Cosmogenic 32P and 33P in Rainwater and Plankton off Bermuda and Implications for Phosphorus Cycling
Measurements of 32P (half-life: 14.28 days) and 33P (half-life: 25.3 days), two cosmic ray-produced phosphorus radioisotopes, were made at the Bermuda site in rainwater and Zooplankton. The activity ratio 33P/32P was close to 1 in rainwater collected from precipitation which occurred during the spring of 1991. In contrast, the plankton collected offshore of the island with a 125-μm net had ratios ranging from 1.4 to 3. Assuming that isotopic fractionation is small and that precipitation is the only supply of the cosmogenic isotopes to the ocean, the increase in the 33P/32P ratio in plankton relative to rain can be interpreted as aging of the phosphorus in that pool, or in the pool from which it is derived. This is because of the differential radioactive decay rates of 32P and 33P. From this preliminary data set, we suggest that 33P/32P could be used as an in situ tracer to estimate rates of phosphorus export and the cycling among the various phosphorus pools in the upper ocean.