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Saturday, February 28, 2009

”Climate flicker” at the end of the last glacial period

From ETH in Zurich, this interesting essay on the last glacial period has some interesting points to ponder. h/t to Sid Stafford - Anthony


The last glacial period was characterised by strong climatic fluctuations. Scientists have now been able to prove very frequent and rapid climate change, particularly at the end of the Younger Dryas period, around 12,000 years ago. These fluctuations were accompanied by rapid changes in circulation in the oceans and the atmosphere.

Researchers are able to determine when glaciers were stable and when they melted by studying titanium content in glacial lake sediments. (Picture: siyublog/flickr)
Researchers are able to determine when glaciers were stable and when they melted by studying titanium content in glacial lake sediments. (Picture: siyublog/flickr)

Sediment deposits in lakes are the climate archives of the past. An international team of researchers from Norway, Switzerland and Germany have now examined sediments originating from the Younger Dryas period from the Kråkenes Lake in northwest Norway. In the sediments, they found clues that point to a “climate flicker” at the end of the last glacial period, oscillating between colder and warmer phases until the transition to the stable climate of the Holocene, our current interglacial period. The short-term, strong fluctuations of the Younger Dryas would have dwarfed the “extreme weather phenomena” seen today, according to Gerald Haug, professor at the Department for Earth Sciences at ETH Zürich and co-author of the study, which was published online yesterday in “Nature Geoscience”.


Seasonal sediment deposits

Seasonal sediment accumulation, for example, gave scientists clues to these strong climate fluctuations. They can be read in lakes in a similar way to reading rings on trees. In warmer phases and melting glaciers, the accumulation of sediments increases. More clues on the changes in glacier growth were given by the element titanium, which is present in the sediments. Glaciers erode their bedrock, and in doing so concentrate the titanium contained in the sediments they are carrying. The sediments containing titanium are washed into the glacier’s draining lakes in the meltwater. The amount of sediment and the titanium content can therefore allow us to deduce when the glaciers were stable and when they melted. The researchers interpreted the maxims, recurring every 10 years, as phases of strong glacier activity caused by temperature fluctuations and thus as warmer times.


A seemingly self-preserving cycle

The scientists also examined a sediment core from seabed deposits of the same age in the North Atlantic. They reconstructed the original temperature and salt concentration of the water based on microfossils and the oxygen isotope ratio in the sediment. It was shown that the results from the lake sediments corresponded to those from the sea sediments. “The melting of glaciers was caused by the warm Gulf stream advancing into this region,” Gerald Haug explains. This increase in temperature caused the west winds to shift to the north and brought warm air to northern Europe. However, the meltwater draining into the Atlantic lowered the salt concentration and the density of the surface water, changing the convection in the ocean, which in turn allowed new sea ice to form. Subsequently, the Gulf Stream and the west winds were again forced out of the North Atlantic area and the region cooled down once again. These processes were repeated for around 400 years, until the current interglacial period was able to stabilise itself.


The Würm glaciation began around 100,000 years ago and lasted until around 10,000 years ago. In this period, there were strong fluctuations between warm and cold phases, particularly in the North Atlantic area. The Younger Dryas, which ushered in the current interglacial period, is one of the best-known and best-researched abrupt climate changes of that glaciation. It began around 12,900 years ago and at first caused an abrupt temperature drop in the northern hemisphere, as well as a temperature rise of up to 10°C in less than 20 years towards the end, around 11,700 years ago.


Unclear mechanisms

Up until now, there have been several studies which document the glacial conditions during the Younger Dryas period of 1,200 years. However, the mechanisms which caused it, sustained it and finally led to an interglacial period have yet to be fully understood. The researchers believe that further high-resolution studies of this type could give insights into how glacial periods are triggered and how they are brought to an end.

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