A 3-DIMENSIONAL TIME-DEPENDENT APPROACH TO CALIBRATING SEDIMENT TRAP FLUXES

We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle-reactive radionuclide Th-234 (t1/2=24.1 days). Two independent VERTEX-style multitrap arrays were used for collection of sinking particles at 95 m and 97 m depths over a four-day period in May 1992 at the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time-series Study (BATS) site. Samples for total Th-234 were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of Th-234. We collected a total of 27 samples over the four-day period. The Th-234 samples were collected daily at each of the two traps and every other day on a 6 x 6 km grid to characterize the entire source region for particles collected in the traps. In situ flow sensors at one trap array indicated low horizontal shear at the trap mouth (5-10 cm/s) compared to normal values at BATS. The predicted Th-234 flux from the watercolumn profiles was not significantly different from zero (-30 +/- 140 disintegrations per minute/m2/d). The measured trap Th-234 flux at both arrays was significantly higher (290 +/- 15 dpm/m/d). We hypothesize that upper ocean traps at Bermuda may overcollect during low-flux periods and undercollect during high-flux periods, thus recording a biased signal of the true particle flux.