Bølling–Allerød Interstadial

The Bølling–Allerød Interstadial, also called the Late Glacial Interstadial (LGI), was an interstadial period which occurred from 14,690 to c. 12,890 years Before Present, during the final stages of the Last Glacial Period. It was defined by abrupt warming in the Northern Hemisphere, and a corresponding cooling in the Southern Hemisphere, as well as a period of major ice sheet collapse and corresponding sea level rise known as Meltwater pulse 1A. This period was named after two sites in Denmark where paleoclimate evidence for it was first found, in the form of vegetation fossils that could have only survived during a comparatively warm period in Northern Europe. It is also referred to as Interstadial 1 or Dansgaard–Oeschger event 1.

This interstadial followed the Oldest Dryas period, which lasted from ~18,000 to 14,700 BP. While Oldest Dryas was still significantly colder than the current epoch, the Holocene, globally it was a period of warming from the very cold Last Glacial Maximum, caused by a gradual increase in concentrations. A warming of around 2 C-change had occurred during this period, nearly of half of which had taken place during its last couple of centuries. In contrast, the entire Bølling–Allerød Interstadial experienced very little change in global temperature. Instead, the rapid warming was limited to the Northern Hemisphere, while the Southern Hemipshere had experienced equivalent cooling. This "polar seesaw" pattern had occurred due to the strengthening of the Atlantic meridional overturning circulation (and the corresponding weakening of the Southern Ocean overturning circulation). These changes in thermohaline circulation had caused far more heat to be transferred from the Southern Hemisphere to the North.

This interstadial is commonly divided into three stages. The initial, Bølling, stage had experienced the largest hemispheric temperature change, and it is also the stage when Meltwater Pulse 1A had occurred. It was then interrupted by a geologically brief (around two centuries) stage known as the Older Dryas (after Dryas octopetala, the Arctic plant widespread during such cold periods in the Northern Hemisphere), before NH warming returned during the Allerød stage. In the Northern Hemisphere, Allerød is likely to have been less warm yet more wet than the Bølling. There may have also been another brief cold stage during Allerød. The entire interstadial had abruptly ended with the onset of the Younger Dryas (YD), when the AMOC pattern had reversed, and the Northern Hemisphere temperatures fell back to near-glacial levels within a decade. Global temperatures declined only slightly during YD, and they had steadily climbed alongside the concentrations once that period had transitioned to Holocene.

For human populations of the Northern Hemisphere, Bølling–Allerød Interstadial had represented the first pronounced warming since the end of the Last Glacial Maximum (LGM). The cold had previously forced them into refuge areas, but the warming of the interstadial enabled them to begin repopulating the Eurasian landmass.

Discovery


In 1901, Danish geologists Nikolaj Hartz (1867–1937) and Vilhelm Milthers (1865–1962) found deposits of birch trees in a clay pit near Allerød Municipality on Zealand island and later in the drained peat deposits at Bølling Lake in Jutland peninsula (both parts of Denmark). This provided proxy evidence for consistent warming at these sites during the last glacial period, because the temperatures were warm enough to support these trees. In contrast, the rest of the glacial period was so cold that the dominant plant in the area was a small, cold-adapted flower called Dryas octopetala. Thus, the cold period which preceded this interstadial is known as the Oldest Dryas, and the two subsequent cold periods as the Older and Younger Dryas.

Additional evidence for this period involves the gathering of oxygen isotope stages (OIS) from stratified deep-sea sediment cores. Samples are gathered and measured for change in isotope levels to determine temperature fluctuation for given periods of time.

Causes
The strengthening of the Atlantic meridional overturning circulation is considered to be the primary cause for the Bølling–Allerød warming of the Northern Hemisphere, while its weakening is considered responsible for the inverse pattern during the Older and Younger Dryas. While increase had also occurred during this interstadial, it was at a rate of 20–35 ppmv within 200 years, or less than half of the increase of the recent 50 years, and role in global warming was dwarved by the opposing hemispheric changes caused by thermohaline circulation.

Some research shows that a warming of 3-5 C-change had occurred at intermediate depths in the North Atlantic over the preceding several millennia during Heinrich stadial 1 (HS1). The authors postulated that this warm salty water (WSW) layer, situated beneath the colder surface freshwater in the North Atlantic, generated ocean convective available potential energy (OCAPE) over decades at the end of HS1. According to fluid modelling, at one point the accumulation of OCAPE was released abruptly (c. 1 month) into kinetic energy of thermobaric cabbeling convection (TCC), resulting in the warmer salty waters getting to the surface and subsequently warming the sea surface by approximately 2 C-change.

Oceans and the cryosphere
Records obtained from the Gulf of Alaska show abrupt sea-surface warming of about 3 °C (in less than 90 years), matching ice-core records that register this transition as occurring within decades. Antarctic Intermediate Water (AAIW) cooled slightly during this interstadial. The Meltwater pulse 1A event coincides with or closely follows the abrupt onset of the Bølling–Allerød (BA), when global sea level rose about 16 m during this event at rates of 26–53 mm/yr. In the Great Barrier Reef, the Bølling–Allerød period is associated with a substantial accumulation of calcium carbonate, which is consistent with the modelled cooling of the region.

A 2017 study attributed the second Weichselian Icelandic ice sheet collapse, onshore (est. net wastage 221 Gt a−1 over 750 years) and similar to today's Greenland rates of mass loss, to atmospheric Bølling–Allerød warming. The study's authors noted: "Geothermal conditions impart a significant control on the ice sheet's transient response, particularly during phases of rapid retreat. Insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions, such as Siple Coast, Antarctica, and northeastern Greenland, have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated." The melting of the glaciers of Hardangerfjord began during this interstadial. Boknafjord had already begun to deglaciate before the onset of the Bølling–Allerød interstadial. Some research suggests that isostatic rebound in response to glacier retreat (unloading) and an increase in local salinity (i.e., δ18Osw) was associated with increased volcanic activity at the onset of Bølling–Allerød. Notably, volcanic ash fallout on glacier surfaces could have had enhanced their melting through ice-albedo feedback.

In the Southern Hemisphere, the weakened Southern Ocean overturning circulation caused the expansion of Antarctic Intermediate Water, which sequesters less effectively than the Antarctic bottom water, and this was likely the main reason for the increase in  concentrations during the interstadial.

Western Europe and the North European Plain
The climate began to improve rapidly throughout Western Europe and the North European Plain c. 16,000-15,000 years ago. The environmental landscape became increasingly boreal, except in the far north, where conditions remained arctic. Sites of human occupation reappeared in northern France, Belgium, northwest Germany, and southern Britain between 15,500 and 14,000 years ago. Many of these sites are classified as Magdalenian, though other industries containing distinctive curved back and tanged points appeared as well. As the Fennoscandian ice sheet continued to shrink, plants and people began to repopulate the freshly deglaciated areas of southern Scandinavia.

Between 12,000 and 10,000 years ago, the western coast of Norway and southern Sweden to latitude 65° north became occupied by sites belonging to the Fosna-Hensbacka complex. They are defined by the appearance of tanged points and other artifacts similar to those found earlier in Northwest Germany. Komsa sites, dated to about 7,000 years ago, are found along Norway's Finnmark county above 70° north and further east on the Kola Peninsula. They are defined by surface finds of tanged points, burins, scrapers, and adzes. The primary game of Magdalenian hunters appears to have been reindeer, though evidence of bird and shellfish consumption persist, as well.

East European Plain
Periglacial loess-steppe environments prevailed across the East European Plain, but climates improved slightly during several brief interstadials and began to warm significantly after the beginning of the Late Glacial Maximum. Pollen profiles for this time indicate a pine-birch woodland interspersed with the steppe in the deglaciated northern plain, birch-pine forest with some broadleaf trees in the central region, and steppe in the south. The pattern reflects the reemergence of a marked zonation of biomes with the decline of glacial conditions. Human site occupation density was most prevalent in the Crimea region and increased as early as around 16,000 years ago.

Reoccupation of northern territories of the East European Plain did not occur until 13,000 years ago. Settlement of the central portion of the East European Plain had significantly reduced during a period of maximum cold around 21,000 to 17,000 years ago.

Overall, little archaeological evidence suggests major shifting settlement pattern during this time on the East European plain. That is unlike what was occurring in Western Europe, where Magdalenian industry producers were rapidly repopulating much of Europe. Evidence of this can be found as far east at Kunda sites (about 10,000 years ago, throughout the Baltics, where tanged point and other tool making traditions reminiscent of the northwestern European Magdalenian persist).

Generally, lithic technology is dominated by blade production and typical Upper Paleolithic tool forms such as burins and backed blades (the most persistent). Kostenki archaeological sites of multiple occupation layers persist from the Last Glacial Maximum and into the Late Glacial Maximum on the eastern edge of the Central Russian Upland, along the Don River. Epigravettian archaeological sites, similar to Eastern Gravettian sites, are common in the southwest, central, and southern regions of the East European Plain about 17,000 to 10,000 years BP and are also present in the Crimea and Northern Caucasus.

The time of the Epigravettian also reveals evidence for tailored clothing production, a tradition persisting from preceding Upper Paleolithic archaeological horizons. Fur-bearing small mammal remains abound such as Arctic fox and paw bones of hares, reflecting pelt removal. Large and diverse inventories of bone, antler, and ivory implements are common, and ornamentation and art are associated with all major industries. Insights into the technology of the time can also be seen in features such as structures, pits, and hearths mapped on open-air occupation areas scattered across the East European Plain.

Mammoths were typically hunted for fur, bone shelter, and bone fuel. In the southwest region around the middle Dnestr Valley, sites are dominated by reindeer and horse, accounting for 80 to 90% of the identifiable large mammal remains. Mammoth is less common, typically 15% or less, as the availability of wood eliminated the need for heavy consumption of bone fuel and collection of large bones for construction. Mammoth remains may have been collected for other raw material, namely ivory. Other large mammals in modest numbers include steppe bison and red deer.

Plant foods more likely played an increasing role in the southwest region than in the central and southern plains since southwest sites consistently yield grinding stones widely thought to have been used for preparation of seeds, roots, and other plant parts.

Siberian Plain
Southern Siberia supported little vegetation, but some trees, mainly pine, persisted. Evidence comes not only from pollen-spore data but also from wood charcoal in former hearths at archaeological sites. Pollen samples around Chukotka and the Taimyr Peninsula indicate a forest zone emerging roughly 7000 years ago and slightly warmer climates than now.

The earliest human reoccupation of Siberia did not begin until 21,000 years ago. Evidence continues to be found mainly in the south around Lake Baikal, as at the Studenoe site, for example. Later sites include Kokorevo in the Yenisei Valley and Chernoozer'e in the Ob River basin. The sites are confined to latitudes below 57°N and most are C14 dated from 19,000 to 14,000 years ago. Settlements differed from those of the East European Plain as they reflected a more mobile lifestyle by the absence of mammoth-bone houses and storage pits, all indicators of long-term settlement. Visual art was uncommon. Fauna remained red deer, reindeer, and moose and indicate a mainly meat-oriented diet.

The habitat of Siberia was far harsher than anywhere else and often did not provide enough survival opportunities for its human inhabitants. That is what forced human groups to remain dispersed and mobile, as is reflected in the lithic technology, as tiny blades were typically manufactured, often termed microblades less than 8 mm wide with unusually sharp edges indicating frugality from low resource levels. They were fixed into grooves along one or both edges of a sharpened bone or antler point. Specimens of complete microblade-inset points have been recovered from both Kokorevo and Chernoozer'e. At Kokorevo, one was found embedded in a bison shoulder blade.

As climates warmed further around 15,000 years ago, fish began to populate rivers, and technology used to harvest them, such as barbed harpoons, first appeared on the Upper Angara River. People expanded northwards into the Middle Lena Basin. By 11,000 years ago, settlement size increased as discovered at the Ust'-Belaya site, where fauna remains consisted of entirely modern-type remains of deer, moose, fish, and traces of domesticated dogs. New technology such as fish hooks appear among bone and antler implements.

The Dyuktai culture, near Dyuktai Cave, on the Aldan River at 59°N, is similar to southern Siberian sites and includes the wedge-shaped cores and microblades, along with some bifacial tools, burins, and scrapers. The site likely represents the material remains of the people who spread across the Bering Land Bridge and into the New World. Around 12,000 years ago, the Sumnagin culture appears over large portions of northern and eastern Siberia. The sites are small and yield few artifacts of small blades struck off thin cylindrical cores. Bone tools and fishing equipment are absent.

Most Sumnagin sites were located in the forest zone so most tools were likely created from wood, which would help explain a sparse archaeological record. Another factor may be low levels of human settlement, since the region of the Sumnagin culture could likely support a considerably lower biomass than the rest of Eurasia. That is still true along the Middle Lena Basin among current human populations. The Sumnagin diet consisted of large mammals such as deer, moose, and even brown bear, as revealed by the fauna remains found. Nevertheless, Sumnagin culture representatives moved northward and became the first to populate Siberia's Arctic tundra around 10,000 years ago.

At around 9500 to 9000 years ago, Sumnagin sites spread to Zhokhov Island, where slotted bone and antler points, antler and ivory mattocks, and bone handles for cutting tools were found. Few wooden artifacts were also found, including a large shovel or scoop, arrow shafts, and a sledge-runner fragment. Fauna remains consist of reindeer and polar bear. Only isolated bones of walrus, seal, and birds were identified. Further settlement proceeded eastward and westward into Chukotka and the Taimyr Peninsula.

Asia
δ18O records from Valmiki Cave in southern India indicate extreme shifts in Indian Summer Monsoon intensity at Termination 1a, which marks the start of the Bølling–Allerød and occurred about 14,800 BP.

North America
Over the land between the Lena Basin and northwest Canada, increased aridity occurred during the Last Glacial Maximum. Sea level fell to about 120 m below its present position, exposing a dry plain between Chukotka and western Alaska. Clear skies reduced precipitation, and loess deposition promoted well-drained, nutrient-rich soils that supported diverse steppic plant communities and herds of large grazing mammals. The wet tundra soils and spruce bogs that exist today were absent.

Cold temperatures and massive ice sheets covered most of Canada and the northwest coast, thus preventing human colonization of North America prior to 16,000 years ago. An "ice-free corridor" through western Canada to the northern plains is thought to have opened up no earlier than 13,500 years ago. However, deglaciation in the Pacific northwest may have taken place more rapidly and a coastal route could have been available by 17,000 years ago. Rising temperatures and increased moisture accelerated environmental change after 14,000 years ago, as shrub tundra replaced dry steppe in many parts of Beringia.

Camp settlement sites are found along Tanana River in central Alaska by 14,000 years ago and some evidence suggests human exploration at the Bluefish Caves in the Yukon as early as 15,500 years ago. Earliest occupation levels at the Tanana Valley sites contain artifacts similar to the Siberian Dyuktai culture. At Swan Point, these comprise microblades, burins, and flakes struck from bifacial tools. Artifacts at the nearby site of Broken Mammoth are few, but include several rods of mammoth ivory. The diet was of large mammals and birds, as indicated by faunal remains.

Earliest site occupation at Ushki sites of central Kamchatka (about 13,000 years ago) display evidence of small oval houses and bifacial points. Stone pendants, beads, and a burial pit are present. In central Alaska up the northern foothills at the Dry Creek site c. 13,500-13,000 years ago near Nenana Valley, small bifacial points were found. People were thought to have moved into this area to hunt elk and sheep on a seasonal basis. Microblade sites typologically similar to Dyuktai appear about 13,000 years ago in central Kamchatka and throughout many parts of Alaska.

Around 12,000 years ago, the rising sea level reached a position less than 60 m below today's level and flooded the lowlands between Chukotka and western Alaska. The ensuing increase in moisture accelerated Alaska's transition to wet tundra and coniferous forests. The Bering Land Bridge had closed, thus Beringia ceased to exist. About this time, sites that comprise the Denali complex appeared and persisted to about 7,500 years ago. Denali complex sites indicate high yields of caribou remains c. 8,000 years ago and corresponds with an increase in settlement size.

Human genetics
The European distribution of Y-chromosome haplogroup I and various associated subclades has also been explained as resulting from male postglacial recolonization of Europe from refugia in the Balkans, Iberia, and the Ukraine/Central Russian Plain.

Males possessing haplogroup Q are postulated as representing a significant portion of the population who crossed Beringia and populated North America for the first time.

The distribution of mtDNA haplogroup H has been postulated as representing the major female repopulating of Europe from the Franco-Cantabrian region after the Last Glacial Maximum. mtDNA haplogroups A, B, C, D and X are interpreted according to some as supporting a single pre-Clovis populating of the Americas via a coastal route.

<!-- The Bølling oscillation, also Bølling interstadial, was a cool temperate climatic interstadial between the glacial Oldest Dryas and Older Dryas stadials, between 14,700 and 14,100 BP, near to the end of the last glacial period. It is named after a peat sequence discovered at Bølling lake in central Jutland, Denmark. It is used to describe a period of time in relation to Pollen zone Ib—in regions where the Older Dryas is not detected in climatological evidence, the Bølling–Allerød is considered a single interstadial period.

Dates
The beginning of the Bølling is also the high-resolution date for the sharp temperature rise marking the end of the Oldest Dryas at 14,670 BP. Roberts (1998) uses 15,000. A range of 14,650–14,000 BP calibrated has been assigned to the Bølling layer of the excavation at Lake Neuchâtel, Switzerland, 1992–1993. The Oxygen isotope record from Greenland ice includes the Bølling warm peak between 14,600 and 14,100 BP. Most of the recent dates available fall within a few hundred years of these.

Flora
Of the two periods, Bølling and Allerød, Bølling is the warmer and came on more suddenly. During it sea level rose about 35 m due to glacial melt. Ice uncovered large parts of north Europe and temperate forests covered Europe from 29 deg. to 41 deg. north latitude. After some pioneer vegetation, such as Salix polaris and Dryas octopetala, hardwoods, such as Quercus, and softwoods, Betula and Pinus, spread northward for a brief few hundred years.

Fauna
During this time late Pleistocene animals spread northward from refugia in the three peninsulas, Spain, Italy and the Balkans. Geneticists can identify the general location by studying degrees of consanguinity in the modern animals of Europe. The hunting camps of ancient humans remain a major source of faunal fossils.

Animals hunted by humans are predominantly the big-game mammals: reindeer, horse, saiga, antelope, bison, woolly mammoth and woolly rhinoceros. In the alpine regions ibex and chamois were hunted. Throughout the forest were red deer. Smaller animals, such as fox, wolf, hare and squirrel also appear. Salmon was fished. For more details, see also the references to fauna under Oldest Dryas and Older Dryas.

Human cultures
Humans reentered the forests of Europe in search of big game, which they were beginning to hunt relentlessly, many to extinction. Their cultures were the last of the Late Upper Palaeolithic. Magdalenian hunters moved up the Loire into the Paris Basin. In the drainage basin of the Dordogne, the Perigordian prevailed. The Epigravettian dominated Italy. In the north, the Hamburgian, Creswellian and Federmesser cultures are found. In the Middle East, the pre-agricultural Natufian settled around the Eastern Mediterranean coast to exploit wild cereals, such as emmer and two-row barley. In the Allerød they would begin to domesticate these plants. --> < <!-- The Allerød oscillation (Allerødtiden) was a warm and moist global interstadial that occurred c.13,900 to 12,900 BP. It was preceded by the Older Dryas and followed by the Younger Dryas cold periods. The Allerød was nearly at the end of the Last Glacial Period. It raised temperatures in the northern Atlantic region to almost present-day levels, before they declined again in the Younger Dryas, which was followed by the present warm Holocene.

The Allerød period was named after a type site in Furesø Municipality in Sjælland, Denmark (near Copenhagen), where deposits created during the period were first identified in work published in 1901 by Hartz and Milthers. This Blytt–Sernander period corresponds to Pollen zone II.

Dating
The Greenland oxygen isotope record shows the warming identified with the Allerød to be after about 14,100 BP and before about 12,900 BP. C-14 dates from an excavation on the shore of Lake Neuchâtel, Switzerland, furnish a date of 14,000 BP, calibrated, for the start of the Allerød. Pollen cores from Berezina plain, Belarus, give 11,800–10,900 BP uncal. Various researchers have similar ranges: 12,000–11,000, 11,700–11,000, etc. They all seem to roughly concur. The interstadial ended abruptly with a decline in temperatures within a decade and the onset of the glacial Younger Dryas.

The Allerød occurred during the last interstadial of the Pleistocene: the Windermere interstadial of Britain, the Woodgrange interstadial of Ireland and the Two Creeks Interval of North America. Although interstadials are defined by region, the Allerød period is not, being global in its effects; that is, the temperature and sea level rose everywhere, not just in north Europe.

Flora
During the Allerød, which foreshadowed the modern climate, mixed evergreen and deciduous forests prevailed in Eurasia, more deciduous toward the south, just as today. Birch, aspen, spruce, pine, larch and juniper were to be found extensively, mixed with oak and hazel. Grasses were to be found in more open regions.

Fauna
Some animals hunted were the red deer, moose, horse, Irish elk and beaver. The ubiquitous brown bear was present as well.

Humans
Humans in north Eurasia were still in the hunter-gatherer stage. A variety of Palaeolithic cultures prevailed in Europe: the Federmesser, Lyngby, Bromme, Ahrensburg and Swiderian. In the Levant, early alcohol use may have occurred during this time. -->