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

We conducted an experiment to test explicitly the accuracy of upper oc ean sediment trap fluxes using the particle-reactive radionuclide Th-2 34 (t1/2=24.1 days). Two independent VERTEX-style multitrap arrays wer e used for collection of sinking particles at 95 m and 97 m depths ove r a four-day period in May 1992 at the U.S. Joint Global Ocean Flux St udy Bermuda Atlantic Time-series Study (BATS) site. Samples for total Th-234 were collected every 8 m between the surface and 96 m and immed iately combined for analysis to obtain the vertically integrated activ ity of Th-234. We collected a total of 27 samples over the four-day pe riod. 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 sou rce 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 zer o (-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 duri ng low-flux periods and undercollect during high-flux periods, thus re cording a biased signal of the true particle flux.