The use of Thorium 234 to calibrate
drifting sediment traps

James W. Murray
University of Washington

Friday, June 26, 1998
12:00 noon—Pacific Forum

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The accuracy of drifting sediment traps has been suspect because of biases due to hydrodynamic effects. Our recent work during the Equatorial Pacific Process Study of the Joint Global Ocean Flux Program suggests that Thorium 234 (234Th) may be a viable tracer to use to correct observed trap fluxes for this bias. During this study we deployed drifting traps with very different size and shape on separate arrays at the same depth— Internal Rotating Sphere (IRS) and Particle Interceptor Traps (PIT) designs. Thorium 234 was measured on both traps and also in the water column. The water column values were used to calculate an expected 234Th flux.

The IRS trap had very low 234Th (and carbon) fluxes relative to the calculated fluxes. The fluxes were as much as 5 to 10 times too low, suggesting that these traps undercollected 234Th. The PIT traps appeared to over-collect 234Th (and carbon) by about a factor of two. When the carbon fluxes were corrected for the under/overtrapping, they were in excellent agreement for these two trap designs.

For some (but not all) conditions we observed strong correlations of the 234Th corrected particulate carbon export with the particulate silica flux and new production measured with 15N. At these times there appears to be tight coupling between C, N, and Si fluxes in the euphotic zone.

Traps are still the best approach for collecting sinking particles but new designs are needed that minimize or eliminate the effects of hydrodynamics. Neutrally buoyant traps should be designed and tested using the 234Th method.

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Last updated: December 19, 2000