36Cl deposition rate reconstruction from bomb pulse until present: A study based on groundwater records
Résumé
The radioactive isotope 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 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 chro-nometer 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 36 Cl deposition curve, free from latitudinal and seasonal variations of the signal. Three old groundwater samples, with residence times comprised in the range À900 to À8000 BP, allow the estimation of a mean natural deposition of 49 at m À2 s À1 and are in good agreement with 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 À2 s À1 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 À2 s À1. 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 À2 s À1. This persistence of high deposition rate was classically attributed to biological and atmo-spherical recycling processes or underestimation of the natural atmospheric production of the 36 Cl. Additional source of 36 Cl production has been envisaged through the activation of chlorine radicals from stratospherical CFCs, leading to a 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.