IDENTIFICATION OF BOMB-PRODUCED CL-36 IN MIDLATITUDE GLACIAL ICE OF NORTH-AMERICA

In 1991, the U.S. Geological Survey collected a 160-meter (m) ice core from the Upper Fremont Glacier (43 degrees 07'N, 109 degrees 36'W) in the Wind River Mountain Range of Wyoming in the western United States [1]. In 1994-95, ice from this core was processed at the National Ice Core Laboratory in Denver, Colorado, and analyzed for chlorine-36 (Cl -36) by accelerator mass spectrometry at PRIME Laboratory, Purdue Univ ersity. A tritium bomb peak identified in the work by [1] was used as a marker to estimate the depth of bomb-produced Cl-36. Tritium concent rations ranged from 0 tritium units (TU) for older ice to more than 30 0 TU at 29 m below the surface of the glacier, a depth that includes i ce that was deposited as snow during nuclear-weapons tests through the early 1960's. Maximum Cl-36 production during nuclear-weapons tests w as in the late 1950's; therefore, the analyses were performed on ice f rom a depth of 29.8 to 32 m. Calculated flux for Cl-36 in ice deposite d in the late 1950's ranged from 1.2 +/- 0.1 x 10(-1) atoms/cm(2) s fo r ice from 29.8 to 30.4 m, to 2.9 +/- 0.1 x 10(-1) atoms/cm(2) s for i ce from 31.5 to 32.0 m. Ice samples from a depth of 104.7 to 106.3 m w ere selected to represent pre-weapons tests Cl-36 flux. Calculated flu x for Cl-36. in this deeper ice was 4.6 +/- 0.8 x 10(-3) atoms/cm(2) s for ice from 104.7 to 105.5 m and 2.0 +/- 0.2 x 10(-2) atoms/cm(2) s for ice from 105.5 to 106.3 m. These flux calculations from the Upper Fremont Glacier analyses are the first for bomb-produced Cl-36 in ice from a mid-latitude glacier in North America. It may now be possible t o fully quantify the flux of Cl-36 from nuclear-weapons tests archived in mid-latitude glacial ice and to gain a better understanding of the distribution of Cl-36 and other cosmogenic nuclides.