Identification of bomb-produced chlorine-36 in mid-latitude glacial ice of North America

Abstract

In 1991, the U.S. Geological Survey collected a 160-meter (m) ice core from the Upper Fremont Glacier (43°07′N, 109°36′W) in the Wind River Mountain Range of Wyoming in the western United States [1]. In 1994–1995, ice from this core was processed at the National Ice Core Laboratory in Denver, Colorado, and analyzed for chlorine-36 (36Cl) by accelerator mass spectrometry at PRIME Laboratory, Purdue University. A tritium bomb peak identified in the work by [1] was used as a marker to estimate the depth of bomb-produced 36Cl. Tritium concentrations ranged from 0 tritium units (TU) for older ice to more than 300 TU at 29 m below the surface of the glacier, a depth that includes ice that was deposited as snow during nuclear-weapons tests through the early 1960's. Maximum 36Cl production during nuclear-weapons tests was in the late 1950's; therefore, the analyses were performed on ice from a depth of 29.8 to 32 m. Calculated flux for 36Cl in ice deposited in the late 1950's ranged from 1.2 ± 0.1 × 10−1 atoms/cm2 s for ice from 29.8 to 30.4 m, to 2.9 ± 0.1 × 10−1 atoms/cm2 s for ice from 31.5 to 32.0 m. Ice samples from a depth of 104.7 to 106.3 m were selected to represent pre-weapons tests 36Cl flux. Calculated flux for 36Cl in this deeper ice was 4.6 ± 0.8 × 10−3 atoms/cm2 s for ice from 104.7 to 105.5 m and 2.0 ± 0.2 × 10−2 atoms/cm2 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 36Cl in ice from a mid-latitude glacier in North America. It may now be possible to fully quantify the flux of 36Cl from nuclear-weapons tests archived in mid-latitude glacial ice and to gain a better understanding of the distribution of 36Cl and other cosmogenic nuclides.