Measurements of 36CI in Polar Ice

Abstract

We are measuring 36Cl in the upper part of the 1966 Camp Century deep core, using the tandem-accelerator mass-spectrometry facility at the University of Rochester. There are two primary motivations for measuring 36Cl in ice. The first is to look for correlations between 36Cl concentrations and changes in solar activity. Previous studies of 10Be and 14C (Stuiver and Quay 1980, Raisbeck and others 1981, Beer and others 1983) have demonstrated that the production of these radio-isotopes increases during periods of low solar activity. When the sun is active, the magnetic field induced by the solar wind deflects galactic cosmic rays from the inner solar system and thereby lowers the production of radio-isotopes. Our study of 36Cl in ice dating back to A.D. 1550, from Camp Century, Greenland, shows that there was an increase in 36Cl production during the Maunder Minimum (1650–1715). However, climatic variations introduce fluctuations of a factor of 2 or more over short periods and the Maunder Minimum peak is only readily apparent when the data are smoothed mathematically. The second motivation for measuring 36Cl in ice is to test the possibility that the ratio of 36Cl to 10Be will provide a means for dating ice over 50 000 years old (Nishiizumi and others 1983). The dependence on both solar activity and climatic fluctuations mentioned above precludes dating with a single radio-isotope. If, however, the production rates of 36Cl and 10Be are each proportional to the cosmic-ray flux and are transported identically to the ice sheet, these fluctuations would cancel and the ratio of 36Cl to 10Be would provide a radiometric means of dating very old ice. The half-lives for 36Cl (3.0 × 105 years) and 10Be (1.6 × 106 years), combine to give a “half-life” for the 36Cl/10Be ratio of 3.7 × 105 years. Unfortunately, preliminary results from Camp Century, Greenland, show that it will almost certainly not be possible to use the 10Be/36Cl ratio for dating. 10Be and 36Cl have been measured in more than 20 samples; the 10Be/36Cl ratio varied, without any obvious pattern, from below 5 to over 15. Additional evidence comes from Antarctica. We found that the 10Be/36Cl ratio changed significantly within a short depth interval within a single block of ice and that the calculated age of ice based on 10Be/36Cl differed from the age of an embedded meteorite, ALHA82102 (personal communication from K. Nishiizumi). A variation in the 10Be/36Cl ratio may be caused by climatic effects: for example, changes in air circulation between the stratosphere and the troposphere in the polar region, in combination with chemical processes that affect aerosol chlorine and beryllium differently. Also, the 10Be/36Cl ratio is smaller than expected, indicating that there may be additional sources of 36Cl. If there is a source of 36Cl that varies independently from the galactic cosmic-ray flux, this would result in variations in the isotope ratio. Discovery of the cause of the variations may result in a new tool for understanding paleoclimate.