A SEDIMENT TRAP EXPERIMENT IN THE VEMA CHANNEL TO EVALUATE THE EFFECTOF HORIZONTAL PARTICLE FLUXES ON MEASURED VERTICAL FLUXES

Sediment traps are used to measure fluxes and collect samples for stud ies in biology, chemistry and geology, yet we have much to learn about factors that influence particle collection rates. Toward this end, we deployed cylindrical sediment traps on five current meter moorings ac ross the Vema Channel to field-test the effect of different horizontal particle fluxes on the collection rate of the traps-instruments inten ded for the collection of vertically settling particles. The asymmetri c flow of Antarctic Bottom Water through the Vema Channel created an e xcellent natural flume environment in which there were vertical and la teral gradients in the distribution of both horizontal velocity and pa rticle concentration and, therefore, the resulting horizontal flux. Ho rizontal effects were examined by comparing quantities of collected ma terial (apparent vertical fluxes) with the horizontal fluxes of partic les past each trap. We also looked for evidence of hydrodynamic biases by comparing and contrasting the composition of trap material based o n particle size and the concentration of Al, Si, Ca, Mg, Mn, C-org and CaCO3. Experimental inverted traps and traps with only side openings were deployed to test a hypothesis of how particles are collected in t raps. The vertical flux of surface-water particles should have been re latively uniform over the 45 km region of the mooring locations, so if horizontal transport contributed significantly to collection rates in traps, the calculated trap fluxes should be correlated positively wit h the horizontal flux. If the horizontal flow caused undertrapping, th ere should be a negative correlation with velocity or Reynolds number. The gross horizontal flux past different traps varied by a factor of 37, yet the quantity collected by the traps differed by only a factor of 1.4. The calculated horizontal fluxes were 2-4 orders of magnitude larger than the measured apparent vertical fluxes. Mean velocities pas t the traps ranged from 1-22 cm s(-1) (Reynolds numbers of 3,500-43,00 0 for these traps with a diameter of 30.5 cm and an aspect ratio of si milar to 3) and showed no statistically significant relationship to th e apparent vertical flux. We conclude that at current speeds measured in a very large portion of the world's oceans, vertical fluxes measure d with moored, cylindrical traps should exhibit little effect from hor izontal currents.