COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP
1.3 SPECIALTY GROUP REPORT: COSMOGENIC ISOTOPES
Cosmogenic isotopes in ice cores can be used to determine a chronology for the core, to study the history of terrestrial magnetic field and of solar activity and to investigate global atmospheric mixing. The cosmogenic isotopes to be measured in ice include 14C, 1OBe, 26Al, 36Cl and 81Kr. Because the applications are somewhat different, these isotopes are discussed in separate groups.
14C is a powerful tool for determining ice core chronology beyond ages which can be reached by d180 annual layer counting, i.e., for ages older than about 10 kyr in Greenland. Current analytical techniques utilizing accelerator mass spectrometry (AMS), will, with minor improvements, allow dating of 1-2 kg of ice +/1 800 years at 15 kyr and +/- 2500 years at 25 kyr. The precision could be improved by taking larger samples. Samples up to 30 kyr can be dated at present. Calibration studies win have to be completed, before 14C dating can be fully implement. The time resolution depends both on the precision with which the 14C measurements can be made and on the time span over which close-off of the firn occurs. 'Me second may be an important factor in Antarctic cores from, low accumulation rates areas.
10Be, 26Al and 36Cl
The concentrations of 1OBe, 26Al and 36Cl in polar ice can be effected by changes in the production rate of these isotopes in the atmosphere, by changes in atmospheric transport processes and by changes in precipitation mechanisms. Studies to date have allowed the following conclusions to be drawn.
A. Because 1OBe, 26Al, and 36Cl have short atmospheric residence times, their ratios to their stable isotopes are not uniform in the atmosphere. Therefore a simple radioactive to stable isotope ratio cannot be used to given an age as it can for 14C. In addition to radioactive decay, climate, atmospheric chemistry and atmospheric circulation also effect the concentrations of these isotopes in polar precipitation. It was hoped that ratioing 36Cl to 1OBe might normalize these effects so that dating would be possible. It is now known that the 36C/10Be ratio varies by up to a factor of ten in samples of essentially the same age. Because aluminum and beryllium chemistries are more similar than chlorine and beryllium, it may be that 26A1/10Be ratio measurements will prove useful for dating ice. This ratio has a half-life of 1.4 x 106 y and so would be useful for dating very old ice.
B. Measurements at Milcent in central Greenland have shown that during the Maunder Minimum (1645-1715 A.D.), a period of quiet sun when there were almost no sunspots, the 1OBe concentration was substantially higher than during other periods. Recent measurements at Camp Century have shown a good correlation between the main short-term variations of the 1OBe record in ice and the 14C record in tree rings for the last 5000 years. This data strongly supports the explanation that solar modulation of the galactic cosmic ray flux causes these fluctuations.
C. Detailed studies at Milcent and at Dye 3 have revealed that the 11 year Schwabe sunspot cycle can be observed in the 1OBe data.
D. Holocene 14C dam from tree ring studies suggests there is a correlation between the geomagnetic: field and the atmospheric 14C concentrations. The dam support a picture in which there was a slow variation of the geomagnetic: field over the past 10,000 years, with a peak roughly 2000 years BP. 1OBe studies of Holocene ice, which show a different pattern than 14C, have led to a reexamination of this question. The isotope pattern can also be interpreted as indicating that production rates were higher during glacial time.
E. During the last glaciation and at the transition between the Glacial and Holocene there are large changes in the 1OBe concentration of polar ice that seem to be due mainly to changes of the precipitation rate. The quantitative form of this relation has not yet been worked out.
Future Work in Polar Cores
The major uncertainty in interpreting cosmogenic isotope levels in ice cores is the role of atmospheric transport and chemistry. If these processes can be understood, it will be possible to use cosmogenic isotope concentrations in dated cores to deduce the history of several important geophysical parameters such as solar activity and the strength of the geomagnetic field. One way to understand the effects of transport is to examine cores from several geographic areas. A Summit Core in Greenland and Antarctic cores as they become available will be important in this comparison. It will be important to study cosmogenic isotope concentrations in both Pleistocene and Holocene sections of these polar cores in order to unravel the various influence or to find which is dominant. The object of this work can be grouped into three broad categories.
1. Production rate effects: Studies under this category will require measurements of isotope concentrations during the following critical periods: Schwabe 11 year cycle (recent samples with one to two year time resolution); quiet sun periods (Maunder, Spoerer and Wolff minima and correlations with 14C wiggles in tree rings); geomagnetic field variations (multi-year averages throughout the Holocene). The ultimate goal of this work is to be able to extend studies of changes in cosmic ray flux caused by these effects to periods beyond the reach of 14C.
2. Climate: IOBe concentrations show an approximate inverse correlation with precipitation rate. Pit and shallow core studies combined with analysis of meteorological records can be used to investigate the current fallout pattern. Changes in isotopic concentration at the Pleistocene-Holocene transition and at periods of rapid climate change in the Pleistocene, as shown by rapid 180 changes should be examined and correlations of these measurements with those of chemical impurities which would also be affected by climate change looked for.
3. Dating: Work on meteorological effects may give some insight to the causes of the large variations observed in the 10Be/36C1 ratio. In addition, development of 26Al measurements in ice and studying the utility of 26Al/10Be (tl/2= 1.4x 106 y) as a dating pair for old ice should be undertaken. It is possible to date cores by matching the 1OBe variations which correspond to the Suess 14C wiggles. Such dating would require continuous measurement of IOBe with 5-10 years time resolution. Since 1OBe measurements require 1 kg samples this technique should be applied only if other dating techniques are not available or if core material becomes available in abundance. A second technique of relative dating relies on finding the large 1OBe spikes which have now been observed in a number of Antarctic cores. The spikes should be looked for in cores from both Greenland and the Antarctic.
Mid-latitude cores sample a different part of the global atmosphere and can provide a very useful comparison for determining the importance of circulation effects in controlling isotope concentrations. Only cold glaciers and ice sheets are suitable for this work (e.g., several sites can be found in Asia, South America, Europe and North America.)
81Kr (t l/2=2.1 x 105 y) could be very useful for dating old ice at the bottom of the ice sheet where model-based estimates are very uncertain. At the moment 50 kg of ice are required for a 30% precision. Work now in progress may reduce the sample size and increase the precision significantly.
Specific Sample Requirements
2-3 kg of ice required for an AMS dating. Sampling will concentrate on pre-
Holocene ice where annual layer counting is not longer reliable.
IOBe, 26Al, 36Cl:
The amount of ice required depends on the snow accumulation rate and is on the order 1 kg for 1OBe, 1.5 kg for 360, and 10 kg for 26Al The high
resolution comparison with the tree-ring 14C record requires sampling on a one to
two year interval for the last 1000 years. This means that about 1/3 of the core
would be required at Summit. It may be necessary to drill more than one core to intermediate depths to provide enough material for the cosmogenic isotope
studies. It is important, however, that the cosmogenic: isotope measurements be directly comparable with other chemical and isotopic data in the same core.
For ice older than 1000 years strip samples of total mass 1-2 kg will be required.
These samples would be chosen to coincide with regions of the core exhibiting changes in climate parameters in order to provide information about changes in
precipitation rate. In addition the IOBe spikes observed in the Antarctic cores would be searched for. Finding these in Greenland would allow direct correlation
between the northern and southern hemisphere records.
If the 26Al/1OBe ratio proves to be useful for dating, 10 to 20 kg of ice would be
required. It is probable that only one or two measurements would be required from the bottom of deep ice cores.
Currently 50 kg of ice is required for 81Kr dating, which has been used
successfully on Canadian aquifer samples in producing an age of 100 kyr. Several
samples from the bottom of the ice core could be obtained from the core itself and
possibly from side-track drilling. Improvements in enrichment of the isotope will
allow for smaller samples and better precision.
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