The Younger Dryas

Cathryn Mallonee
ES 767 Quaternary Geology, Emporia State University
Fall 2011

Photo of the Dryas Octopetala from plantsciences.montana.edu


Abstract
Northern Hemisphere
Southern Hemisphere
Possible Causes
Conclusions
References


Abstract

The Younger Dryas cold reversal event is one of the best known instances of an abrupt climate change on record. Theories about what caused the shift from global warming to a distinctly cooling period are varied. Debate continues amongst scientists as to whether the Younger Dryas was a regional or global occurrence. Evidence from the Northern Hemisphere strongly supports the existence of the event, but evidence from the Southern Hemisphere is less compelling. Concern about the impact of a similar abrupt climate change episode in today's world has prompted further research to better understand the Younger Dryas.

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The alteration of the Earth's climate from the Last Glacial Maximum to the current Holocene interglacial period began approximately 14,500 years ago. During this transitional period from approximately 12,900 to 11,600 years ago, many parts of the world experienced an abrupt reversal in its warming trend to nearly fully glacial conditions (Lowell and Kelly, 2008). This event was named after the Dryas Octopetala, a cold loving flower found in alpine glacial areas. The pollen of this flower was found in layers of low lying European forest bog samples as well as Arctic ice cores (Alley, 2000). The presence of this pollen indicates colder alpine-like environmental conditions. Three periods of dryas pollen have been noted in these types of records and the corresponding periods are named the Younger, Older, and Oldest Dryas periods. The Younger Dryas climatic event is one of the most extreme since the Last Glacial Maximum and has thus had many workers focus their research on this period, so a good deal of supporting evidence has been gathered.

The Northern Hemisphere Evidence

Europe: The effects of the Younger Dryas are quite notable in Europe, particularly in areas of Scandinavia. Evidence supporting this cold period includes studies of moraines, radiocarbon dating, geomorphology of the area, and fossil pollen extract from lake and bog sediments (Grove, 2004). Lithostratigraphic and biostratigraphic evidence from many sites throughout Europe including Iberia, the Pyrenees, and Northern Italy support this event., and that previously forested areas of Scandinavia were replaced with tundra. Chronology from varves in Lake Gosciaz in Poland indicate the colder transition into and the warmer transition out of the Younger Dryas period (Grove, 2004).

North America: Evidence for the Younger Dryas in North America includes events such as replacement of boreal forests by tundra, renewed glaciation in New Brunswick, glacial and periglacial sediments overlying peats in Nova Scotia, a mean annual drop of 3 to 4 degrees Celcius in New Jersey and Connecticut all corresponding to the Northern Hemisphere experiencing a cold period at the time of the Younger Dryas. Some have even suggested that the decline of the Clovis peoples in Southwest Asia and North America and the extinction of animal species at this time are likely related to the Younger Dryas event (Moore and Hillman (1992).

Asia:Cores from western Tibet's Lake Sumix indicate cold, dry conditions between 11,000 and 10,000 years ago. Aquatic organisms and organic matter were absent and decrease in Oxygen 18 isotope indicated a cooling period. (Grove 2004)

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The Southern Hemisphere Evidence

Evidence for the Younger Dryas event in the Southern Hemisphere is sketchier. Coral samples collected in the southwest Pacific indicate that temperatures were 4 to 6 degrees C cooler during the Younger Dryas period (Grove, 2004). Evidence from New Zealand includes the advancement of the Franz Josef Glacier over the Canavan Knob as occuring during the same period as the Younger Dryas advances in Europe, but pollen evidence from this same area does not support a significant cooling during this period. It has been argued that the glacial advances of the Late Glacial in New Zealand was not related to the Younger Dryas cooling event in the Northern Hemisphere. (McGlone, 1995) Evidence also indicates that in the mid to low latitudes of the Southern Hemisphere and the southwest Pacific ocean actually warmed throughout the time of the Younger Dryas (Barrows, et. al., 2007).

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Possible Causes

Thermohaline circulation interuption is one of the leading hypotheses for explaining the cause of the Younger Dryas climate change. Circulation in the modern North Atlantic ocean is due to a flow of warm salty surface waters from the tropical Atlantic to the high latitudes of the North Atlantic. The cooling and higher salinity of these water triggers a sinking of the dense polar surface water which creates a self sustainig circulation loop known as the Atlantic Meridional Overturning Circulation, AMOC, which is part of the larger Great Ocean Conveyor Belt circulation. There were two pulses of meltwater discharge into the North Atlantic between 14000 and 8000 years ago which were caused by the disintegration of the Laurentide Ice Sheet. A glacial systems model by Lev Tarasov and W.R. Peltier, found that the largest combined meltwater and iceberg discharge was directied into the Arctic Ocean and that the only outlet was via the Fram Strait into the Greenland-Iceland-Norwegian seas where the North Atlantic deep ocean circulation is formed. They hypothesize that the influx of freshwater into the Arctic at that time triggered the Younger Dryas Event (Tarazov and Peltier, 2005). The second period of discharge did not cause a similar climatic change when it occurred, possibly due to a stronger thermohaline circulation in existence at that time.

The Great Ocean Conveyor Belt (From NASA.gov)

The Fram Strait (modified from Google Maps)

Other scientists question this hypothesis. They favor and "extraterrestrial" explanation for the Younger Dryas. Some scientists have theorized that an asteroid or comet crashing into Earth may have been responsible for triggering the cold event. In his "Palaeolithic extinctions and the Taurid Complex", W.M. Napier of the Cardiff Centre for Astrobiology in the United Kingdom notes that the onset of the Younger Dryas was "marked by intense wildfires over North America, major disruption of human culture, and a rapid extiction of 35 genera of North American mammals." The paper points to the presence of nanodiamonds at several Younger Dryas boundary sites which are embedded within melted plant resins. Though he does not claim that the debris from the break up of a large comet was actually responsible for the Younger Dryas, he presents evidence to support that such a scenario was possible.

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Conclusions

The Younger Dryas was a complex event which may or may not have been felt globally. New and improved dating techniques as well as new evidence continues to write the story of this extraordinary reversal event. The specter of abrupt climate change looms large in the minds of many with today's increased awareness of global warming. A better understanding of the mechanisms controlling similar accelerated climatic events in history may be more pertinent in the contemporary world than in the past. Would an interruption in the thermohaline cycle today cause a corresponding cooldown in global temperatures, or were the conditions at that time ideal to induce such an occurrence? What are the other possible causes that could lead our world down a similar path? Conflicting evidence concerning the global significance of the Younger Dryas continues to be an area of great interest to scientists and will doubtless be the subject of many future studies. A full explanation will likely prove to be multicausal with atmospheric, oceanic and continental changes all contributing to the ultimate cause and the explanation of the global effects of the Younger Dryas event.

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References

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Quaternary Geology 767 Syllabus Emporia State University