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

Abstract

Abstract. During the Bonus-GoodHope (BGH) expedition (Jan–Mar 2008) we studied the water column distribution of total 234Th and biogenic particulate Ba (Baxs) 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 fluxes, 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 100 m 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/234Thp 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−1 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−1 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 100 m ranged from 0.9 ± 0.2 mmolC m−2 d−1 to 5.1 ± 2.1 mmolC m−2 d−1 and from 0.3 ± 0.0 mmolC m−2 d−1 to 4.9 ± 3.2 mmolC 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. Estimated POC export represented between 6 to 54 % of the potential export as represented by new production, indicating that export efficiency was particularly low throughout the studied area, except close to the SACCF. Below the export layer, in the mesopelagic zone, 234Th activities generally reached equilibrium with 238U, but sometimes were in large excess relative to 238U (234Th/238U ratio>1.1), reflecting intense remineralisation/disaggregation of 234Th-bearing particles. The accumulation of excess 234Th in the 100–600 m depth interval ranged from 458 ± 55 dpm m−2 d−1 to 3068 ± 368 dpm m−2 d−1. Using POC to 234Th ratio of sinking particles, we converted this 234Th flux into a POC remineralisation flux, which ranged between 0.9 to 9.2 mmolC m−2 d−1. Mesopelagic remineralisation was also evidenced by Baxs inventories which are related to bacterial degradation of sinking material and offer a means to quantify the flux of respired C. Highest biogenic particulate Ba (Baxs) contents were generally observed in the 200–400 m depth interval with values reaching up to >1000 pM in the northern PFZ. Depth weighted average mesopelagic Baxs (meso-Baxs) was high in the PFZ and low in the northernmost (STZ-SAZ) and the southernmost (SACCF-AZ-WG) parts of the BGH section; conversion into respired C flux yielded a range of –0.23 to 6.4 mmolC m−2 d−1. Excluding two outliers, we found a significant positive correlation for mesopelagic waters between POC remineralisation estimated from meso-Baxs and from 234Th excess (R2 = 0.73). Our results indicate that POC export production in this area of the Southern Ocean was strongly attenuated in the mesopelagic waters due to remineralisation, a process which thus appears to strongly impact on longer term bathypelagic zone sequestration of POC.