The Summit Ice Cores (GISP2 and GRIP)

On 1 July 1993 the Greenland Ice Sheet Project Two (GISP2) successfully completed drilling through the base of the Greenland Ice Sheet and another 1.55 m into bedrock at a site in the Summit region of central Greenland (72.6N; 38.5W; 3200 masl) [ Mayewski et al., 1994a]. In so doing, GISP2 recovered the deepest ice core record in the world (3053.44 meters). GISP2 and its European companion project, GRIP (the Greenland Ice Core Project, which one year earlier penetrated the ice sheet to a depth of 3028.8 m, 30 km to the east of GISP2), have now developed the longest ice-core-derived paleoenvironmental record, >110,000 years, available from the Northern Hemisphere.

The Summit region has proven to be an ideal site from which to recover deep ice cores. The C mean annual air temperature at Summit and minimal occurrence of melt layers throughout the record assure the in-situ preservation of a broad range of gaseous, soluble and insoluble measures of the paleoenvironment. The C ice temperature measured at the base of the two cores (W. Hancock and M. Wumkes, personal communication, 1993; N. Gundestrup and L. Hansen, personal communication, 1993) assures that the ice sheet in this region is frozen to its bed. This, in combination with only gently sloping local bedrock topography [ Hodge et al., 1990] and surface siting close to the current ice divide, minimizes the possibility, throughout most of the thickness of the ice sheet in this region, of any ice flow deformation (other than vertical thinning) that would disrupt the original depositional order of the record.

The similarity (discussed below) of the GISP2 and GRIP records is compelling evidence that the stratigraphy of the ice is reliable and unaffected by extensive folding, intrusion, or hiatuses from the surface to 2790 m (110,000 years ago). This agreement (between the two cores separated by 30 km, 10 ice thicknesses) provides strong support of climatic origin for even the minor features of the records and implies that investigations of subtle environmental signals (e.g., rapid climate change events with 1-2 year onset and termination) can be rigorously pursued.

The O of ice has classically provided the basic stratigraphy and paleoclimatology of ice cores. Ice originates by evaporation of sea water. As an air mass travels away from the site of evaporation towards higher latitudes, it cools and is able to hold less water. Water is lost from the air mass as precipitation (rain or snow). When liquid and gaseous HO interact, the heavy isotope of O (O, 0.2% natural abundance) is depleted in the gas phase relative to the light, major isotope (O, 99.8% natural abundance). Along the flow path of the air mass, residual gas becomes progressively more depleted in O as temperature becomes colder and more and more of the original HO content is lost as precipitation. The O value is determined from:

where subscript s refers to sample, std refers to standard mean ocean water (SMOW), and is parts per thousand.

Independent calibrations of the oxygen isotope-temperature relationship have been developed through the analysis of GISP2 borehole temperature, allowing conversion of isotope-derived surface-temperature histories to temperature-depth profiles [ Cuffey et al., 1992]. Thus it follows that variations with depth in the O of ice in a core reflect past variations of temperature with time at a study site. Changes in moisture sources feeding central Greenland may provide additional complications in the interpretation of the record [ Charles et al., 1994]. Grootes et al. [1993] measured the O of ice in the GISP2 core and compared their record with the previously published record for GRIP [ Dansgaard et al., 1993]. Down to a depth of 2790 m in GISP2 (corresponding to an age of about 110 kyr BP (kyr before present, where present is AD1950)), the GISP2 and GRIP records are nearly identical in general shape and in most of the details.

The same interpretation was developed by Taylor et al. [1993a] based on a comparison of the electrical conductivity records from the two sites. The electrical conductivity of ice has been widely measured in ice coring programs because it allows the rapid characterization of certain chemical properties of the core. Electrical measurements can be performed on the entire core, although generally the first 2 m of a core are too fragile to handle. The electrical conductivity method (ECM) [ Hammer, 1980; Schwander and Stauffer, 1984; Taylor et al., 1992] measures the current flowing between two electrodes that have a potential difference of a few thousand volts. The electrodes are on the order of a centimeter apart and a direct current is used. The measurement has a spatial resolution of a few millimeters and is made as the electrodes are moved along a prepared smooth surface. The ECM is exclusively related to the acid/base balance in the ice [ Hammer, 1980; Taylor et al., 1992], with higher currents indicating more acidic conditions. Because of the sensitivity to sulfuric acid and the ability to resolve features that are less than a centimeter thick, the electrical conductivity method (ECM) is well suited to locate volcanic fallout in a core [ Hammer, 1980; Taylor et al., 1992]. Under the right circumstance, the ECM method can also detect annual layers [ Neftel, 1985; Taylor et al., 1992] and possibly ammonia associated with biomass burning [ Legrand et al., 1992].