Twilight zone organic carbon remineralization in the Polar Front Zone and Subantarctic Zone south of Tasmania S.H.M. Jacquet a,n,2 , F. Dehairs a , I. Dumont b , S. Becquevort b , A.-J. Cavagna a , D. Cardinal c,1 a Vrije Universiteit Brussel, Earth Systems Sciences Research Group and Analytical and Environmental Chemistry Department, Pleinlaan 2, Brussels, Belgium b Universite´ Libre de Bruxelles, Laboratoire d’Ecologie des Syst  emes Aquatiques, CP 221, Boulevard du Triomphe, Brussels, Belgium c Royal Museum for Central Africa, Geology Department, Tervuren, Belgium article info Available online 30 May 2011 Keywords: Biological pump Particulate biogenic barium Mesopelagic carbon mineralization Southern Ocean Carbon transfer efficiency abstract We report on the distribution of excess, non-lithogenic particulate barium (Ba xs ), a proxy for twilight zone remineralization of organic matter, in the Australian sector of the Southern Ocean during the mid- austral summer 2007. This study was part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Mesopelagic particulate excess Ba (Ba xs ) in the twilight zone (focus was on the 100–600 m depth layer) proved to be significantly controlled by the vertical distribution of bacterial activity, with higher Ba xs contents in situations where significant bacterial activity extended deeper in the water column. However, despite this covariation, the magnitude of the carbon fluxes as estimated from Ba xs and bacterial activity did not match well, with carbon demand based on bacterial activity largely exceeding organic carbon remineralization estimated from Ba xs . Possible reasons for this discrepancy are discussed. It appeared that the magnitude of the mesopelagic carbon remineralization flux obtained from Ba xs was realistic when weighted against primary, new and export productions. Our findings corroborate earlier results indicating that mesopelagic carbon remineralization in the 100–600 m depth layer increases from the Subantarctic Zone (SAZ) toward the Polar Front, and from spring to late summer. We observed furthermore that the iron-replete Subantarctic Zone east of the Tasman Plateau had a higher mesopelagic remineralization efficiency (on average 91 720% of the carbon exported from the upper 100 m was remineralized before reaching 600 m) compared to the Fe-limited SAZ area west of the plateau and the Polar Front Zone, where mesopelagic remineralization efficiencies ranged between 21 75% and 64 724%, respectively. SAZ-East and SAZ-West sites thus differed in their efficiency of carbon sequestration into the deep ( 4600 m) water column, with SAZ-West exceeding the sequestra- tion capacity of SAZ-East. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The efficiency of the biological pump in transferring carbon (C) to the deep sea with the potential for longer-term sequestration is strongly influenced by the processing of organic matter in the mesopelagic zone, also called twilight zone (  100–1000 m) (Buesseler et al., 2007). One of the different possible approaches to study mesopelagic organic matter breakdown consists of asses- sing the excess particulate biogenic barium Ba (Ba xs ) stocks in this depth zone (Cardinal et al., 2005; Jacquet et al., 2007a, 2008a, 2008b; Dehairs et al., 2008). In the twilight zone, particulate biogenic Ba is present mainly as micro-crystalline barite (BaSO 4 ) (Dehairs et al., 1980; Stroobants et al., 1991; Jacquet et al., 2007b). The link between barite and C remineralization resides in the fact that this mineral precipitates inside oversaturated micro-environ- ments (biogenic aggregates) during the process of bacterial (pro- karyotic) degradation of sinking particulate organic matter (Dehairs et al., 1980, 1992, 1997, 2000; Collier and Edmond, 1984; Bishop, 1988; Bertram and Cowen, 1997; Ganeshram et al., 2003). Once the aggregates have been remineralized, barites are released and spread over the mesopelagic layer. Carbon remineralization fluxes are assessed via a transfer function relating Ba xs with oxygen utilization rate (Shopova et al., 1995; Dehairs et al., 1997). Overall this earlier work highlights the fact that suspended barite in mesopelagic waters builds up over the growing season and reflects integrated past remineralization activity. Mesopelagic Ba xs therefore also informs on the efficiency of the system toward carbon sequestra- tion. While in this earlier work any direct comparison between Ba xs Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.05.029 n Corresponding author at: Laboratoire d’Oce ´ anographie Physique et Bioge ´ ochi- mique, COM, UMR 6535, Marseille, France. Tel.: þ33 4 91 82 93 37. E-mail address: stephanie.jacquet@univmed.fr (S.H.M. Jacquet). 1 Now at: Laboratoire d’Oce ´ nagraphie et du Climat: Expe ´ rimentations et Approches Nume ´ riques, Universite ´ Pierre & Marie Curie, Paris, France. 2 Now at: Laboratoire d’Oce ´ anographie Physique et Bioge ´ ochimique, COM, UMR 6535, Marseille, France. Deep-Sea Research II 58 (2011) 2222–2234