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

• Published in: Biogeosciences
• Main Authors: F. Planchon, A.-J. Cavagna, D. Cardinal, L. André, F. Dehairs
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2013
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Antarctic; Southern Ocean; Antarc*
• Online Access: https://doi.org/10.5194/bg-10-803-2013; https://doaj.org/article/d1a11737917f400fabb0c84865fb3020

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 234 Th and biogenic particulate Ba (Ba xs ). Surface POC export production was estimated from steady state and non steady state modelling of 234 Th fluxes, which were converted into POC fluxes, using the POC/ 234 Th ratio of large, potentially sinking particles (> 53 μm) collected via in situ pumps. Deficits in 234 Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding 234 Th 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/ 234 Th 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 234 Th activities and from particulate biogenic Ba (Ba xs ) accumulation. Excess 234 Th activities, reflected by 234 Th/ 238 U ratios as large as 1.21 ± 0.05, are attributed to ...