GISP2 Notebook Vol.1 No. 1

GISP2 Scientific Goals

The ~3000 m deep GISP2 core will yield a high resolution, ~200,000 year history of global change - to include two interglacials and two glacials - the longest such record available from the Northern Hemisphere.

During the past decades, linkages between the atmosphere, biosphere, anthrosphere, hydrosphere, cryosphere, lithosphere and cosmogenic input have been revealed. These linkages are characterized by complex integration and feedback. While the existence of this complex global system is now recognized, how it functions is poorly understood. The recent increase in CO2 and other greenhouse gases as a result of human activity and in turn concern over the climate change which may result require that we gain a detailed knowledge of the total global system. The history which will be derived from the GISP2 core will provide the detailed record of interaction needed to unravel the functioning of the global system. Further, the wealth of this history will help to fuel the next generation of visionary thought in Earth System Science.

Signals which are generated by interactions within the global system can be seen in the atmosphere, "captured" in the cryosphere and are available for our viewing by the sophisticated analyses of ice cores. Properties which will be studied in the GISP2 core include: Gases, stable isotopes of occluded gases, stable isotopes in ice, particles, major and trace element chemistry of ice, cosmogenic isotopes, conductivity, physical properties.

The detailed view of global system history revealed from these ice core properties will expand significantly our understanding of global change- EVOLUTION, PROCESS, TREND, AND COHERENCE. It will provide the type of PERSPECTIVE necessary to assess PREDICTIVE global change models. Specifically, a GISP2 global change history will include:

1. High resolution views of Holocene and pre-Holocene climate (temperature, precipitation rate, air mass source, humidity, wind strength, and dustiness) and atmospheric chemistry (gases, soluble and insoluble constituents). Particular attention will be paid to: Little Ice Age events, the Climatic Optimum, the Younger Dryas, glacial/interglacial transitions, and rapid climate transitions, plus detailed scans of time series to search for cycling and comparison with proxy climate data sets from other media and latitudes.
2. Documentation of the partitioning, reservoir exchange, and production rates for anthropogenic, volcanic, biogenic, oceanic, terrestrial, cosmogenic, etc. sources of the gaseous, soluble and insoluble constituents in the atmosphere. Particular attention will be paid to radiatively active gases and aerosols, as well as biogeochemical cycling (e.g. C, N, O, and S) including fractionation from various important biogenic reservoirs (e.g. terrestrial, oceanic).
3. Investigation of timing and forcing dependency as revealed by comparison of climate (e.g. temperature, precipitation amount), radiatively important gases (e.g. CO2, CH4, etc.) and aerosols (e.g. , volcanic, marine, crustal, and anthropogenic), biogeochemical cycling (e.g. C, N, O, and S), and other system boundaries (e.g. ocean temperature, level, and circulation, sea ice extent, ice volume, volcanic activity, atmospheric turbidity, etc.).
4. Development of accurate dating techniques and flow modelling for pre-Holocene ice, with particular attention to the identification of global stratigraphic markers (e.g. isotopic ratios, cosmogenic events, volcanic events).

*GISP2 is comprised of scientists from: the Cold Regions Research and Engineering Laboratory, the Desert Research Institute, Ohio State University, Pennsylvania State University, the New York State Department of Health, the State University of New York at Albany, Lamont-Doherty Geological Observatory, University of Arizona, University of Colorado, University of Miami, University of New Hampshire, University of Rhode Island, University of Washington and the University of Wisconsin under the scientific management of the University of New Hampshire and with logistic and drilling support from the University of Alaska in Fairbanks.

Abstracts of Current GISP2 Principal Investigators

Continuous visual logging of the core, systematic measurement of density, texture, (size, shape and arrangement of grains and pores) and fabric, (crystallographic orientation), and detailed study of these properties in especially interesting intervals. The first such detailed study is now being investigated. The results of this research will provide the necessary physical framework in which to interpret the paleoclimatic record in the core, important data needed to understand ice dynamics, to date the core by flow modelling, and improve understanding of the physical properties and processes of ice as a material and as part of an ice sheet.

Specifically this study will verify the validity of paleoclimatic and other results from GISP2 through study of the physical properties of the core. Physical processes during formation and flow of glacial ice and recovery and relaxation of ice cores can affect the stratigraphic record of trace impurities profoundly under some circumstances; such effects must be characterized or demonstrated to be negligible to validate paleoclimatic analyses.

This study will also advance our understanding of the formation and behavior of glacial ice. Ice is a widespread and economically important material. It is central to glacier dynamics (and thus to ice-sheet stability and future sea levels), and can be used as an analog for geological (mantle creep) and engineering (hot pressing, high temperature creep) processes. Fundamental physical knowledge gained from ice-core studies is thus of considerable value; such knowledge also can be used to improve the ice-dynamical time scale and thus the interpretation of paleoclimatic data.

The d¹⁸O of atmospheric O2 is influenced by d¹⁸O of seawater (the ultimate source of all photosynthetic O2), isotopic fractionations during photosynthesis, respiration, and hydrologic processes. They thus reflect the changing nature of planetary interactions between the biosphere, hydrosphere, and atmosphere (Fireman and Norris, 1982; Horibe et al., 1985). Further, the curve of d¹⁸O (O2) vs. time indicates the rate of primary productivity on the planet, as well as relative rates of primary production in the oceans and on land. We also propose to use d¹⁸O of O2, which is constant throughout the atmosphere at any one time, as a time-stratigraphic marker for the correlation of Greenland and Antarctic ice cores.

Results of our studies to date, extending back to about 30 kyr B.P., indicate that the O2 content of the ice ace atmosphere was about 8% less than at present. We propose to check our existing results with additional measurements, and to extend the record back to 200,000 years before present. We will use the results to test, or generate, hypotheses explaining the causes of glacial/ interglacial changes in the atmospheric CO2 content. The N2/Ar ratio of ice core trapped gases provides for a test of sample integrity. The atmospheric concentrations of N2 and Ar in air cannot have changed significantly during the last few million years, because these gases are relatively inert and consequently have very long atmospheric turn-over times. Absent any physical fractionation of gases, the N2/Ar ratio in ice core trapped gas samples will be identical to that in the modern atmosphere. Differences would indicate that physical fractionation processes have changed concentrations of the gases trapped in the ice, including those which are radiatively active. The results will provide a boundary condition on the reliability of radiatively active gas concentrations, to be measured by other investigators in samples from the GISP2 core.

Work from our lab shows that the isotopic composition of N2 and O2 in ice core trapped gases differs from that of modern air, due to mass-dependent fractionation during the gas trapping process. This fractionation would affect concentrations as well as isotopic compositions of all constituents in trapped air. A fractionation correction, based on the d¹⁵N of N2 in the trapped gas can and must be made for all isotopic analyses of trapped gases.

The Central Greenland Glaciological Survey was begun in 1987 and completed this field season (1989). A 150 x 150 km grid was established the first year, centered at the Summit camp (37Á 55' W, 72Á 18' N), and survey markers were installed at 20 sites. Using Doppler satellite surveying methods, positions were measured in both 1987 and 1989, thus enabling the larger scale pattern of ice flow to be determined. At 9 sites around the grid (which straddled the ice crest), shallow cores to about 17 m depth were recovered. The oxygen isotope stratigraphy, along with measurements of gross beta activity, has enabled the mean accumulation rate of the past 30 years or so to be calculated at each of the 9 sites. In addition, these detailed records are being studied to assess the spatial correlation in the isotopic signal. Measurements of deuterium and oxygen isotope variation in pit samples taken from the drill sites may enable specific source areas for precipitation to be identified. Also, the mean annual temperature (i.e., the temperature at 10 m depth) was measured this field season at a number of sites. This temperature field, along with the spatial distribution of accumulation rate and velocity, are fundamental parameters needed for realistic numerical models of ice sheet flow in Central Greenland.

This proposal is for the measurement of helium concentrations and isotope ratios, Kr, Ar ratios, and Krypton 81 ages in ice from GISP2. The helium program is a search for nulls and/or reversals of the Earth's magnetic field, which should leave a strong signal in the helium 3 to helium 4 isotope ratios and the helium 3 concentrations in ice-core helium. Although the diffusivity of helium in ice is about the same as in liquid water, model calculations show that the signal should be well-preserved because of the high vertical advection velocity of the ice relative to the helium diffusion rate. The Kr/Ar ratios in Greenland ice down to ca. 150 meters depth are enriched relative to the ratios in air about 1.3%, and the ratio of this enrichment to the isotopic enrichment in nitrogen shows that the effect is due to gravitational equilibrium in the air column in the firn. The magnitude of these enrichments is about 85% of that calculated from the firn depth and temperature, probably because of turbulent mixing in the uppermost 10 meters of firn. The Kr/Ar enrichments may vary with depth, and it is proposed to measure the ratio on gas-splits from the helium samples collected. The helium samples must be collected in the field immediately on core retrieval because of the high diffusivity of helium in ice

If a geomagnetic helium event is recorded in the ice sheets, it will provide a world-wide traceable horizon in both Greenland and Antarctica, for chronology and for model studies of ice flow, using the known diffusivity if He in ice and modeling the tracer horizon in two dimensions. Moreover, the event can be correlated with the sedimentary record and with lava flows during the past 100,000 years. There are at least six possible events to look for: the Blake and Laschamp known events, the proposed Mono Lake and Lake Mungo excursions, and in addition, two Be 10 possible events observed in the Vostok core, that may in fact be markers of geomagnetic events.

Krypton 81 (half-life =213,000 years) is the only choice for dating ice older than 50,000 years. We have just reported the first Krypton age on ice, measured on 224 Kg of ice from the Allan Hills. The age was measured on 9000 Kr-81 atoms concentrated from an initial 90,000 atoms: the value obtained is 108,000 years with an uncertainty of 28,000 years. We have every hope that the amount of ice required will come down to some 50 Kg or less, and that the precision will increase, and it is hoped therefore to measure ages on two deep (>2500m) GISP2 samples.

This project will include atmospheric radionuclides among the species being measured at the solar-powered atmospheric camp(ATM) that was established in 1989 near Summit, Greenland as part of the Greenland Ice Sheet Project Two(GISP2). The PI will determine the concentrations of ⁷Be and ²¹⁰Pb (and perhaps gamma emitting anthropogenic radionuclides) in high volume aerosol samples (collected continuously during the field season at a nominal 24 hour interval) and fresh and aging surface and near surface snow samples. ¹⁰Be and ³⁶ will be measured in these samples by Dr. Robert Finkel (Lawrence Livermore National Laboratory) at no cost to NSF.

All of the research at ATM has the primary goal of fostering the interpretation of paleoatmospheric and paleoclimatic signals recorded in the ice of the Greenland Ice Sheet. This will be accomplished by improving our understanding of the current day transport proceeded, air-snow transfer processes, and early post-depositional mechanisms resulting in the incorporation of atmospheric constituents into the snowpack. Preliminary results indicate that measurements of the atmospheric radionuclides in the atmosphere and snow near Summit will illuminate the importance of atmospheric boundary layer dynamics, and post-depositional modifications of the snow, on the delivery and subsequent preservation of submicron aerosol-associated species in the region. Under the provisions of the GISP2 data exchange agreement, the results of this study will combined with those of other researchers at ATM (as well as various fresh and aging snow studies ongoing as part of GISP2) in a collaborative effort to begin to quantify the fidelity with which polar snow and ice record signals from the overlying atmosphere.

This program will investigate the stratigraphy, relaxation characteristics and crystalline structure of ice core. Studies will include: 1) delineation of annual layering to as great a depth as the visible stratigraphy can be deciphered 2) precision density measurements to monitor the relaxation characteristics of ice cores as they age 3) determination of the principal mechanisms by which cores relax-bubble decompression, fracturing, microcracking and exsolved gas cavitation, and complementary measurements of total gas volume in the ice and gas pressure measurements inside original bubbles and exsolution cavities 4) crystal size measurements as a function of the depth and age of the ice 5) c-axis fabric measurements and 6) analysis of debris in basal ice cores. Relaxation of ice cores results in significant changes in their mechanical condition that must be considered in relation to the preparation and analysis of core samples for entrapped gas and chemical studies; c-axis fabrics constitute the primary source of information for interpreting the strain history of the ice column that vertically drilled cores represent. Careful documentation of these key properties is essential to accurate assessments of the depth-age relationship and confidence in paleoclimate reconstructions based on geochemical and entrapped gas analysis.