EXPORT FLUX OF PARTICULATE ORGANIC-CARBON FROM THE CENTRAL EQUATORIALPACIFIC DETERMINED USING A COMBINED DRIFTING TRAP TH-234 APPROACH

The export flux of particulate organic carbon from the euphotic zone i n the central equatorial Pacific was measured using an approach that u tilizes Th-234 and organic carbon analyses on water column and driftin g sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12 degrees N to 12 degrees S at 140 degrees W. Samples were collected du ring the Survey I (February-March 1992) and Survey II (August-Septembe r 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of Th-234 with the flux calculated fro m the deficiency of Th-234 relative to U-238 in the water column. Calc ulated Th-234 fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these Th-234 fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and cal culated Th-234 fluxes revealed evidence for overtrapping, especially i n the shallow traps (less than or equal to 100 m). Measured and calcul ated Th-234 fluxes agreed to within 50% for traps at 150-250 m. Good c orrelation was obtained between measured fluxes of organic carbon and Th-234 except for some shallow samples high in organic carbon, suggest ing that Th-234 was a good tracer for organic carbon. The flux of part iculate organic carbon (POC) was calculated as the product of the calc ulated flux of Th-234 times the organic carbon/Th-234 ratio in trap sa mples. Assuming that the organic carbon/Th-234 ratio in trap samples w as representative of sinking particles, we used an average Value for t he organic carbon/Th-234 ratio for each station. The variability in th e station-averaged POC/Th-234 ratio ranged from 10% to 30%. The POC fl uxes calculated using our combined Th-234-trap approach ranged from 1 to 6 mmol C m(-2) day(-1) during Survey I, and from 2 to 30 mmol C m(- 2) day(-1) during Survey II. The average uncertainty for the POC fluxe s was +/-60%. Primary and new production integrated to the depth of th e 0.1% light level varied by factors of 2-3 for Survey I and Survey II , respectively. The export of particulate organic carbon from the euph otic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10 deg rees N-10 degrees S; 90 degrees W-180 degrees E) of new (0.47 Gt C yea r(-1)) and particulate export (0.42 Gt C year(-1)) production were in good agreement, suggesting that, on an annual basis, significant expor t of DOC need not be invoked to balance new and export production in t his region. Copyright (C) 1996 Elsevier Science Ltd.