CARBON AND NITROGEN EXPORT DURING THE JGOFS NORTH-ATLANTIC BLOOM EXPERIMENT ESTIMATED FROM TH-234U-238 DISEQUILIBRIA

The disequilibrium between the particle-reactive tracer Th-234 (t1/2 = 24.1 days) and its soluble parent, U-238, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April-30 May 1989) at approximately 47-degrees-N, 20-degrees-W. Four profiles of dissolved and particulate 234 Th in the upper 300 m and a non-steady state box model were used to quantify dissolved Th-234 uptake and particle export rates. The highest export fluxes occurred during the first half of May. From POC/Th-234 and PON/Th-234 ratios, particulate organic C and N fluxes were calculated. Results were 5-41 mmol C m-2 day-1 and 0.9-6.5 mmol N m-2 day-1 from the 0-35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.