Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean.

During the Bonus-GoodHope (BGH) expedition (Jan-Mar 2008) we studied the water column distribution of total ²³⁴Th and biogenic particulate Ba (Ba_xs) in the Atlantic sector of the Southern Ocean. The objective was to assess the export flux of particulate organic carbon (POC) from the surface to the mesopelagic twilight zone along a section between the Cape Basin and Weddell Gyre. Export production of POC was estimated from steady state and non steady state export fluxes of ²³⁴Th which were converted into POC ?uxes, using the POC/²³⁴Th ratio of large (>53 µm) suspended particles, collected via in-situ pumps. Deficits in ²³⁴Th activities were observed at all stations from the surface to the bottom of the mixed- layer. ²³⁴Th export fluxes from the upper 100m ranged from 496±57 dpm m^-2 d^-1 to 1195±120 dpm m^-2 d^-1 for the steady state model and from 149±18 dpm m^-2 d^-1 to 1217±146 dpm m^-2 d^-1 for the non steady state model calculated for a time window of 15 to 22 days preceding the timing of the present cruise. The POC/²³⁴Th_p ratio of large, potentially sinking particles (>53 µm), was observed to increase with latitude, from 1.9±0.2 µmol dpm^-1 and 1.7±0.3 µmol dpm^-1 in the Subtropical Zone (STZ) and Subantarctic Zone (SAZ), respectively, to 3.0±0.2 µmol dpm in the Polar Front Zone (PFZ), 4.8±1.9 µmol dpm^-1 at the Southern Antarctic Circumpolar Current Front (SACCF) to 4.1±1.7 µmol dpm in the northern Weddell Gyre, in line with an increasing contribution of larger cell diatoms. Steady state and non steady state POC export from the upper 100m ranged from 0.9±0.2mmolC m^-2 d to 5.1±2.1mmolC m^-2 d^-1 and from 0.3±0.0mmolC m^-2 d^-1 to 4.9±3.2mmolC m^-2 d^-1 , respectively. From the SAZ to the SACCF, non steady state POC export production represented only 15 to 54% of the steady state POC flux, suggesting that the intensity of export had decreased over time partly due to the fact that regenerated-production based communities of small-sized phytoplankton became predominant. In contrast, for the HNLC area south of the SACCF, we found an excellent agreement between the two modeling approaches indicating that surface POC export remained rather constant there. [ABSTRACT FROM AUTHOR]