistocene, followed by the Atlantic, a warmer and moister period than our most recent climate. The Boreal, transitional between the two periods, varied a great deal, at times having within it climates like today's. Subsequent to the original Blytt-Sernander scheme, the first stage of the Boreal was divided off as a Pre-boreal transitional phase, followed by the Boreal proper; some current schemes based on pollen zones distinguish a pre-Boreal, an early Boreal and a late Boreal. One cited date for the end of the Younger Dryas and the start of the Pre-Boreal is 11,500 Before Present calibrated; the start of the period is sharply defined by a rise of 7 °C in 50 years in South Greenland.
The date is based solidly on Greenland ice cores, which give 11,640 BP for the late Younger Dryas and 11,400 BP for the early Pre-Boreal. But estimates of other dates vary by up to 1000 years, for a number of reasons. First, "Boreal" can identify a paleoclimate, a pollen zone or a temporally-fixed chronozone, those three bases of definition allow quite different dates. Second, different dating methods obtain different dates; the underlying problem is that pollen vary somewhat from region to region. The scientists of each region use the methods available in their region, whether lake varves, the annual layers of sediment from ancient or modern lake bottoms, ice cores or counts of tree rings. Standardization has become of increasing concern to scientists everywhere. Dates from many methods continue to multiply, but it is unclear. Yet, there are some solid dates of the Boreal; the Saksunarvatn tephra is dated in Greenland ice to 10,180±60 BP. The tephra occurs in early Boreal contexts. So, it seems certain that the early Boreal includes the year 10,000 BP.
The late Boreal includes the Kilian/Vasset tephra of Swiss and southwest German lakes at 8200 BP, all calibrated. But the borders are less certain. Studies of bogs in northwest Russia are the basis for a division of the PreBoreal into PB-1, 10,000–9800, PB-2, 9800–9300 BP incal; the scheme goes on to divide the Boreal into BO-1, 9300–9000, BO-2, 9000–8500, BO-3, 8500–8000, incal. CalPal used on these dates suggests overall boundaries of 11,500 and 10,500 BP for the Pre-Boreal, the end of the Boreal at 8900. Dates given are earlier than those given more than 10 years ago. For example and Rud give dates of 10,000–9000 BP for the PreBoreal and 9000–8000 BP for the Boreal, which are uncalibrated C-14 dates based on Scandinavian pollen stratigraphy. More-recent dates are more accurate, as technology improves with time quite rapidly, yet and climate phases to some degree may depend on latitude, so no date can be regarded as wrong. Scientists look for the overall pattern of the dates, but that technique is not 100% reliable, either.
Before the Pre-Boreal, Eurasia was locked in the chill of the Younger Dryas and was a continuous tundra belt, with regions of taiga, covered with a blanket of grasses and other low plants typical of open land. Large numbers of herbivores wandered in herds over vast distances; the blanket teemed with small reproducing species, which supported food chains of larger predators. The largest predators and humans hunted the mammals of the open tundra; the Pre-Boreal began with a sudden rise in temperature. Forest replaced the open lands in Europe, forest-dwelling animals spread from southern refugia and replaced the ice-age tundra mammals; the old fauna persisted in Central Asia, but were soon hunted out, as they were not replenished by the larger areas nourishing the ecosystem. The sea brought additional isolation by rising and drowning the entire coast. Ireland was cut off early in the Boreal, it is home to only two-thirds of the species present in Britain. Britain was cut off by the end of the Boreal.
Forest had closed over the former European tundra. Humans had to move east with the large mammals; those who stayed became hunter-gatherers of the forests and fishers of the numerous bays and shallow waters around the thousands of islands that now spangled the seas of Europe. They were encouraged to enter the pre-productive phase that we call the Mesolithic; those who moved east hunted out the last of wild big game and turned their best efforts into learning to herd what was left. In the Americas, humans were now in the Archaic. Meanwhile humanity toward the south of the north temperate zone had turned to food production in a number of separated locations and were on the brink of civilization. There is no evidence of any extensive contact with the cultures of the north during the Boreal; the producers tended to live in dense centers without any interest in moving from there except when motivated to find new lands. The gath
A glacial period is an interval of time within an ice age, marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate between glacial periods; the last glacial period ended about 15,000 years ago. The Holocene epoch is the current interglacial. A time with no glaciers on Earth is considered a greenhouse climate state. Within the Quaternary, there have been a number of interglacials; the last glacial period was the most recent glacial period within the Quaternary Ice Age, occurring in the Pleistocene epoch, which began about 110,000 years ago and ended about 15,000 years ago. The glaciations that occurred during this glacial period covered many areas of the Northern Hemisphere and have different names, depending on their geographic distributions: Wisconsin, Midlandian, Würm, Dali, Taibai Luojishan, Tianchi Qomolangma, Llanquihue; the glacial advance reached its maximum extent about 18,000 BP. In Europe, the ice sheet reached Northern Germany.
In the last 650,000 years, there were, on average, seven cycles of glacial retreat. Since orbital variations are predictable, computer models that relate orbital variations to climate can predict future climate possibilities. Work by Berger and Loutre suggests; the amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next glacial period, which otherwise would begin in around 1,000 years, more glacial cycles
The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago to the beginning of the present Quaternary Period 2.58 Mya. The Neogene is sub-divided into two epochs, the earlier Miocene and the Pliocene; some geologists assert that the Neogene cannot be delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes. During this period and birds continued to evolve into modern forms, while other groups of life remained unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of the period; some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean; the global climate cooled over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary Period that follows.
In ICS terminology, from upper to lower: The Pliocene Epoch is subdivided into 2 ages: Piacenzian Age, preceded by Zanclean AgeThe Miocene Epoch is subdivided into 6 ages: Messinian Age, preceded by Tortonian Age Serravallian Age Langhian Age Burdigalian Age Aquitanian AgeIn different geophysical regions of the world, other regional names are used for the same or overlapping ages and other timeline subdivisions. The terms Neogene System and upper Tertiary System describe the rocks deposited during the Neogene Period; the continents in the Neogene were close to their current positions. The Isthmus of Panama formed, connecting South America; the Indian subcontinent continued forming the Himalayas. Sea levels fell, creating land bridges between Africa and Eurasia and between Eurasia and North America; the global climate became seasonal and continued an overall drying and cooling trend which began at the start of the Paleogene. The ice caps on both poles began to grow and thicken, by the end of the period the first of a series of glaciations of the current Ice Age began.
Marine and continental flora and fauna have a modern appearance. The reptile group Choristodera became extinct in the early part of the period, while the amphibians known as Allocaudata disappeared at the end. Mammals and birds continued to be the dominant terrestrial vertebrates, took many forms as they adapted to various habitats; the first hominins, the ancestors of humans, may have appeared in southern Europe and migrated into Africa. In response to the cooler, seasonal climate, tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore diversified, herbivorous mammals evolved alongside it, creating the many grazing animals of today such as horses and bison. Eucalyptus fossil leaves occur in the Miocene of New Zealand, where the genus is not native today, but have been introduced from Australia; the Neogene traditionally ended at the end of the Pliocene Epoch, just before the older definition of the beginning of the Quaternary Period. However, there was a movement amongst geologists to include ongoing geological time in the Neogene, while others insist the Quaternary to be a separate period of distinctly different record.
The somewhat confusing terminology and disagreement amongst geologists on where to draw what hierarchical boundaries is due to the comparatively fine divisibility of time units as time approaches the present, due to geological preservation that causes the youngest sedimentary geological record to be preserved over a much larger area and to reflect many more environments than the older geological record. By dividing the Cenozoic Era into three periods instead of seven epochs, the periods are more comparable to the duration of periods in the Mesozoic and Paleozoic eras; the International Commission on Stratigraphy once proposed that the Quaternary be considered a sub-era of the Neogene, with a beginning date of 2.58 Ma, namely the start of the Gelasian Stage. In the 2004 proposal of the ICS, the Neogene would have consisted of the Miocene and Pliocene epochs; the International Union for Quaternary Research counterproposed that the Neogene and the Pliocene end at 2.58 Ma, that the Gelasian be transferred to the Pleistocene, the Quaternary be recognized as the third period in the Cenozoic, citing key changes in Earth's climate and biota that occurred 2.58 Ma and its correspondence to the Gauss-Matuyama magnetostratigraphic boundary.
In 2006 ICS and INQUA reached a compromise that made Quaternary a subera, subdividing Cenozoic into the old classical Tertiary and Quaternary, a compromise, rejected by International Union of Geological Sciences because it split both Neogene and Pliocene in two. Following formal discussions at the 2008 International Geological Congress in Oslo, the ICS decided in May 2009 to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.58 Mya and including the Gelasian age, considered part of the Neogene Period and Pliocene Epoch. Thus the Neogene Period ends bounding the succeeding Quaternary Period at 2.58 Mya. "Digital Atlas of Neogene Life for the Southeastern United States". San Jose State University. Archived from the original on 2013-04-23. Retrieved 21 September 2018
The Atlantic in palaeoclimatology was the warmest and moistest Blytt-Sernander period, pollen zone and chronozone of Holocene northern Europe. The climate was warmer than today, it was preceded by the Boreal, with a climate similar to today’s, was followed by the Subboreal, a transition to the modern. Because it was the warmest period of the Holocene, the Atlantic is referenced more directly as the Holocene climatic optimum, or just climatic optimum; the Atlantic is equivalent to Pollen Zone VII. Sometimes a Pre-atlantic or early Atlantic is distinguished, on the basis of an early dividing cold snap. Other scientists place the Atlantic after the cold snap, assigning the latter to the Boreal; the period is still in the process of definition. It is a question of definition and the criteria: Beginning with the temperatures, as derivable from Greenland ice core data, it is possible to define an'Early' or'Pre-Atlantic' period at around 8040 BC, where the 18O isotope line remains above 33 ppm in the combined curve after Rasmussen et al. which would end at the well-known 6.2 ka BC -cold-event.
Or one single Atlantic period is defined, starting at that just mentioned cold-event. After a lake-level criterion, Kul’kova and others define the Atlantic as running from 8000 to 5000 BP. Early Atlantic, or AT1, was a time of high lake levels, 8000–7000 BP; each period has its distinctive ratios of species. According to the ice-core criterion it is difficult to find a clear boundary, because the measurements still differ too much and alignments are still under construction. Many find a decline of temperature significant enough after 4800 BC. Another criterion comes from bio-stratigraphy: the elm-decline. However, this appears in different regions between 4300 and 3100 BC; the Atlantic was a time of rising temperature and marine transgression on the islands of Denmark and elsewhere. The sea rose to 3 m above its present level by the end of the period; the oysters found. Tides of up to 1 m were present. Inland, lake levels in all north Europe were higher, with fluctuations; the temperature rise had the effect of extending southern climates northward in a short period.
The treelines on northern mountains rose by 600 to 900 m. Thermophilous species migrated northward, they did not replace the species that shifted the percentages in their favor. Across middle Europe, the boreal forests were replaced by climax or "old growth" deciduous ones, though providing a denser canopy, were more open at the base; the dense canopy theory, has been questioned by F. Vera. Oak and hazel require more light. Vera hypothesizes that the lowlands were more open and that the low frequency of grass pollen was caused by the browsing of large herbivores, such as Bos primigenius and Equus ferus. During the Atlantic period the deciduous temperate zone forests of south and central Europe extended northward to replace the boreal mixed forest, which found refugia on the mountain slopes. Mistletoe, Water Chestnut and Ivy were present in Denmark. Grass pollen decreased. Softwood forests were replaced by hardwood. Quercus, both cordata and platyphyllos, oak, linden, Ulmus glabra and ash replaced Betula and Pinus, spreading to the north from further south.
The period is sometimes called "the alder-elm-lime period". In northeast Europe, the Early Atlantic forest was but affected by the rise in temperature; the forest had been pine with an underbrush of hazel, alder and willow. Only about 7% of the forest became broad-leaved deciduous, dropping to Boreal levels in the cooling of the Middle Atlantic. In the warmer Late Atlantic, the broad-leaved trees became 34% of the forest. Along the line of the Danube and the Rhine, extending northward in tributary drainage systems, a new factor entered the forest country: the Linear Pottery culture, clearing the arable land by slash and burn methods, it flourished about 5500–4500 BC, falling within the Atlantic. By the end of the Atlantic and pasture lands extended over much of Europe and the once virgin forests were contained within refugia; the end of the Atlantic is signaled by the "Elm decline", a sharp drop in Elm pollen, thought to be the result of climate, disease or human food-producing activities. In the subsequent cooler Sub-Boreal, forested country gave way to open range once more.
The best picture of Atlantic Period fauna comes from the kitchen middens of the Ertebølle culture of Denmark and others like it. Denmark was more of an archipelago. Humans lived on the shorelines, exploiting waters rich in marine life, marshes teeming with birds, forests where cervids and suids as well as numerous small species were plentiful; the higher water levels offset the effects of the submarine toxic zone in the Baltic Sea. It contained fish now rare there, such as the anchovy, Engraulis encrasicolus, the three-spined stickleback, Gasterosteus aculeatus. Available were pike, whitefish and ling. Three kinds of seals were found there, the ringed and grey. Mesolithic man whales in the estuaries; the main birds were maritime: the red-throated diver, the black-throated diver, the gannet. The Dalmatian pelican, now found only as far north as south-eastern Europe, has been found in Denmark; the capercaillie, as is the case now, was found in forested areas. In the lofty canopy could be found a continuous zone of smaller animals, such as the ubiquitous squirrel, Sciur
The Preboreal is a stage of the Holocene epoch. It is succeeded by the Boreal, it lasted from 10,300 to 9,000 BP in radiocarbon years or 8350 BC to 7050 BC in Gregorian calendar years. It is the first stage of the Holocene epoch; the Holocene has not been formally divided by the IUGS. As a result the Preboreal is only a proposal, as stratigraphy and dating techniques have improved since this 1972 proposal the dates would be different if proposed today. Instead others have begun to use the terms Early and Late, which should be Lower and Upper for the Holocene. If this terminology were to be used the preboreal would be replaced by Lower Holocene which would be dated 11.7 - 8.2 ka B2K. In July 2018 the International Commission on Stratigraphy ratified Greenlandian as the globally recognised first age of the Holocene, much overlapping with the North European regional term Preboreal
The Younger Dryas was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum started receding around 20,000 BP. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are abundant in the Late Glacial minerogenic-rich, like the lake sediments of Scandinavian lakes. Physical evidence of a sharp decline in temperature over most of the Northern Hemisphere has been discovered by geological research; this temperature change occurred at the end of what the earth sciences refer to as the Pleistocene epoch and before the current, warmer Holocene epoch. In archaeology, this time frame coincides with the final stages of the Upper Paleolithic in many areas; the Younger Dryas was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 years BP. The change was sudden, taking place in decades, it resulted in a decline of 2 to 6 degrees Celsius and advances of glaciers and drier conditions, over much of the temperate northern hemisphere.
It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the Equator towards the North Pole, in turn thought to have been caused by an influx of fresh cold water from North America to the Atlantic. The Younger Dryas was a period of climatic change. In the Southern Hemisphere and some areas of the Northern Hemisphere, such as southeastern North America, there was a slight warming; the presence of a distinct cold period at the end of the Late Glacial interval has been known for a long time. Paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, like in the Allerød clay pit in Denmark, first recognized and described the Younger Dryas; the Younger Dryas is the youngest and longest of three stadials, which resulted from abrupt climatic changes that took place over the last 16,000 calendar years. Within the Blytt–Sernander classification of north European climatic phases, the prefix "Younger" refers to the recognition that this original "Dryas" period was preceded by a warmer stage, the Allerød oscillation, which, in turn, was preceded by the Older Dryas, around 14,000 calendar years BP.
That is not securely dated, estimates vary by 400 years, but it is accepted that it lasted around 200 years. In northern Scotland, the glaciers were more extensive than during the Younger Dryas; the Older Dryas, in turn, was preceded by another warmer stage, the Bølling oscillation, that separated it from a third and older stadial known as the Oldest Dryas. The Oldest Dryas occurred 1,770 calendar years before the Younger Dryas and lasted about 400 calendar years. According to the GISP2 ice core from Greenland, the Oldest Dryas occurred between about 15,070 and 14,670 calendar years BP. In Ireland, the Younger Dryas has been known as the Nahanagan Stadial, in Great Britain, it has been called the Loch Lomond Stadial. In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1; the preceding Allerød warm period is subdivided into three events: Greenland Interstadial-1c to 1a. Since 1916 and the onset and the refinement of pollen analytical techniques and a steadily-growing number of pollen diagrams, palynologists have concluded that the Younger Dryas was a distinct period of vegetational change in large parts of Europe during which vegetation of a warmer climate was replaced by that of a generally-cold climate, a glacial plant succession that contained Dryas octopetala.
The drastic change in vegetation is interpreted to be an effect of a sudden decrease in temperature, unfavorable for the forest vegetation, spreading northward rapidly. The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna but led to regional glacial advances in Scandinavia and a lowering of the regional snow line; the change to glacial conditions at the onset of the Younger Dryas in the higher latitudes of the Northern Hemisphere, between 12,900–11,500 calendar years BP, has been argued to have been quite abrupt. It is in sharp contrast to the warming of the preceding Older Dryas interstadial, it has been inferred that its end occurred over a period of a decade or so, but the onset may have been faster. Thermally-fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that its summit was 15 °C colder during the Younger Dryas than today. In Great Britain, beetle fossil evidence suggests that mean annual temperature dropped to −5 °C, periglacial conditions prevailed in lowland areas, icefields and glaciers formed in upland areas.
Nothing of the period's size, extent, or rapidity of abrupt climate change has been experienced since its end. In addition to the Younger and Oldest Dryases, a century-long period of colder climate, similar to the Younger Dryas in abruptness, has occurred within both the Bølling oscillation and the Allerød oscillation interstadials; the cold period which occurred within the Bølling oscillation is known as the intra-Bølling cold period, the cold period which occurred within the Allerød oscillation is known as the intra-Allerød cold period. Both cold periods are comparable in duration and intensity with the Older Dryas and began and ended quite abruptly; the cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, Europe