Wet deposition fluxes of cosmogenic32P and33P and variations in the33P/32P ratios at Bermuda

A study was undertaken to quantify the supply of cosmogenic32P (halflife 14.3 days) and33P (halflife 25.3 days) on the island of Bermuda in order to use the natural radioisotopes as tracers of the phosphorus biodynamic in the ocean. The activities of32P and33P were measured in individual precipitation events over a 1 yr period in 1991–1992 at the Bermuda Biological Station for Research (32° 30′N, 64° 40′W). For the first time, wet deposition rates were determined at a marine site and averaged 8.6 ± 1.5 × 10−2 dpm cm−2 yr−1 for32P and 8.2 ± 2.0 × 10−2 dpm cm−2 yr−1 for33P. The33P/32P activity ratios in individual precipitation events were found to be remarkably constant, ranging from 0.66 to 1.22. The monthly means of the activity ratio in rainwater showed variations about the annual mean of 0.96. Higher ratios were observed during the combined period March–April 1991 and January–March 1992, contrasting with the lower ratios found during July–November 1991. The individual rain samples of the two contrasting periods belonged to two different populations with mean activity ratios of 1.02 and 0.89 for the high and low ratio period, respectively. tivity ratios in individual precipitation events allowed determinations of the mean irradiation period based on a simple non-steady-state model. The deduced irradiation periods averaged 27 days in the July–November 1991 period and 50 days in the combined March–June 1991 and January–March 1992 period, assuming a well-mixed trophosphere, efficient scavenging of the aerosols and a33P/32P activity ratio of 0.7 for the ratio of the tropospheric production rates. The annual average irradiation time was estimated at 40 days, consistent with the calculation of the aerosol residence time in the upper troposphere. Furthermore, the tropospheric production rates of32P and33P were determined from the ratios, wet deposition rates, and aerosol residence time, and were consistent within the range of previously published estimates.