A three dimensional time‐dependent approach to calibrating sediment trap fluxes

Abstract

We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle‐reactive radionuclide 234Th (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 234Th were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of 234Th. We collected a total of 27 samples over the four‐day period. The234Th samples were collected daily at each of the two traps and every other day on a 6 × 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 234Th flux from the watercolumn profiles was not significantly different from zero (−30 ± 140 disintegrations per minute/m2/d). The measured trap 234Th flux at both arrays was significantly higher (290 ± 15 dpm/m2/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.