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

Abstract

The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th 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°N to 12°S at 140°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 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/ 234Th ratio in trap samples. Assuming that the organic carbon/ 234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/ 234Th ratio for each station. The variability in the station-averaged POC/ 234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-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°N–10°S; 90°W–180°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.