8.2-kiloyear event

In climatology, the 8.2-kiloyear event was a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BC, and which lasted for the next two to four centuries. It defines the start of the Northgrippian age in the Holocene epoch. The cooling was significantly less pronounced than during the Younger Dryas cold period that preceded the beginning of the Holocene. During the event, atmospheric methane concentration decreased by 80 ppb, an emission reduction of 15%, by cooling and drying at a hemispheric scale.^[2][3]

Identification

A rapid cooling around 6200 BC was first identified by Swiss botanist

Researchers suggest that the discharge was probably superimposed upon a longer episode of cooler climate lasting up to 600 years, and it was merely one contributing factor to the event as a whole.[21]

Further afield from the Laurentide Ice Sheet, some tropical records report a 3 °C (5.4 °F) cooling, based on cores drilled into an ancient coral reef in Indonesia.^[22] The event also caused a global CO₂ decline of about 25 ppm over about 300 years.^[23] However, dating and interpretation of other tropical sites are more ambiguous than the North Atlantic sites. In addition, climate modeling shows that the amount of meltwater and the pathway of meltwater are both important in perturbing the North Atlantic thermohaline circulation.^[24]

The initial meltwater pulse caused between 0.5 and 4 m (1 ft 8 in and 13 ft 1 in) of sea-level rise. Based on estimates of lake volume and decaying ice cap size, values of 0.4–1.2 m (1 ft 4 in – 3 ft 11 in) circulate. Based on sea-level data from the Mississippi Delta, the end of the Lake Agassiz–Ojibway (LAO) drainage occurred at 8.31 to 8.18 ka and ranges from 0.8 to 2.2 m.^[25] The sea-level data from the Rhine–Meuse Delta indicate a 2–4 m (6 ft 7 in – 13 ft 1 in) of near-instantaneous rise at 8.54 to 8.2 ka, in addition to 'normal' post-glacial sea-level rise.^[26] Meltwater pulse sea-level rise was experienced fully at great distance from the release area. Gravity and rebound effects associated with the shifting of water masses meant that the sea-level fingerprint^{[colloquialism]} was smaller in areas closer to the Hudson Bay. The Mississippi Delta records around 20%, Northwestern Europe 70% and Asia records 105% of the globally averaged amount.^[27] The cooling of the 8.2-kiloyear event was a temporary feature, but the sea-level rise of the meltwater pulse was permanent.

In 2003, the Office of Net Assessment (ONA) at the United States Department of Defense was commissioned to produce a study on the likely and potential effects of a modern climate change.^[28] The study, conducted under ONA head Andrew Marshall, modeled its prospective climate change on the 8.2 ka event, precisely because it was the middle alternative between the Younger Dryas and the milder Little Ice Age.^[29]

Effects

Across much of the world, the 8.2 ka event engendered drier environmental conditions.^[30] Northern Hemisphere monsoon precipitation declined by 12.4% for every °C of global mean temperature change, while Southern Hemisphere monsoon precipitation rose by 4.2%/°C.^[31] The 8.2 ka event was also associated with an increase in ocean salinity and terrestrial dust flux.^[32]

North Africa and Mesopotamia

Drier conditions were notable in

In particular, in Tell Sabi Abyad, Syria, significant cultural changes are observed at c. 6200 BC; the settlement was not abandoned at the time.^[34]

Madagascar

In northwestern Madagascar, the 8.2 ka event is associated with a negative δ¹⁸O excursion and calcite deposition, indicating wet, humid conditions caused by the southward migration of the ITCZ.^[35] Summer monsoons in the Southern Hemisphere likely became stronger, contributing to precipitation increases.^[36] Humidification was two-phased, with an 8.3 kiloyear sub-event preceding the 8.2 kiloyear sub-event by about 20 years.^[37]

Europe

The sediment core records of the Fram Strait show a short-lived cooling during the 8.2 ka event superimposed on a broader interval of warm climate.^[38] In western Scotland, the 8.2 ka event coincided with a dramatic reduction in the Mesolithic population.^[39] In the Iberian Peninsula, the 8.2 ka event is linked to greater summer aridity that caused an increase in the frequency of fires and a consequent expansion of fire-resistant evergreen oak trees.^[40]

North Asia

Lacustrine sediment records show that Western Siberia underwent humidification during the 8.2 ka event.^[41]

South Asia

Carbonates from Riwasa Palaeolake show a weakening of the Indian Summer Monsoon (ISM) synchronous with the 8.2 ka event.^[42] Stalagmites from Kotumsar Cave^[43] and from Socotra and Oman further confirm the ISM precipitously diminished in strength.^[44]

East Asia

A sediment core from

Evidence from the Gulf of Thailand reveals that a sea level drop occurred concordantly with the 8.2 ka event. Also detectable from palynological and sedimentological records is an increase in runoff.^[47]

North America

In Greenland, the 8.2 ka event is associated with a large negative spike in ice core δ¹⁸O values.^[48][49] The waters off Cape Hatteras experienced a major salinity increase.^[50] Bat guano δ¹³C and δD values in the Grand Canyon declined.^[51] Southwestern Mexico became significantly drier, evidenced by the interruption of stalagmite growth.^[52] In the Gulf of Mexico, bay-head deltas back stepped as sea levels rose.^[53] Mustang Island was breached and ceased to be an effective salinity barrier.^[54] Gulf of Mexico δ¹⁸O_seawater values dropped by 0.8%.^[55]

South America

The South American Summer Monsoon (SASM) drastically intensified during the 8.2 ka event as revealed by sediment records from Juréia Paleolagoon.^[56]

See also

References