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Younger Dryas

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Three temperature proxies showing the Younger Dryas event at around 12,000 years BP. The NGRIP sequence (red – mislabelled as GRIP) uses the water molecule isotopic composition δ18
O. The Vostok and EPICA Dome C series show delta-deuterium. All 3 proxies use the same vertical axis.

The Younger Dryas is a climatic event from c. 12,900 to c. 11,700 calendar years ago (BP). It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are occasionally abundant in the Late Glacial, often minerogenic-rich, like the lake sediments of Scandinavian lakes.

The Younger Dryas saw a sharp decline in temperature over most of the Northern Hemisphere, at the end of the Pleistocene epoch, immediately before the current, warmer Holocene. 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 calendar years BP. The change was relatively sudden, taking place in decades, and 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, but the effects were complex and variable. In the Southern Hemisphere and some areas of the Northern Hemisphere, such as southeastern North America, there was a slight warming.[1]

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.[2][3][4][5]

The Younger Dryas is the youngest and longest of three stadials, which resulted from typically abrupt climatic changes that took place over the last 16,000 calendar years.[6] 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, and estimates vary by 400 years, but it is generally accepted that it lasted around 200 years. In northern Scotland, the glaciers were thicker and more extensive than during the Younger Dryas.[7] The Older Dryas, in turn, was preceded by another warmer stage, the Bølling oscillation, that separated it from a third and even older stadial, often known as the Oldest Dryas. The Oldest Dryas occurred approximately 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.[8]

In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain, it has been called the Loch Lomond Stadial.[9][10] In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a).[11]

Abrupt climate change

Since 1916 and the onset and then 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 often contained Dryas octopetala. The drastic change in vegetation is typically interpreted to be an effect of a sudden decrease in (annual) temperature, unfavorable for the forest vegetation that had been speaking northward rapidly. The cooling not only favored the expansion of cold-tolerant, light-demanding plants and associated steppe fauna but also led to regional glacial advances in Scandinavia and a lowering of the regional snow line.[2]

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.[12] 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,[13] but the onset may have even been faster.[14] Thermally-fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that its summit was approximately 15 °C (27 °F) colder during the Younger Dryas[12][15] than today.

In Great Britain, beetle fossil evidence suggests that mean annual temperature dropped to −5 °C (23 °F),[15] and periglacial conditions prevailed in lowland areas, and icefields and glaciers formed in upland areas.[16] Nothing of the period's size, extent, or rapidity of abrupt climate change has been experienced since its end.[12]

In addition to the Younger, Older, 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 (ICBP), and the cold period, which occurred within the Allerød oscillation, is known as the intra-Allerød cold period (ICAP ). Both cold periods are comparable in duration and intensity with the Older Dyras and began and ended quite abruptly. The cold periods have been recognized in sequence and relative magnitude in paleoclimatic records from Greenland ice cores, European lacustrine sediments, Atlantic Ocean sediments, and the Cariaco Basin, Venezuela.[17]

Examples of older Younger Dryas-like events have been reported from the ends (called terminations)[18] of older glacial periods. Temperature-sensitive lipids, long chain alkenones, found in lake and marine sediments, are well-regarded as a powerful paleothermometer for the quantitative reconstruction of past continental climates.[19] The application of alkenone paleothermometers to high-resolution paleotemperature reconstructions of older glacial terminations have found that very similar, Younger Dryas-like paleoclimatic oscillations occurred during Terminations II and IV. If so, the Younger Dryas is not the unique paleoclimatic event, in terms of size, extent, and rapidity, as it is often regarded to be.[19][20] Furthermore, paleoclimatologists and Quaternary geologists reported finding what they characterized as well-expressed Younger Dryas events in the Chinese δ18
O records of Termination III in stalagmites from high-altitude caves in Shennongjia area, Hubei Province, China.[21] Various paleoclimatic records from ice cores, deep sea sediments, speleothems, continental paleobotanical data, and loesses show similar abrupt climate events, which are consistent with Younger Dryas events, during the terminations of the last four glacial periods. They argue that Younger Dryas events might be an intrinsic feature of deglaciations that occur at the end of glacial periods.[21][22][23]

Timing

Analyses of stable isotopes from Greenland ice cores provide estimates for the start and end of the Younger Dryas. The analysis of Greenland Summit ice cores, as part of the Greenland Ice Sheet Project-2 (GISP-2) and Greenland Icecore Project (GRIP), estimated that the Younger Dryas started about 12,800 ice (calendar) years BP. Depending on the specific ice core analysis consulted, the Younger Dryas is estimated to have lasted 1,150–1,300 years.[2][3] Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40 to 50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration and snow accumulation, suggest an even more rapid transition, which would require about 7 °C (13 °F) of warming in just a few years.[12][13][24][25] Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F).[26]

The end of the Younger Dryas has been dated to around 11,550 years ago, occurring at 10,000 years ago (uncalibrated radiocarbon year), a "radiocarbon plateau" by a variety of methods, mostly with consistent results:

years ago place
11500 ± 50  GRIP ice core, Greenland[27]
11530 + 40
− 60 		 Krakenes Lake, western Norway[28]
11570  Cariaco Basin core, Venezuela[29]
11570  German oak/pine dendrochronology[30]
11640 ± 280  GISP2 ice core, Greenland[24]

Although the start of the Younger Dryas is regarded to be synchronous across the North Atlantic region, recent research concluded that the start of the Younger Dryas might be time-trangressive even within there. After an examination of laminated varve sequences, Muschitiello and Wohlfarth found that the environmental changes that define the beginning of the Younger Dryas are diachronous in their time of occurrence according to latitude. According to the changes, the Younger Dryas occurred as early as c. 12,900–13,100 calendar years ago along latitude 56–54°N. Further north, they found that the changes occurred at c. 12,600–12,750 calendar years ago.[31]

According to the analyses of varved sediments from Lake Suigetsu, Japan, and other paleonenvironmental records from Asia, a substantial delay occurred in the onset and the end of the Younger Dryas between Asia and the North Atlantic. For example, paleoenvironmental analysis of sediment cores from Lake Suigetsu in Japan found the Younger Dryas temperature decline of 2–4 °C between 12,300 and 11,250 varve (calendar) years BP, instead of about 12,900 calendar years BP in the North Atlantic region.

In contrast, the abrupt shift in the radiocarbon signal from apparent radiocarbon dates of 11,000 radiocarbon years to radiocarbon dates of 10,700–10,600 radiocarbon years BP in terrestrial macrofossils and tree rings in Europe over a 50-year period occurred at the same time in the varved sediments of Lake Suigetsu. However, this same shift in the radiocarbon signal predates the start of Younger Dryas at Lake Suigetsu by a few hundred years. Interpretations of data from Chinese also confirm that the Younger Dryas East Asia lags the North Atlantic Younger Dryas cooling by at least 200 to 300 years. Although the interpretation of the data is more murky and ambiguous, it is likely that end of the Younger Dryas and the start of Holocene warming were similarly delayed in Japan and in other parts of East Asia.[32]

Similarly, an analysis of a stalagmite growing from a cave in Puerto Princesa Subterranean River National Park, Palawan, Philippines, found that the onset of the Younger Dryas was also delayed there. Proxy data recorded in the stalagmite indicated that it took more than 550 calendar years for Younger Dryas drought conditions to reach their full extent in the region and about 450 calendar years to return to pre-Younger Dryas levels after it ended.[33]

Global effects

In Western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period.[34] Cooling in the tropical North Atlantic may, however, have preceded it by a few hundred years; South America shows a less well-defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas and has no clearly defined start or end; Peter Huybers has argued that there is a fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania.[35] Timing of the tropical counterpart to the Younger Dryas, the Deglaciation Climate Reversal (DCR), is difficult to establish as low latitude ice core records generally lack independent dating over the interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and was characterized by a shift to much wetter and likely colder conditions.[36] The magnitude and abruptness of the changes would suggest that low latitude climate did not respond passively during the YD/DCR.

Effects of the Younger Dryas were of varying intensity throughout North America.[37] In western North America, its effects were less intense than in Europe or northeast North America;[38] however, evidence of a glacial re-advance[39] indicates that Younger Dryas cooling occurred in the Pacific Northwest. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon's Klamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas.[40]

Other features include the following:

North America

East

The Younger Dryas is a period significant to the study of the response of biota to abrupt climate change and to the study of how humans coped with such rapid changes.[42] The effects of sudden cooling in the North Atlantic had strongly regional effects in North America, with some areas experiencing more abrupt changes than others.[43]

The effects of the Younger Dryas cooling impacted New England and parts of maritime Canada more rapidly than the rest of the United States at the beginning and the end of the Younger Dryas chronozone.[44][45][46][47] Proxy indicators show that summer temperature conditions in Maine decreased by up to 7.5oC. Cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north.[48]

Vegetation in the central Appalachian Mountains east towards the Atlantic Ocean was dominated by spruce (Picea spp.) and tamarack (Larix laricina) boreal forests that later changed rapidly to temperate, more broad-leaf tree forest conditions at the end of the Younger Dryas period.[49][50] Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene.[50] East of the Appalachians, in the Ohio River Valley and south to Florida rapid, no-analog vegetation responses seem to have been the result rapid climate changes, but the area remained generally cool, with hardwood forest dominating.[49] During the Younger Dryas, the Southeastern United States was warmer and wetter than the region had been during the Pleistocene[50][43][51] because of trapped heat from the Caribbean within the North Atlantic Gyre caused by a weakened Atlantic meridional overturning circulation (AMOC).[52]

Central

There was also a gradient of changing effects from the Great Lakes region south to Texas and Louisiana. Climatic forcing moved cold air into the northern portion of the American interior, much as it did the Northeast.[53][54] Although there was not as abrupt a delineation as seen on the Eastern Seaboard, the Midwest was significantly colder in the northern interior than it was south, towards the warmer climatic influence of the Gulf of Mexico.[43][55] In the north, the Laurentide Ice Sheet re-advanced during the Younger Dryas, depositing a moraine from west Lake Superior to southeast Quebec.[56] Along the southern margins of the Great Lakes, spruce dropped rapidly while pine increased, and herbaceous prairie vegetation decreased in abundance but increased west of the region.[57][54]

Rocky Mountains

Effects in the Rocky Mountain region were varied.[58][59] In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places.[60][61][62][63] That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation[60][64] as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.[65] There were minor re-advancements of glaciers in place, particularly in the northern ranges,[66][67] but several sites in the Rocky Mountain ranges show little to no changes in vegetation during the Younger Dryas.[61] Evidence also indicates an increase in precipitation in New Mexico because of the same Gulf conditions that were influencing Texas.[68]

West

The Pacific Northwest region of experienced 2o to 3oC of cooling and an increase in precipitation.[69][51][70][71][72] Glacial re-advancement has been recorded in British Columbia[73][74] as well as in the Cascade Range.[75] An increase of pine pollen indicates cooler winters within the central Cascades.[76] Speleothem records indicate an increase in precipitation in southern Oregon,[72][77] the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin.[78] A pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range,[79] but the pollen record is less chronologically constrained than the aforementioned speleothem record. The Southwest appears to have seen an increase in precipitation as well, also with an average 2o of cooling.[80]

Effects on agriculture

The Younger Dryas is often linked to the Neolithic Revolution, the adoption of agriculture in the Levant.[81][82] It is argued that the cold and dry Younger Dryas lowered the carrying capacity of the area and forced the sedentary Early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While there is relative consensus regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.[83][84]

Sea level

Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in the rates of sea level rise have been reconstructed for the postglacial period. For the early part of the sea level rise that is associated with deglaciation, three major periods of accelerated sea level rise, called meltwater pulses, occurred. They are commonly called meltwater pulse 1A0 for the one between 19,000 and 19,500 calendar years ago; meltwater pulse 1A for the one between 14,600 and 14,300 calendar years ago and meltwater pulse 1B for the one 11,400 and 11,100 calendar years ago. The Younger Dryas occurred after meltwater pulse 1A, a 13.5 m rise over about 290 years, centered at about 14,200 calendar years ago, and before meltwater pulse 1B, a 7.5 m rise over about 160 years, centered at about 11,000 calendar years ago.[85][86][87] Finally, not only did the Younger Dryas postdate both all of meltwater pulse 1A and predate all of meltwater pulse 1B, it was a period of significantly-reduced rate of sea level rise relative to the periods of time immediately before and after it.[85][88]

Possible evidence of short-term sea level changes has been reported for the beginning of the Younger Dryas. First, the plotting of data by Bard and others suggests a small step, less than 6 m, in sea level near the onset of the Younger Dryas. There is a possible corresponding change in the rate of change of sea level rise seen in the data from both Barbados and Tahiti. Given that this change is "within the overall uncertainty of the approach," it was concluded that a relatively smooth sea-level rise, with no significant accelerations, occurred then.[88] Finally, research by Lohe and others in western Norway has reported a sea-level low-stand at 13,640 calendar years ago and a subsequent Younger Dryas transgression starting at 13,080 calendar years ago. They concluded that the timing of the Allerød low-stand and the subsequent transgression were the result of increased regional loading of the crust, and geoid changes were caused by an expanding ice sheet, which started growing and advancing in the early Allerød about 13,600 calendar years ago, well before the start of the Younger Dryas.[89]

Causes

The prevailing theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. Geological evidence for such an event is so far lacking.[90] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic. An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation.[91]

There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, but that cannot explain the apparent variability in the extinction across all continents.[92]

Impact hypothesis

Main article: Younger Dryas impact hypothesis

A hypothesized Younger Dryas impact event, presumed to have occurred in North America about 12,900 calendar years ago, has been proposed as the mechanism that initiated the Younger Dryas cooling.[93] Among other things, findings of melt-glass material in sediments in Pennsylvania, South Carolina and Syria have been reported. The researchers argue that the material, which dates back nearly 13,000 years, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as the result of a bolide impact. They argue that these findings support the controversial Younger Dryas Boundary (YDB) hypothesis: that the bolide impact occurred at the onset of the Younger Dryas.[94] The hypothesis has been questioned in research that concluded that most of the results cannot be confirmed by other scientists and that the authors misinterpreted the data.[95][96][97]

After a review of the sediments found at the sites, new research has found that the sediments claimed by hypothesis proponents to be deposits resulting from a bolide impact date from much later or much earlier times than the proposed date of the cosmic impact. The researchers examined 29 sites commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only three of tho sites actually date from then.[98]

In a study published in the Journal of Geology issue of September 2014, Charles R. Kinzie (et al.) looked at the distribution of nanodiamonds produced during extraterrestrial collisions: 50 million square kilometers of the Northern Hemisphere at the YDB were found to have the nanodiamonds.[99] Only two layers exist showing these nanodiamonds: the YDB 12,800 calendar years ago and the Cretaceous-Tertiary boundary, 65 million years ago, which, in addition, is marked by mass extinctions.[100]

Laacher See eruption hypothesis

Laacher See erupted at about the same time as the Younger Dryas started and has been suggested as a possible cause. Laacher See is a maar lake, a lake within a broad low-relief volcanic crater about 2 km (1.2 mi) diameter. It is in Rhineland-Palatinate, Germany, about 24 km (15 mi) northwest of Koblenz and 37 km (23 mi) south of Bonn. The maar lake is within the Eifel mountain range, and is part of the East Eifel volcanic field within the larger Vulkaneifel.[101][102] This eruption was of sufficient size, VEI 6, with over 10 km3 (2.4 cu mi) tephra ejected, to have caused significant temperature changes in the Northern Hemisphere.[103][104]

However, the hypothesis has been generally dismissed. Firstly, detailed dating of varved and other lake deposits in other maar lakes determined an age of at least 11,230 ± 40 radiocarbon years before present for the Laacher See eruption, about 200 radiocarbon years older than the start of the Younger Dryas.[105] More recently, the radiocarbon dating of trees killed by the Laacher See eruption and an examination of Swiss dendrochronology and volcanic sulphur in Greenland ice cores both concluded that the Laacher See eruption predated the onset of the Younger Dryas by some 203 calendar years on average.[106] Second, Late-Glacial sediment cores from Switzerland demonstrate that a significant period of time separate the Laacher See eruption and the start of the Younger Dryas.[105][107][108] Even if the Laacher See eruption happened at the start of the Younger Dryas, it was not large enough to initiate a new stadial. The paleoclimatic proxies found in Greenland ice cores show only a short-term climatic deterioration.[106][109]

Cultural references

The failure of North Atlantic thermohaline circulation is used to explain rapid climate change in some science fiction writings as early as Stanley G. Weinbaum's 1937 short story "Shifting Seas" in which the author describes the freezing of Europe after the Gulf Stream is disrupted. It was used more recently in Kim Stanley Robinson's novels, particularly Fifty Degrees Below. It also underpinned the 1999 book, The Coming Global Superstorm.

Likewise, the idea of rapid climate change caused by disruption of North Atlantic Ocean currents creates the setting for the 2004 apocalyptic science-fiction film The Day After Tomorrow. Similar sudden cooling events have featured in other novels, such as John Christopher's The World in Winter but not always with the same explicit links to the Younger Dryas event as in the case of Robinson's work.

See also

References

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