Chlorine-36 and the initial value problem

Chlorine-36 is a radionuclide with a half-life of 3.0110 super(5)a. Most super(36)Cl 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 super(-) as well as super(36)Cl have been important in numerous hydrologic studies. In almost all applications of super(36)Cl, a knowledge of the initial, or pre-anthropogenic, levels of super(36)Cl 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 super(36)Cl 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 super(36)Cl from vertical depth profiles in desert soils; (e) recover super(36)Cl from cores of glacial ice; and (f) calculate subsurface production of super(36)Cl 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 super(36)Cl 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 super(36)Cl.Original Abstract: Le chlore-36 est un radionucleide de periode 3.0110 super(5)a. Pour l'essentiel, le super(36)Cl dans l'hydrosphere provient des effets du rayonnement cosmique sur les gaz atmospheriques. De grandes quantites de super(36)Cl ont aussi ete produites au cours des essais thermonucleaires entre 1952 et 1958. Du fait que l'anion chlorure est la forme la plus courante de chlore rencontree dans l'hydrosphere et parce qu'il est extremement mobile en solution aqueuse, les analyses a la fois de Cl super(-) total et de super(36)Cl ont montre leur interet dans de nombreuses etudes hydrologiques. Dans presque toutes les applications du super(36)Cl, une connaissanc