A Neutrally Buoyant, Upper Ocean Sediment Trap

Abstract

The authors have designed and deployed a neutrally buoyant sediment trap (NBST) intended for use in the upper ocean. The aim was to minimize hydrodynamic flow interference by making a sediment trap that drifted freely with the ambient current. The principal design problem was to make the NBST descend to and stay near a prescribed depth. For a variety of reasons, the most success has been with NBSTs that were autoballasted by means of a microprocessor-controlled volume changer. Autoballasting NBSTs has demonstrated an ability to hold a prescribed depth to within 10 m. There have been two successful, concurrent deployments of NBSTs and conventional surface-tethered sediment traps (STSTs) at the Bermuda Atlantic Times Series site. During both periods the observed flow past the STSTs was low, about 0.05 m s?1, so that hydrodynamic effects on the STSTs would have been minimized. Comparisons of the trap results (described in a companion paper by Buesseler et al.) indicate that the total mass of collected material was generally similar in the two traps. Other variables, including the composition of the material and the fraction contributed by swimmers, were markedly different (swimmers are small animals that enter a trap intact and presumably alive). These are intriguing results but could not be conclusive since there is no absolute standard for such measurements. Future field work that includes comprehensive geochemical sampling will be required to learn which sediment trapping method yields the more useful observations.