Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap Th-234 approach

The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes Th-234 and organic carbon analyses on water column and drifting 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 during the Survey I (February-March 1992) and Survey II (August-September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of Th-234 with the flux calculated from the deficiency of Th-234 relative to U-238 in the water column. Calculated 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 calculated Th-234 fluxes revealed evidence for overtrapping, especially in the shallow traps (less than or equal to 100 m). Measured and calculated Th-234 fluxes agreed to within 50% for traps at 150-250 m. Good correlation was obtained between measured fluxes of organic carbon and Th-234 except for some shallow samples high in organic carbon, suggesting that Th-234 was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of Th-234 times the organic carbon/Th-234 ratio in trap samples. Assuming that the organic carbon/Th-234 ratio in trap samples was representative of sinking particles, we used an average Value for the organic carbon/Th-234 ratio for each station. The variability in the station-averaged POC/Th-234 ratio ranged from 10% to 30%. The POC fluxes 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 fluxes was +/-60%. Primary and new production integrated to the depth of the 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 euphotic 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 degrees N-10 degrees S; 90 degrees W-180 degrees E) of new (0.47 Gt C year(-1)) and particulate export (0.42 Gt C year(-1)) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region. Copyright (C) 1996 Elsevier Science Ltd.