Ice core
An ice core is a tube of ice removed from an ice sheet. The ice is older the further down it gets, so an ice core contains ice formed over a range of years.Ice cores are collected by driving a hollow tube deep into the ice sheets of Antarctica and Greenland and in glaciers elsewhere.
Ice cores contain an abundance of climate information, more so than any other natural recorder of climate such as tree rings or sediment layers. Although their record is short (in geologic terms), it can be highly detailed.
Upper layers of ice in a core corresponds to a single year, sometimes even a single season and almost everything that fell in the snow that year remains behind, including wind-blown dust, ash, atmospheric gases, even radioactivity. Deeper into the ice the layers thin and annual layers become indistinguishable.
An ice core from the right site can contain an uninterrupted, detailed climate record extending back hundreds of thousands of years. This record can include (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity and a variety of other climate indicators.
It is the simultaneity of these properties recorded in the ice that makes ice cores such a powerful tool in paleoclimate research.
[[image:Vostok-ice-core-petit.png|thumb|right|Graph of CO2 (black), reconstructed
temperature (blue) and dust (red) from the Vostok ice core for the past 420,000 years]]
Isotopic analysis of the ice in the core can be linked to temperature and global sea level variations. Analysis of the air contained in bubbles in the ice can reveal the palaeocomposition of the atmosphere, in particular CO2 variations. Volcanic eruptions leave identifiable ash layers. Beryllium 10 concentrations are linked to cosmic ray intensity which can be a proxy for solar strength. Dust in the core can be linked to increased desert area or wind speed. See proxy.
Shallow cores, or the upper parts of cores in high-accumulation areas, can be dated exactly by counting individual layers, each representing a year. Deeper into the core the layers thin out due to ice flow and eventually individual years cannot be distinguished. It may be possible to identify events - atom bomb test radioisotope layers in the upper levels; ash layers corresponding to known volcanic eruptions. Lower down the ages are reconstructed by modelling accumulation rate variations and ice flow.
Up to 2003, the longest core drilled was at Vostok station. It reached back 420,000 years and revealed 4 past glacial cycles. Drilling stopped just above Lake Vostok. The Vostok core was not drilled at a summit; hence ice from deeper down has flowed from upslope; this slightly complicates dating and interpretation. Vostok core data is available [1].
These two cores were drilled by European and US teams on the summit of Greenland. Their usable record stretches back more than 100,000 years. They agree (in the cliamtic history recovered) to a few meters above bedrock. However the lowest portion of these cores cannot be interpreted, probably due to disturbed flow close to the bedrock [1].
The EPICA core in Antarctica has now gone back 720,000 years and revealed 8 previous glacial cycles.
Ice core data
Dating cores
Famous ice cores
Vostok
GRIP/GISP
EPICA/Dome C
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