Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap- super(2) super(3) super(4) th approach

The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes super(2) super(3) super(4) Th 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 super(2) super(3) super(4) Th with the flux calculated from the deficiency of super(2) super(3) super(4) Th relative to super(2) super(3) super(8) U in the water column. Calculated super(2) super(3) super(4) Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these super(2) super(3) super(4) Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated super(2) super(3) super(4) Th fluxes revealed evidence for overtrapping, especially in the shallow traps ( less than or equal to 100 m). Measured and calculated super(2) super(3) super(4) Th fluxes agreed to within 50% for traps at 150-250 m. Good correlation was obtained between measured fluxes of organic carbon and super(2) super(3) super(4) Th except for some shallow samples high in organic carbon, suggesting that super(2) super(3) super(4) Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of super(2) super(3) super(4) Th times the organic carbon/ super(2) super(3) super(4) Th ratio in trap samples. Assuming that the organic carbon/ super(2) super(3) super(4) Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/ super(2) super(3) super(4) Th ratio for each station. The variability in the station-averaged POC/ super(2) super(3) super(4) Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined super(2) super(3) super(4) Th-trap approach ranged from 1 to 6 mmol C m super(-) super(2) day super(-) super(1) during Survey I, and from 2 to 30 mmol C m super(-) super(2) day super(-) super(1) during Survey II. The average uncertainty for the POC fluxes was plus or minus 60%. Primary and new production integrated to the depth of the 0.1% light level varied by f