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

Abstract. As part of the GEOTRACES Bonus-GoodHope (BGH) expedition (January–March 2008) in the Atlantic sector of the Southern Ocean, particulate organic carbon (POC) export was examined from the surface to the mesopelagic twilight zone using water column distributions of total ²³⁴Th and biogenic particulate Ba (Ba_xs). Surface POC export production was estimated from steady state and non steady state modelling of ²³⁴Th fluxes, which were converted into POC fluxes, using the POC/²³⁴Th ratio of large, potentially sinking particles (> 53 μm) collected via in situ pumps. Deficits in ²³⁴Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding ²³⁴Th export fluxes from the upper 100 m of 496 ± 214 dpm m⁻² d⁻¹ to 1195 ± 158 dpm m⁻² d⁻¹ for the steady state model and of 149 ±517 dpm m⁻² d⁻¹ to 1217 ± 231 dpm m⁻² d⁻¹ for the non steady state model. Using the POC/²³⁴Th_p ratio of sinking particles (ratios varied from 1.7 ± 0.2 μmol dpm⁻¹ to 4.8 ± 1.9 μmol dpm⁻¹) POC export production at 100 m was calculated to range between 0.9 ± 0.4 and 5.1 ± 2.1 mmol C m⁻² d⁻¹,assuming steady state and between 0.3 ± 0.9 m⁻² d⁻¹ and 4.9 ± 3.3 mmol C m⁻² d⁻¹, assuming non steady state. From the comparison of both approaches, it appears that during late summer export decreased by 56 to 16% for the area between the sub-Antarctic zone and the southern Antarctic Circumpolar Current Front (SACCF), whereas it remained rather constant over time in the HNLC area south of the SACCF. POC export represented only 6 to 54% of new production, indicating that export efficiency was, in general, low, except in the vicinity of the SACCF, where export represented 56% of new production. Attenuation of the POC sinking flux in the upper mesopelagic waters (100–600 m depth interval) was evidenced both, from excess ²³⁴Th activities and from particulate biogenic Ba (Ba_xs) accumulation. Excess ²³⁴Th activities, reflected by ²³⁴Th/²³⁸U ratios as large as 1.21 ± 0.05, are attributed to remineralisation/disaggregation of ²³⁴Th-bearing particles. The accumulation of excess ²³⁴Th in the 100–600 m depth interval ranged from 458 ± 633 dpm m⁻² d⁻¹ to 3068 ± 897 dpm m⁻² d⁻¹, assuming steady state. Using the POC/²³⁴Th_p ratio of sinking particles (> 53 μm), this ²³⁴Th accumulation flux was converted into a POC remineralisation flux which ranged between 0.9 ± 1.2 mmol C m⁻² d⁻¹ and 9.2 ± 2.9 mmol C m⁻² d⁻¹. Mesopelagic particulate biogenic Ba has been reported to reflect bacterial degradation of organic matter and to be related to oxygen consumption and bacterial carbon respiration. We observed that the highest Ba_xs contents (reaching up to > 1000 pM), in general, occurred between 200 and 400 m. Depth-weighted average mesopelagic Ba_xs (meso-Ba_xs) values were converted into respired C fluxes, which ranged between 0.23 and 6.4 mmol C m⁻² d⁻¹, in good agreement with ²³⁴Th-based remineralisation fluxes. A major outcome from this study is the observed significant positive correlation between POC remineralisation as estimated from meso-Ba_xs contents and from ²³⁴Th excess (R² = 0.73; excluding 2 outliers). Remineralisation of POC in the twilight zone was particularly efficient relative to POC export resulting in negligible bathypelagic (> 600 m) POC export fluxes in the sub-Antarctic zone, the Polar Front zone and the northern Weddell Gyre, while the subtropical zone as well as the vicinity of the SACCF had significant deep POC fluxes.