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Analysis of the annual deposition rates revealed abrupt changes within only a few years, declining for example by approximately 30% in the transition period between the Oldest Dryas and the Bølling.
Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.
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The bubbles disappear and the ice becomes more transparent.
The weight above makes deeper layers of ice thin and flow outwards.
These data can be combined to find the climate model that best fits all the available data. Coastal areas are more likely to include material of marine origin, such as sea salt ions.
Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind.
At Summit Camp in Greenland, the depth is 77 m and the ice is 230 years old; at Dome C in Antarctica the depth is 95 m and the age 2500 years.
As further layers build up, the pressure increases, and at about 1500 m the crystal structure of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a clathrate.
One of the main sites yielding representational objects is Abri Cellier, which has long been on the short-list of major art-bearing sites attributed to the European Aurignacian.
Collie's 1927 excavations yielded 15 engraved and/or pierced limestone blocks that have served as a key point of reference for the study of Aurignacian art in SW France.
In summer 1990, during the third international expedition to Lake Van, eastern Anatolia, 10 sediment cores were retrieved from depths up to 446 m.