Chlorine-36 and the initial value problem |
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| Authors: | Stanley N Davis DeWayne Cecil Marek Zreda Pankaj Sharma |
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| Institution: | (1) Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, USA Fax: +1-520-621-1422 e-mail: sdavis@ccit.arizona.edu, US;(2) Water Resources Division, U.S. Geological Survey, INEL, P.O. Box 2230, Idaho Falls, ID 83403, USA, US;(3) Department of Physics, PRIME Lab, Purdue University, West Lafayette, IN 47907, USA, US |
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| Abstract: | Chlorine-36 is a radionuclide with a half-life of 3.01×105a. Most 36Cl in the hydrosphere originates from cosmic radiation interacting with atmospheric gases. Large amounts were also produced
by testing thermonuclear devices during 1952–58. Because the monovalent anion, chloride, is the most common form of chlorine
found in the hydrosphere and because it is extremely mobile in aqueous systems, analyses of both total Cl– as well as 36Cl have been important in numerous hydrologic studies. In almost all applications of 36Cl, a knowledge of the initial, or pre-anthropogenic, levels of 36Cl is useful, as well as essential in some cases. Standard approaches to the determination of initial values have been to:
(a) calculate the theoretical cosmogenic production and fallout, which varies according to latitude; (b) measure 36Cl in present-day precipitation and assume that anthropogenic components can be neglected; (c) assume that shallow groundwater
retains a record of the initial concentration; (d) extract 36Cl from vertical depth profiles in desert soils; (e) recover 36Cl from cores of glacial ice; and (f) calculate subsurface production of 36Cl for water that has been isolated from the atmosphere for more than one million years. The initial value from soil profiles
and ice cores is taken as the value that occurs directly below the depth of the easily defined bomb peak. All six methods
have serious weaknesses. Complicating factors include 36Cl concentrations not related to cosmogenic sources, changes in cosmogenic production with time, mixed sources of chloride
in groundwater, melting and refreezing of water in glaciers, and seasonal groundwater recharge that does not contain average
year-long concentrations of 36Cl.
Received, December 1996 · Revised, August 1997 · Accepted, August 1997 |
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| Keywords: | isotopes USA groundwater age |
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