Ice core

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 mountain 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.

Structure of ice sheets and cores

Most ice sheets are formed from snow. Because an ice sheet survives summer, the temperature in that location usually does not warm much above freezing. In many locations in Antarctica the air temperature is always well below the freezing point of water.

The surface layer is snow in various forms, with various air gaps between snowflakes. As snow continues to accumulate, the buried snow is compressed and forms firn, a grainy material with a texture similar to granulated sugar. There are no longer pores allowing circulation of air. This often takes less than a year.

Under increasing pressure, at some depth the firn is compressed into ice. This depth may range between a few to several tens of meters. Below this level material is frozen in the ice. Ice may appear clear or blue.

Layers can be visually distinguished in firn and in ice to significant depths. In a location on the summit of an ice sheet where there is little flow, accumulation tends to move down and away, creating layers with minimal disturbance. In a location where underlying ice is flowing, deeper layers may have increasingly different characteristics and distortion. Drill cores near bedrock often are challenging to analyze due to distorted flow patterns and composition likely to include materials from the underlying surface.

Characteristics of firn

The layer of porous firn on Antarctic ice sheets is 50–100m deep, with air which is up to 30 years old. Air in the atmosphere and firn are slowly exchanged by molecular diffusion through pore spaces, because gases move toward regions of lower concentration. Thermal diffusion causes isotope fractionation in firn when there is rapid temperature variation, creating isotope differences which are captured in bubbles when ice is created at the base of firn. There is gas movement due to diffusion in firn, but not convection except very near the surface.

Below the firn is a zone in which seasonal layers alternately have open and closed porosity. These layers are sealed with respect to diffusion. Gas ages increase rapidly with depth in these layers. Various gases are fractionated while bubbles are trapped where firn is converted to ice. [1]

Coring

A core is collected by separating it from the surrounding material. For material which is sufficiently soft, coring may be done with a hollow tube. Deep core drilling into hard ice, and perhaps underlying bedrock, involves using a hollow drill which actively cuts a cylindrical pathway downward around the core. Because deep ice is under pressure and easily deforms, the hole will tend to close if there is nothing to supply back pressure. The hole may be filled with a fluid to keep the hole from closing; in the case of GISP2, the borehole was filled with butyl acetate, an organic compound, to within 100m of the surface.

When a drill is used, the cutting apparatus is on the bottom end of a drill barrel, the tube which surrounds the core as the drill cuts downward around the edge of the cylindrical core. The length of the drill barrel determines the maximum length of a core sample (6 m at GISP2 and Vostok). Collection of a long core record thus requires many cycles of lowering a drill/barrel assembly, cutting a core 4–6m in length, raising the assembly to the surface, emptying the core barrel, and preparing a drill/barrel for drilling.

Core processing

The core is carefully extruded from the barrel; often facilities are designed to accomodate the entire length of the core on a horizontal surface. Drilling fluid will be cleaned off before the core is cut into 1–2 meter sections. Various measurements may be taken during preliminary core processing.

Current practices to avoid contamination of ice include:

• Keeping ice well below the freezing point.
  • At Greenland and Antarctic sites, temperature is maintained by having storage and work areas under the snow/ice surface.
  • At GISP2, cores were never allowed to rise above -15°C, partly to prevent microcracks from forming and allowing present-day air to contaminate the fossil air trapped in the ice fabric, and partly to inhibit recrystallization of the ice structure.
• Wearing special clean suits over cold weather clothing.
• Mittens or gloves.
• Filtered respirators.

Due to the many types of analysis done on core samples, sections of the core are scheduled for specific uses. After the core is ready for further analysis, each section is cut as required for tests. Some testing is done on site, other study will be done later, and a significant fraction of each core segment is reserved for archival storage for future needs.

Ice core data

Many materials can appear in an ice core. Layers can be measured in several ways to identify changes in composition. Small meteorites may be embedded in the ice. Volcanic eruptions leave identifiable ash layers. Dust in the core can be linked to increased desert area or wind speed.

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. Beryllium 10 concentrations are linked to cosmic ray intensity which can be a proxy for solar strength. See proxy.

Core contamination

Some contamination has been detected in ice cores. The levels of lead on the outside of ice cores is much higher than on the inside.^[2]

Paleoatmospheric sampling

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.

In Law Dome ice cores, the trapping depth at DE08 was found to be 72 m where the age of the ice is 40±1 years; at DE08–2 to be 72 m depth and 40 years; and at DSS to be 66 m depth and 68 years.[3]

Paleoatmospheric firn studies

At the South Pole, the firn-ice transition depth is at 122 m, with a CO₂ age of about 100 years. Gases involved in ozone depletion, CFCs, chlorocarbons, and bromocarbons, were measured in firn and levels were almost zero at around 1880 except for CH₃Br, which is known to have natural sources.[4] Similar study of Greenland firn found that CFCs vanished at a depth of 69m (CO₂ age of 1929).[5]

Analysis of the Upper Fremont Glacier ice core showed large levels of chlorine-36 that definitely correspond to the production of that isotope during atmospheric testing of nuclear weapons. This result is interesting because the signal exists despite being on a glacier and undergoing the effects of thawing, refreezing, and associated meltwater percolation.[6] ³⁶Cl has also been detected in the Dye-3 ice core (Greenland)[7], and in firn at Vostok. DOI:10.1111/j.1600–0889.2004.00109.x

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. 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 such as nuclear bomb atmospheric testing's radioisotope layers in the upper levels, and ash layers corresponding to known volcanic eruptions. Some composition changes also are detected by high-resolution scans of electrical resistance. Lower down the ages are reconstructed by modelling accumulation rate variations and ice flow.

Dating is a difficult task. Five different dating methods have been used for Vostok cores, with differences such as 300 years at 100m depth, 600yr at 200m, 7000yr at 400m, 5000yr at 800m, 6000yr at 1600m, and 5000yr at 1934m. [8]

Different dating methods makes comparison and interpretation difficult. Matching peaks by visual examination of Moulton and Vostok ice cores suggests a time difference of about 10,000 years but proper interpretation requires knowing the reasons for the differences. [9]

Famous ice cores

Greenland

GRIP/GISP

Within the framework of the joint European Greenland Ice Core Project (GRIP) a 3029 m long ice core was drilled in Central Greenland from 1989 to 1992 at 72° 35' N, 37° 38' W. Studies of isotopes and various atmospheric constituents in the core have revealed a detailed record of climatic variations reaching more than 100,000 years back in time. The results indicate that Holocene climate has been remarkably stable and have confirmed the occurrence of rapid climatic variation during the last ice age (the Wisconsin). Climatic instability observed in the core part believed to date from the Eemian interglacial has not been confirmed by other climate records. [10]

On July 11993 the Greenland Ice Sheet Project Two (GISP2) successfully completed drilling through the base of the Greenland Ice Sheet and another 1.55m into bedrock at a site in the Summit region of central Greenland (72° 36' N, 38° 30' W; 3200 masl). In so doing GISP2 recovered the deepest ice core record in the northern hemisphere (3053.44 meters). [11]

These GRIP/GISP 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. [12] There is evidence the GISP2 cores contain an increasing structural disturbance which casts suspicion on features lasting centuries or more in the bottom 10% of the ice sheet. ^[13]

NGRIP

In 2003, the North Greenland Ice Core Project (NGRIP) recovered what seem to be plant remnants nearly two miles below the surface, and they may be several million years old. [14]

"Several of the pieces look very much like blades of grass or pine needles," said University of Colorado at Boulder geological sciences Professor James White, an NGRIP principal investigator. "If confirmed, this will be the first organic material ever recovered from a deep ice-core drilling project," he said.

The NGRIP drilling site is near the center of Greenland. Drilling began in 1999 and was completed at bedrock in 2003. The cores are cylinders of ice four inches in diameter that were brought to the surface in 3.5-meter lengths. The cores that contained the possible plant material also contained a high content of trapped gas, which may help researchers determine the area's annual climate history during the past 123,000 years.

Antarctica

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 [15].

EPICA/Dome C

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.

External links

• Ice Core Gateway
• "Frozen time" from Nature (journal)
• "Oldest ever ice core promises climate revelations" – from New Scientist
• "Ice cores unlock climate secrets" from the BBC
• "New Ice Core Record Will Help Understanding of Ice Ages, Global Warming" from NASA
• Ice-core evidence of rapid climate shift during the termination of the Little Ice Age – Upper Fremont Glacier study

References

• 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.

• ^  Gow, A. J., D. A. Meese, R. B. Alley, J. J. Fitzpatrick, S. Anandakrishnan, G. A. Woods, and B. C. Elder (1997). "Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review". J. Geophys. Res. 102 (C12): 26559–26576. DOI:10.1029/97JC00165
• ^  Amy Ng and Clair Patterson (1981). "Natural concentrations of lead in ancient Arctic and Antarctic ice". Geochimica et Cosmochimica Acta 45 (11): 2109–2121. DOI:10.1016/0016–7037(81)90064–8