Younger Dryas

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Examine the evidence for abrupt climate change during the YD stadial and discuss possible mechanisms for this change.

Discuss the major changes in climate during the YD stadial and examine their possible causes.

YD = the cold spell just before the warm Holocene, 12,500-11,500 calendar ya

The nature and chronology of the late glacial:

Last glacial maximum was c.18,000 14C years ago (21,500 calendar ya)

During this time sea levels 120-135m lower than today

The present interglacial became 10,000 14C years ago (11,500 calendar ya)

The transition from the last glacial maximum to the present interglacial is known as the late glacial - was not a smooth transition in climate

Involved a number of abrupt climatic changes at 'sub-Milankovitch' periodicity

Abrupt climate changes involved:

- changes in temperature in many mid- and high-latitude areas

- changes in effective moisture in low-latitude areas

- widespread shifts in ocean circulation

Meltwater forcing:

Rooth (1982): YD may have been cause by diversion of meltwater from glacial Lake Agassiz

Broecker et al. (1989): presented oxygen-isotope evidence for a reduction in meltwater entering the Gulf of Mexico from Lake Agassiz at the time of the YD, and suggested that diversion to a St Lawrence route way led to freshening of the North Atlantic.

Broecker and Denton (1990): discuss the possibility that meltwater outburst before the YD cooling may have made the oceans susceptible to subsequent MOC shutdown, thereby reconciling the apparent mismatch between the YD and the timing of meltwater discharge in the North Atlantic as shown by Fairbanks (1989)

Imputs of meltwater into NA which resulted in changed thermohaline circulation are most likely cause but feedbacks of GHGs also important

If N.Hemisphere cools down rapidly:

1. Sea levels drop 2. It is drier

=shift of vegetation bands

Ice core research indicates that the YD happened in decades or centuries (very rapid)

Can see evidence in Loch Lomond, Scotland which was covered in ice from glaciers

Glacial forcing factors:

- Orbital parameters

- Volcanic eruptions

- Solar radiation

Cold spells/punctuations every 1500 years, unsure what causes it but think its to do with thermohaline water circulation in NA

8.2 event:

= sudden decrease in global temperatures which occurred 8.2ky BP

Pulse of fresh water in NA THC, thought to be causes by giant lakes in North America bursting through the ice sheet

ALTERNATIVE CAUSES:

1. Impact hypothesis:

Theory that large extra-terrestrial object collided with earth and initiated YD (Firestone et al. 2007, Bunch et al. 2012)

Evidence: melt-glass material in sediments in Pennsylvania, South Carolina and Syria. Researchers argue that this material which dates back nearly 13,000y was formed at temperatures of 1,700 to 2,200 degrees C as a result of bolide impact.

Hypothesis has been largely questioned by researchers (Kerr 2010, Pinter et al. 2011) becuase:

-many of the conclusions cannot be repeated by other scientists

-misinterpretation of data

-lack of confirmatory evidence

2. Volcanoes:

Volcanoes that reach the stratosphere block sunlight and cool the Earth

(Berger 1990)

The Laacher See volcano in Germany was big enough to have caused significant changes in temperature in N.Hemisphere - Laacher See tephra found throughout YD boundary layer (Bogaard 1985)

Oppenheimer 2011: Big volcanic eruption will only cool the Earth for 3-4 years

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