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Ice Cores Lakes and Ponds, Varves, and Pollen
An Ice core is a core taken from a very thick ice sheet (glacier), most often found in Antarctica, Greenland, or very high mountains worldwide. The ice sheet, which can be hundreds or thousands of meters thick, is essentially snow and ice that has collected each year on the surface, has been compressed together, put under pressure over many years, and ultimately re-crystallizes into thick glacial ice.

Coring, and curation, of this ice can be a difficult task. In order to get the ice core samples, scientists use a hollow drill which actively cuts around a central cylinder of ice. In order to collect a long core record, one must go through many cycles of lowering the drill, cutting a limited section (usually only 4-6 m long), then raising all equipment to the surface, removing the core section and beginning the process again. Much care is taken to ensure that the core is uncontaminated by modern impurities. The core is immediately stored in airtight plastic bags as soon as it reaches the surface, and analysed only in clean rooms. In order to keep the ice core from degrading, the ice much be kept well below the freezing point, in some cases as low as -15 °C.
  Again, the reason such care is taken to avoid modern contamination is because those ice cores, and the bubbles and inclusions within them, contain an abundance of climate information over many hundreds of thousands of years.  This information is stored in such a fragile way, any modern impurities could change the climate signal or confuse it dramatically.

So why do we go to such trouble to collect and preserve these ice cores?  They can, when analyzed in many different ways, tell us about temperature, ocean volume, rainfall amount, levels of CO2 and other gases in the atmosphere, solar variability, and sea-surface productivity. Scientists do any number of chemical and isotopic studies on the ice itself, to get information about the above, but they can also physically look at inclusions in the ice, like wind-blown dust, ash, or radioactive substances that can tell about desert extent, volcanic eruptions, forest fires, or even meteoritic impact.

The length of the record is extremely variable.  Once can look at cores that only record the last few hundred years, or look at one of the most complete cores ever taken and look at climate change over 800,000 years!
Ice Core Gerry Plumbley from the University of Alaska Fairbanks and Qing Zhang from China prepare to drill one of the first ice cores taken during the expedition. (Photo courtesy of Ian MacDonald.)
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