Difference between radiocarbon dating dendrochronology
In Greenland, a sequence of collaborative projects began in the 1970s with the Greenland Ice Sheet Project; there have been multiple follow-up projects, with the most recent, the East Greenland Ice-Core Project, expected to complete a deep core in east Greenland in 2020.
Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal.
Ice is lost at the edges of the glacier to icebergs, or to summer melting, and the overall shape of the glacier does not change much with time.
Impurities in the ice provide information on the environment from when they were deposited.
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier.
Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper, and an ice core contains ice formed over a range of years.
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice.
Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.
It can be up to about 20 m thick, and though it has scientific value (for example, it may contain subglacial microbial populations), Cores are often drilled in areas such as Antarctica and central Greenland where the temperature is almost never warm enough to cause melting, but the summer sun can still alter the snow.
In polar areas, the sun is visible day and night during the local summer and invisible all winter.
Below this depth, electromechanical or thermal drills are used.
The cutting apparatus of a drill is on the bottom end of a drill barrel, the tube that surrounds the core as the drill cuts downward.