Estimation of POC Export Fluxes Using 234Th/238U Disequilibria in the Amundsen Sea, Antarctica

Abstract

In order to understand the carbon cycle in the Amundsen Sea o f the Southem Ocean, the export t1uxes o f particulate organic carbon from the euphotic zone to deep water estimated using ²³⁴Th²³⁸U disequilibrium method. Seawaters in 14 water columns were collected during February and March 2012, and analyzed for total and dis- solved and organic carbon. Total ²³⁴Th activities in the water column showed deficiency and excess relative to those of ²³⁸U depending on the water depth. Deficiency of total in the euphotic zone showed mirror images both with chlorophyll-a and f1uorescence, and was consistent with the loss of nitrate, which indicated the effect ofbiological activity. In addition,deficiency of total ²³⁴Th from deep water was associated with the increase oftotal dissolvable Fe/Mn concentration. Excess total ²³⁴Th activity below the euphotic zone might be related to ²³⁴Th concentrated in this water depth. Mean export flux of ²³⁴Th estimated using the steady state model was 867 ± 246 dpm m^-2day^-1 Mean export flux of particulate organic carbon, which were estimated by the product of total ²³⁴Th flux and ratio of POC/²³⁴Th (7.08 ± 4.27 umol C dpm^-1)in the sinking particles, was 5.9 ± 3.9 mmol C m^-2day^-1. These fluxes were similar levels to those in the Weddell Sea during February and March 2008. Export ratios (ThE) relative to the primary production in the euphotic zone were in the range o f3-54% (av. 28%).