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 or by core drilling deep into the ice sheets of Antarctica, 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.

Ice core data

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.

Dating cores

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.

Famous ice cores

Vostok

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] (http://www.ngdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok.html).

GRIP/GISP

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 [2] (http://www.agu.org/revgeophys/mayews01/node8.html).

EPICA/Dome C

The EPICA and Vostok cores compared

The EPICA core in Antarctica was drilled at 75S, 123E (560 km from Vostok) at an altitude of 3,233m, near Dome C. The ice thickness is 3,309+/-22m and the core was drilled to 3,190m. Present-day annual average air temperature is -54.5 oC and snow accumulation 25 mm/y. Information about the core was first published in Nature on 2004/June/10.

The core went back 720,000 years and revealed 8 previous glacial cycles. The picture shows delta-O-18 data (a proxy for temperature: more negative values indicate lower temperatures) from both EPICA and Vostok. The upper plot, with x-axis being age (years before 1950) clearly shows the extra information in the EPICA core before the start of the Vostok record. The lower picture, plotted against depth, shows how compressed the deeper parts of the cores are: the earliest 100kyr of the EPICA core are in the bottom 100 m of the core.

Before 400kyr the character of the ice ages are seen to be somewhat different: interglacial warmth is distinctly less than the four most recent interglacials. The interglacial 400kyr ago, which is believed (from arguments about the configuration of the orbital parameters of the earth) to be an approximate analogue to the current interglacial, was quite long: 28kyr. The Nature paper argues that if this analogue is accepted, the current climate would be expected to continue like today's, in the absence of human influence (which it states is unlikely, given the predicted increases in greenhouse gas concentrations).

Further analysis of the core is hoped to extend the record back somewhat further, possibly as far as the Brunhes-Matutama magnetic reversal, believed to be at about 780kyr.

The core time scale is derived from the measured depth scale by a model incorporating surface snow accumulation variations, ice thinning, basal heat fluxes etc, and is empirically "tied" at 4 times by matches to the marine isotopic record.

External links

• http://www.nature.com/nsu/040607/040607-4.html "Frozen time" from Nature (journal)
• http://www.newscientist.com/news/news.jsp?id=ns99994121 "Oldest ever ice core promises climate revelations " - from New Scientist
• http://news.bbc.co.uk/2/hi/science/nature/3792209.stm " Ice cores unlock climate secrets" from the BBC
• http://earthobservatory.nasa.gov/Newsroom/MediaAlerts/2004/2004060917108.html - "New Ice Core Record Will Help Understanding of Ice Ages, Global Warming" from NASA