An ice core is a tube of ice removed from an ice sheet. It is collected by driving a hollow tube or by core drilling deep into an ice sheet, most commonly in the polar ice caps of Antarctica, Greenland or in high moutain glaciers elsewhere. As the ice sheet forms from the incremental buildup of annual layers of snow, lower layers are older than those on top, and an ice core contains ice formed over a range of years. The properties of the ice can then be used to reconstruct a climatic record over the age range of the core.
Ice cores contain an abundance of climate information as almost everything that fell in the snow that year remains behind, including wind-blown dust, ash, atmospheric gases and radioactivity. The variety of climatic proxies is greater than in any other natural recorder of climate such as tree rings or sediment layers. These include (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires.
The length of the record depends on the depth of the ice core and varies from a few years up to 800 kyr for the EPICA core. The time resolution (i.e. the shortest time period which can be accurately distinguished) depends on the amount of annual snowfall, and reduces with depth as the ice compacts under the weight of layers accumulating on top of it. Upper layers of ice in a core corresponds to a single year or sometimes a single season. Deeper into the ice the layers thin and annual layers become indistinguishable.
An ice core from the right site can be used to reconstruct an uninterrupted and detailed climate record extending over hundreds of thousands of years, providing information on a wide variety of aspects of climate at each point in time. 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.
As porous snow consolidates into ice, the air within it is trapped in bubbles in the ice. This process continuously preserves samples of the atmosphere. In order to retrieve these natural samples the ice is ground at low temperatures, allowing the trapped air to escape. It is then condensed for analysis by gas chromatography or mass spectrometry, revealing gas concentrations and their isotopic composition respectively. Apart from the intrinsic importance of knowing relative gas concentrations (e.g. to estimate the extent of greenhouse warming), their isotopic composition can provide information on the sources of gases. For example CO2 from fossil-fuel or biomass burning is relatively depleted in 13C. See Friedli et al., 1986.
Dating the air with respect to the ice it is trapped in is problematic. The consolidation of snow to ice necessary to trap the air takes place at depth (the 'trapping depth') once the pressure of overlying snow is great enough. Since air can freely diffuse from the overlying atmosphere throughout the upper unconsolidated layer (the 'firn'), trapped air is younger than the ice surrounding it. Trapping depth varies with climatic conditions, so the air-ice age difference could vary between 2500 and 6000 years (Barnola et al., 1991). However, air from the overlying atmosphere may not mix uniformly throughout the firn (Battle et al., 1986) as earlier assumed, meaning estimates of the air-ice age difference could be less than imagined. Either way, this age difference is a critical uncertainty in dating ice-core air samples.
Shallow cores, or the upper parts of cores in high-accumulation areas, can be dated exactly by counting individual layers, each representing a year. These layers may be visible, related to the nature of the ice; or they may be chemical, related to differential transport in different seasons; or they may be isotopic, reflecting the annual temperature signal. 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
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 .
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 climatic 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 .
The EPICA core in Antarctica was drilled at 75°S, 123°E (560 km from Vostok) at an altitude of 3,233 m, near Dome C. The ice thickness is 3,309 +/-22 m and the core was drilled to 3,190 m. Present-day annual average air temperature is -54.5°C 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 18O 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 100 kyr of the EPICA core are in the bottom 100 m of the core.
Before 400 kyr 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 400 kyr 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: 28 kyr. 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 780 kyr.
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.
- http://www.ncdc.noaa.gov/paleo/icgate.html Ice Core Gateway
- 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
- http://www.tonderai.co.uk/earth/ice_cores.php "The Chemistry of Ice Cores" literature review
- BARNOLA, J., PIMIENTA, P., RAYNAUD, D. and KOROTKEVICH, Y. CO2-CLIMATE RELATIONSHIP AS DEDUCED FROM THE VOSTOK ICE CORE - A REEXAMINATION BASED ON NEW MEASUREMENts AND ON A REEVALUATION OF THE AIR DATING. Tellus Series B-Chemical and Physical Meteorology, 43(2):83 -- 90, 1991.
- Battle, M., Bender, M., Sowers, T., Tans, P., Butler, J., Elkins, J., Ellis, J., Conway, T., Zhang, N., Lang, P. and Clarke, A. Atmospheric gas concentrations over the past century measured in air from firn at the South Pole. Nature, 383(6597):231 -- 235, 1996.
- FRIEDLI, H., LOtsCHER, H., OESCHGER, H., SIEGENTHALER, U. and STAUFFER, B. ICE CORE RECORD OF THE C-13/C-12 RATIO OF ATMOSPHERIC CO2 IN THE PAST 2 CENTURIES. Nature, 324(6094):237 -- 238, 1986.
Last updated: 05-13-2005 07:56:04