Bølling–Allerød Interstadial
The Bølling–Allerød Interstadial, also called the Late Glacial Interstadial, was an interstadial period which occurred from 14,690 to 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 had occurred during this period, nearly 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. These changes in thermohaline circulation had caused far more heat to be transferred from the Southern Hemisphere to the North.
For human populations of the Northern Hemisphere, Bølling–Allerød Interstadial had represented the first pronounced warming since the end of the Last Glacial Maximum. The cold had previously forced them into refuge areas, but the warming of the interstadial enabled them to begin repopulating the Eurasian landmass. The abrupt Northern cooling of the subsequent Younger Dryas may have triggered the Neolithic Revolution, with the adoption of agriculture in the Levant.
Discovery
In 1901, Danish geologists Nikolaj Hartz and Vilhelm Milthers 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. 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 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.
Timing
This interstadial is commonly divided into three stages. The initial Bølling stage had the largest hemispheric temperature change, and it is also the stage when Meltwater Pulse 1A had occurred. The beginning of the Bølling is also end of the Oldest Dryas at approximately 14,600 years BP. The Oxygen isotope record from Greenland ice indicates that the Bølling stage lasted approximately 600 years.It was then interrupted by the Older Dryas. The Older Dryas lasted approximately one century. before northern hemisphere warming returned during the Allerød stage.
The Allerød stage was a warm and moist global interstadial that occurred c.13,900 to 12,900 BP. 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 interstadial stage started abruptly with a decline in temperatures within a decade and the onset of the glacial Younger Dryas. Global temperatures declined only slightly during YD, and they had steadily climbed alongside the concentrations once that period had transitioned to Holocene. There may have also been another brief cold stage during Allerød.
In regions where the Older Dryas is not detected in climatological evidence, the Bølling–Allerød is considered a single interstadial period.
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 dwarfed by the opposing hemispheric changes caused by thermohaline circulation.Some research shows that a warming of had occurred at intermediate depths in the North Atlantic over the preceding several millennia during Heinrich stadial 1. The authors postulated that this warm salty water layer, situated beneath the colder surface freshwater in the North Atlantic, generated ocean convective available potential energy over decades at the end of HS1. According to fluid modelling, at one point the accumulation of OCAPE was released abruptly into kinetic energy of thermobaric cabbeling convection, resulting in the warmer salty waters getting to the surface and subsequently warming the sea surface by approximately.
Geophysical effects
Records obtained from the Gulf of Alaska show abrupt sea-surface warming of about 3 °C, matching ice-core records that register this transition as occurring within decades. Antarctic Intermediate Water cooled slightly during this interstadial. The Meltwater pulse 1A event coincides with or closely follows the abrupt onset of the Bølling–Allerød, 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 and similar to today's Greenland rates of mass loss, to atmospheric Bølling–Allerød warming. 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 and an increase in local salinity 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.
The deep oceans were depleted in radiocarbon during the deglaciation following the LGM, which has been hypothesised to be the result of sluggish meridional overturning circulation or due to a release of volcanic carbon or methane clathrates into abyssal waters. The Eastern Tropical Pacific Oxygen Minimum Zone witnessed high oxygen depletion during the early stages of the deglaciation following the LGM, most likely as a result of a weakened Atlantic meridional overturning circulation and an increased influx of nutrient-rich waters due to intensified upwelling.
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. The Bølling–Allerød was almost completely synchronous across the Northern Hemisphere.
Effects on humans
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. In Britain, the Creswellian culture developed as an offshoot of the Magdalenian.As the Fennoscandian ice sheet continued to shrink, plants and people began to repopulate the freshly deglaciated areas of southern Scandinavia. Large parts of what is now the southern part of the North Sea became dry land, now named Doggerland. This was a low landscape with soft hills, rivers, and lakes. By the end of the interstadial, Doggerland was probably covered by a sparse birch woodland, with flourishing fauna such as mammoths and red deer.
Prey favored by European hunters included reindeer, wild boar, European fallow deer, red deer, and European wild ass.
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.Generally, lithic technology is dominated by blade production and typical Upper Paleolithic tool forms such as burins and backed blades. Kostenki archaeological sites of multiple occupation layers persist from the Last 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.