The GISP2 Ice Coring Effort


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Introduction

The following was developed from the AGU paper 95RG00498, "The GISP2 ice core record - Paleoclimate Highlights" by Paul Mayewski and Michael Bender, 1995."

On 1 July 1993 the Greenland Ice Sheet Project Two (GISP2) successfully completed drilling through the base of the Greenland Ice Sheet and another 1.55m into bedrock at a site in the Summit region of central Greenland (72o 36' N, 38o 30' W; 3200 masl) (Mayewski et al., 1994a). In so doing GISP2 recovered the deepest ice core record in the northern hemisphere (3053.44 meters). The Greenland Ice Core Project (GRIP), GISP2's European companion (30km to the east of GISP2 site) penetrated the ice sheet to a depth of 3028.8m one year earlier. Between these two projects, the longest (ice core) paleo-environmental record (>100,000 years) ever compiled is now available for the Northern Hemisphere.


Michael Morrison, GISP2 SMO, University of New Hampshire

Mark Twickler, GISP2 SMO, University of New Hampshire
The Summit region has proven to be an ideal site from which to recover deep ice cores. The -31o C mean annual air temperature at Summit and minimal occurrence of melt layers throughout the record (Alley and Anandakrishnan, 1995) assure the in-situ preservation of a broad range of gaseous, soluble and insoluble measures of the paleo-environment. The -9o C ice temperature measured at the base of the two cores (W. Hancock and M. Wumkes, pers. comm., 1993; N. Gundestrup and L. Hansen, pers. comm., 1993) assures that the ice sheet in this region is frozen to its bed. This, in combination with the gently sloping local bedrock topography (Hodge et al., 1990) and the surface site close to the current ice divide, minimizes (but does not eliminate) the possibility of any ice flow deformation (other than vertical thinning) that would disrupt the original depositional order of the record throughout most of the thickness of the ice sheet in this region.

The d18O of ice has classically provided the basic stratigraphy and paleoclimatology of ice cores. Independent calibrations of the oxygen isotope-temperature relationship have been developed through the analysis of GISP2 borehole temperatures. These calibrations allow conversion of isotope-derived surface-temperature histories to temperature-depth profiles and indicate exceptionally large climate changes, with more than 20o C warming since the glacial maximum (Cuffey et al., 1992;1994; 1995; Shuman et al, 1995). Grootes et al. (1993) measured the d18O 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 B.P.), the GISP2 and GRIP records are nearly identical in shape and in many of the details. The same interpretation was first 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.


Mark Twickler, GISP2 SMO, University of New Hampshire
The similarity 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. This agreement (between the two cores, separated by 30 km, roughly 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.

Dating GISP2

The depth/age relationship for the GISP2 core has been developed from a variety of core parameters including: annual layering of visual stratigraphy; electrical conductivity; laser light scattering of dust; stable isotopes; major anions and cations; insoluble particles; 210Pb; total beta activity; and d14C from occluded CO2 (e.g., Wilson and Donahue, 1990, Dibb, 1992; Taylor et al., 1992; Alley et al., 1993; Meese et al., 1994) plus ice dynamics modeling (Schott et al., 1992). The current conservative estimate of the age error is 2% for 0-11.64 kyr B.P., 5% for 11.64-17.38 kyr B.P. years ago and 10% for 17.38-40.5 kyr B.P. years ago (Alley et al., 1993). While the age scale back to 40.5 kyr B.P. comes from a variety of techniques (Meese et al., 1994) below 40 kyr B.P. the chronology comes from correlating GISP2 into the Vostok chronology of Sowers et al. (1993) derived using the d18O of O2 (Bender et al., 1994). This approach to correlation invokes the fact that the d18O of atmospheric O2 varies with time but, at one time, is constant throughout the atmosphere. Recently annual layer counts based on visual stratigraphy and solid laser light scattering have been extended back to ~110 kyr B.P. (Meese et al. and Ram et al., submitted to the JGR Special Issue).


Mark Twickler, GISP2 SMO, University of New Hampshire

GISP2 Measurements

A variety of projects and a total of forty-two types of measurements composed the GISP2 deep drilling effort. As of completion of drilling a total of nine additional projects provided information necessary to the interpretation of the resultant record (e.g., atmospheric sampling, automatic weather stations, surface glaciology, modeling). A general description of these activities has already been presented (GISP2 Notebook Number 3, 1993; Mayewski et al., 1994a). For purposes of this report we will focus only on highlights of the environmental record thus far deduced.

The Anthropogenic Era

Increases in sulfate and nitrate in south Greenland ice cores attributed to anthropogenic activity (Neftel et al., 1985; Mayewski et al., 1986), have been identified in the GISP2 core and were contrasted to the pre-anthropogenic atmosphere (Mayewski et al., 1990). An observed increase in excess chloride at GISP2 (Mayewski et al., 1993a), as of the 1940's, is believed to be a by-product of the increased levels of anthropogenically-derived HNO3 and H2SO4, since the latter are believed to aid in the volatilization of HCl from sea salt aerosol (Eriksson, 1959). Additional confirmation of the role that anthropogenic pollutants may have on perturbing the chemistry of the atmosphere comes from the coincidence of increased sulfate levels and depression of North Atlantic temperatures between AD 1940-1970 (Wigley, 1990; Charlson et al., 1992). This has been demonstrated by a comparing GISP2, south Greenland and Yukon Territory ice cores with temperature change records (Mayewski et al., 1993b).

Examination of a 217 meter temperature profile developed from a site near the GISP2 borehole reveals a recent warming in near-surface firn which is within the range of natural variability providing no definitive evidence of anthropogenically-induced greenhouse gas warming (Alley and Koci, 1990).

The Little Ice Age and Medieval Warm Period

The Little Ice Age (LIA) and Medieval Warm Period (MWP) environments (the most recent analogs for conditions cooler and warmer, respectively, than the present century) can be characterized by interpreting the multi-parameter GISP2 series (e.g., CO2, stable isotopes, major ions, accumulation rate, particles). The LIA appears to span the period AD 1350 or 1450 to AD 1900 depending upon measurement type (since each may respond to climate change differently).


Brenton Burnett, Eagle Crest High School, Denver, CO
GISP2 temperature modeled from oxygen isotopes reveals a relatively subdued temperature effect at this site for the LIA period (Stuiver et al, 1995; Cuffey et al, 1992; 1994). However, year-to-year correlations between the GISP2 isotopic record and sea surface and land temperatures over the North Atlantic (AD 1840-1970), have revealed changes in atmospheric circulation patterns. One example is the seesaw pattern of the North Atlantic Oscillation (Barlow et al., 1993) which demonstrates the sensitivity of the isotopic record. The accumulation rate, which is indicative of transport distance from the open ocean plus temperature en route, is generally lower during the LIA than the MWP (Meese et al., 1994).

Levels of continental source dusts and marine sea salts increased during the LIA (Mayewski et al., 1993a) in response to increased meridional circulation (O'Brien et al., 1995). The LIA is one of several glacio-chemically identifiable climate events in the Holocene record that correlate with other paleoclimate records and interestingly the LIA is characterized by the most rapid onset of any of these Holocene cold periods (O'Brien et al., 1995). Initial measurements of CO2 in air bubbles of the GISP2 core (Wahlen et al., 1991) indicate that between AD 1530-1810 atmospheric CO2 levels remained relatively constant at 280+/- ppmv. After this period, concentrations rise rather abruptly and smoothly connect to the atmospheric observations at Mauna Loa.


Michael Morrison, GISP2 SMO, University of New Hampshire
Non sea salt (nss) sulfate (reflecting primarily volcanic source SO2) does not appear to be a major forcing agent on multi-decadal scale climate, although volcanoes cause prominent, shorter-lived coolings (Lyons et al., 1990; Stuiver et al, 1995). Individual volcanic event signatures have been studied in the GISP2 core by the measurement of electrical conductivity and the presence of both volcanic source sulfate and particles. Examples of specific events that have been described include: local eruptions (e.g., the AD 1362 Oraefajokull (Iceland) eruption, Palais et al., 1991), intra-hemispherically distributed eruptions (e.g., the AD 1479 Mt. St. Helen's (Washington) eruption, Fiacco et al., in press) and inter-hemispherically distributed eruptions (e.g., the AD 1259 eruption possibly produced by El Chichon (Mexico), Palais et al., 1992). Zielinski et al (1994) provide a complete description of the Holocene volcanic event history developed from continuous, high resolution sampling of sulfate in the GISP2 record which is now available for the entire 110 kyr record and is utilized both as a dating tool and for climate reconstruction (Zielinski et al., JGR Special Issue).

The Younger Dryas and Other Rapid Climate Change Events Over the Last 110 kyr

The Younger Dryas (YD) was the most significant rapid climate change event that occurred during the last deglaciation of the North Atlantic region. Previous ice core studies have focused on the abrupt termination of this event (Dansgaard et al., 1989) because this transition marks the end of the last major climate reorganization during the deglaciation. Most recently the YD has been re-dated using precision, sub-annually resolved multivariate measurements from the GISP2 core as a 1300+/-70 year duration event that terminated abruptly. This was evidenced by an approximate 7o C rise in temperature and a two-fold increase in accumulation rate, at approximately 11.64 kyr BP (Alley et al., 1993). The transition into the Preboreal (PB), the PB/YD transition, and the YD/Holocene transition were all remarkably fast, each occurring over a period of a decade or so (Alley et al., 1993).


Mark Twickler, GISP2 SMO, University of New Hampshire
The isotopic temperature records show 23 interstadial (or Dansgaard/ Oeschger) events first recognized in the GRIP record (Dansgaard et al., 1993) and verified in the GISP2 record (Grootes et al., 1993) between 110 and 15 kyr B.P. These millennial-scale events represent quite large climate deviations; probably many degrees C in temperature, twofold changes in snow accumulation, order-of-magnitude changes in wind-blown dust and sea-salt loading, roughly 100 ppbv in methane concentration, etc., with cold, dry, dusty, and low-methane conditions correlated (Alley et al, 1993; Taylor et al., 1993b; Mayewski et al, 1993c, 1994; Chappellaz et al, 1993).

These events are regional to global since they are observed in local climatic indicators such as snow accumulation rate and the isotopic composition of snow linked to temperature, in regional climatic indicators such as wind-blown sea salt and continental dust, and in regional-to-global indicators such as atmospheric concentrations of methane, nitrate and ammonium. Reorganizations of atmospheric circulation are indicated clearly (Mayewski et al, 1993c; 1994b; Kapsner et al, 1995). Some events are readily identified in the ocean-sediment record in regions critical to global ocean circulation (Bond et al, 1993; Keigwin and Lehman, 1994). Furthermore, new correlation techniques involving the gaseous composition of the atmosphere demonstrate that the major events are also recorded in the isotopic temperature record of the Vostok core from central East Antarctica, although with apparently smaller amplitude and a more ramped appearance than in Greenland (Bender et al, 1994).

GISP2 and GRIP Records Prior to 110 kyr B.P.

The climatic significance of the deeper part of the GISP2 ice core, below 2790 m depth and 110 kyr age, is a matter of considerable investigation and controversy. The isotopic temperature records and electrical conductivity records of GISP2 and GRIP, so similar for younger ice, are very different in the lower part (Grootes et al., 1993; Taylor et al., 1993a). Ice in both cores below 2790 m depth shows evidence of folding or tilting in structures too large to be fully observed in a single core (Gow et al., 1993; Alley et al., 1995). The d18O of O2 in GISP2 above 2790 m matches almost perfectly with the Vostok record (Sowers et al., 1993); below it is far noisier and the smoothed Vostok signal cannot be aligned unambiguously with GISP2 (Bender et al., 1994). These features all suggest that ice age changes discontinuously in the deepest part of GISP2 as a result of folding, extensive boudinage (squeezing out of layers of ice), and/or intrusion. Bender et al. (1994) concluded that the bottom ~ 200 m of ice at GISP2 may be correctly ordered but discontinuous and extremely condensed, perhaps extending back to several hundred kyr B.P. Alternatively, the core may contain a disordered sequence of much younger ice, perhaps largely from marine stages 5c-5e (about 115-130 kyr B.P.).


Jennifer Putscher, GISP2 SMO, University of New Hampshire
GRIP (1993) interpreted the climate proxy records from the deepest part of their core as being properly ordered and continuous. They observed large and rapid changes in isotopic temperature with depth and concluded that these features represented rapid changes in climate during marine isotopic stage 5e, the warmest part of the previous interglacial period. Such a conclusion has extremely important implications for climate because, together with the Dansgaard-Oeschger events, it suggests that rapid cooling events are possible during the current interglacial period.

Evidence for rapid climate change during Stage 5e rests on the assumption that the deep part of the GRIP ice core is continuous, in contrast to GISP2. GRIP is located over the present ice divide, while GISP2 is 30 km to the west. Therefore GRIP may in fact be more likely to be continuous, although a recent modeling study has shown that the divide itself has probably migrated (Anandakrishnan et al., 1993), and GRIP contains deformational features that are similar in many ways to those of GISP2 (Alley et al, 1995). Continuity of the deep part of the GRIP core has not yet been definitively demonstrated. Until this is done, by showing that gas composition records at GRIP are identical with those at Vostok or by some other approach, the evidence for rapid climate change in Greenland during the last interglacial remains equivocal.

Acknowledgments

GISP2 is part of the Office of Polar Programs, National Science Foundation Arctic System Science (ARCSS) initiative. GISP2 science was coordinated by the GISP2 Science Management Office, University of New Hampshire (Dr. P.A. Mayewski, Director; Mr. Michael Morrison, Associate Director (1989-1993); Mr. Mark S. Twickler, Associate Director (1993-1999).

The GISP2 ice core was drilled by the Polar Ice Coring Office, University of Alaska (Dr. John Kelley, Director; Mr. Mark Wumkes, Chief Driller). The core is archived at the National Ice Core Laboratory, Denver Federal Center, United States Geological Survey; Dr. J.J. Fitzpatrick, Technical Director.

GISP2 convened PI meetings at least once every year during the duration of the project. In addition, two joint meetings were held between GRIP and GISP2 in Annecy, France (1993) and Wolfeboro, New Hampshire, USA (1995). The second joint meeting was used to compare results prior to submission to the JGR Special Issue.

Approximately 70% of all GISP2 sample processing was undertaken in the field. Archive samples (~30% of the full core) are stored at the National Ice Core Laboratory (NICL) in Denver, CO.

The GISP2 Executive Committee:

Dr. R.B. Alley (1994-1997)
Dr. P.A. Mayewski (1989-1997, Chair)
Dr. P.M. Grootes (1989-1997)
Dr. M. Wahlen (1989-1997)

The GISP2 Advisory Committee Includes:

Dr. J. Andrews (1994-1997)
Dr. C. Bentley (1989-1997)
Dr. W. Broecker (1989-1997, Chair)
Dr. G. Denton (1991-1997)
Dr. J. Imbrie (1989-1997)
Dr. S. Lehman (1994-1997)