super(36)Cl deposition rate reconstruction from bomb pulse until present: A study based on groundwater records

• Published in: Applied geochemistry Vol. 50; pp. 199 - 208
• Main Authors: Rebeix, Romain, La Salle, Corinne Le Gal, Mayer, Adriano, Finkel, Robert, Claude, Christelle, Sueltenfus, Juergen, Simler, Roland
• Format: Journal Article
• Language: English
• Published: 01-11-2014
• Subjects: Activation; Atmospherics; Attenuation; Deposition; Fallout; Groundwater; Mathematical analysis; Measuring instruments
• ISSN: 0883-2927
• DOI: 10.1016/j.apgeochem.2014.05.005

The radioactive isotope super(36)Cl, with a half-life of 301 ka, is a valuable chronometer for estimation of groundwater residence time up to 2 millions of years. Aerial thermonuclear fission bomb tests, performed during the late 1950s, injected a massive amount of this isotope into the atmosphere, which exceeded the natural fallout signal. Since this bomb pulse, atmospheric super(36)Cl deposition tends to return to natural fallout rate. The monitoring of this attenuation can provide a good opportunity to extend the use of this chronometer to shorter time spans. Venice's lagoon alimentation zone shows groundwaters with residence times distributed over last fifty years. This permits the estimation of a continuous super(36)Cl deposition curve, free from latitudinal and seasonal variations of the signal. Three old groundwater samples, with residence times comprised in the range -900 to -8000 BP, allow the estimation of a mean natural deposition of 49 at m super(-2) s super(-1) and are in good agreement with super(36)Cl fallout observed for the last 40,000 years by (Plummer et al., 1997). For the bomb pulse period, a fallout of 5300 at m super(-2) s super(-1) was calculated. This was followed by a strong attenuation period, taking place until the 1980s, during which the fallout reached values ranging between 167 and 354 at m super(-2) s super(-1). The attenuation reached then a plateau: it experienced a slower lowering until the actual deposition, with fallout values calculated between 124 and 252 at m super(-2) s super(-1). This persistence of high deposition rate was classically attributed to biological and atmospherical recycling processes or underestimation of the natural atmospheric production of the super(36)Cl. Additional source of super(36)Cl production has been envisaged through the activation of chlorine radicals from stratospherical CFCs, leading to a super(36)Cl production rate comparable with that of Ar spallation from the first approximation. Lastly, the latitudinal factor of the attenuation of the fallout rate is discussed and the impact of the jet streams is proposed as an explanation for the discrepancies in the attenuation rate.