Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 107

becomes depleted in D and 18 O, so the last snow to fall will have a different D and
18 O concentration than the first snow that fell. In areas of heavy snowfall this can
cause significant differences in proxy temperature estimates.''

However, the process by which precipitated snow is gradually compressed into
firn and then ice, entrapping gas bubbles and preserving isotope concentrations,
may take hundreds of years or longer. As a result, ice core data typically represent
averages smeared over several hundred (or more) years.
Oard (2005) provided a good description of ice cores. Ice cores longer than
3,000m have been recovered. In Greenland these represent deposition for up to
150,000 years, and in Antarctica they have reached up to about 800,000 years.
In the late 1950s, Dansgaard led several expeditions to Greenland to acquire
ice from icebergs and, although these did not provide historical temperature data
of any significance, the results provided further validation of the dating approach.
The first ice core was taken at Camp Century in 1964. This core was dated back
to about 100,000 years before the present ( ybp ). The results indicated an ice age
from about 100,000 ybp to about 10,000 ybp , permeated by many fluctuations.
Temperature fluctuations during the most recent 10,000 years were much smaller,
but there was evidence of a post-glacial optimum around 5,000 ybp , a Medieval
Warm Period around 1,000 ybp , and a Little Ice Age from around 500 ybp

to

100 ybp .
Since then, a series of ice core studies at Greenland have provided a wealth of
information on historical temperatures. Ice cores from Greenland have the advan-
tage that annual layers are relatively thick, due to relatively high precipitation, and
can be visually observed in many cases. However, the Greenland ice cores only
cover a maximum time range of up to about 150,000 ybp .
One of the early ice core sites was the so-called Dye 3 site which was
previously part of the U. S. Army DEW line, located at 65 N in Greenland.
Coring was conducted at the Dye 3 site and bedrock was reached at a depth of
2,037m. This site was physically accessible but a site higher on the ice with
smooth bedrock below would have been better.
The Greenland Ice Core Project (GRIP) was a multinational European
research project, which involved eight nations (Belgium, Denmark, France,
Germany, Iceland, Italy, Switzerland, and the United Kingdom). GRIP success-
fully drilled a 3,028m ice core into the bed of the Greenland Ice Sheet at Summit
in central Greenland between 1989 to 1992 at 72 N.
The Greenland Ice Sheet Project (GISP) was a decade-long project to drill ice
cores in Greenland and involved scientists and funding agencies from Denmark,
Switzerland, and the United States.
There was also a follow-up U. S. GISP2 project, which drilled at a
geologically better location on the summit. This hit bedrock in 1993 after five
years of drilling, producing an ice core 3,053m in depth.
The North Greenland Ice Core Project (NGRIP) site was near the center of
Greenland (75 N). The core reached the depth of 2,917m. The cores were
cylinders of ice four inches in diameter brought to the surface in 3.5m lengths.