Deep-Sea Research I 52 (2005) 355–370 Variations of carbon remineralisation in the Southern Ocean illustrated by the Ba xs proxy Damien Cardinal a,Ã , Nicolas Savoye b , Thomas W. Trull c , Luc Andre´ a , Elzbieta E. Kopczynska d , Frank Dehairs b a Department of Geology and Mineralogy, Section of Mineralogy and Petrography, Muse´e Royal de l’Afrique Centrale, Leuvensesteenweg 13, Tervuren 3080, Belgium b Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Brussels, Belgium c CSIRO Marine Research, Antarctic Climate and Ecosystem Cooperative Research Centre, University of Tasmania, Hobart, Australia d Department of Antarctic Biology, Polish Academy of Science, Warsaw, Poland Received 1 December 2003; received in revised form 22 June 2004; accepted 21 October 2004 Available online 21 December 2004 Abstract We present water column profiles of excess particulate Ba (Ba xs , an estimate of biogenic Ba from total particulate Ba after small corrections for lithogenic Ba) along a transect in the Australian sector of the Southern Ocean from the Subantarctic Zone (SAZ) to the Sea Ice Zone during spring (October–December 2001). Surface water Ba xs contents appear related to phytoplankton derived particles. Below, in the twilight zone, mesopelagic Ba xs records the changes in plankton biomass from the mixed layer over a time scale of a few weeks and confirms its usefulness as an indicator of the carbon remineralisation process. In comparison to the SAZ, the mesopelagic Ba xs accumulation is larger and begins at shallower depths south of the Polar Front Zone (PFZ), in the Antarctic Zone (AZ), where diatoms are the dominant component of the phytoplankton community. Summer results from 1998, when mesopelagic Ba xs accumulations were larger, also show this latitudinal trend. In contrast, as observed also for deep particulate organic carbon fluxes, the flux of Ba xs to moored deep sea sediment traps was larger in the nano-phytoplankton dominated SAZ than the diatom dominated PFZ. Overall, the results suggest relatively high particulate carbon export to the deep sea in the absence of strong remineralisation in the SAZ, and relatively low export to the deep sea in the presence of strong remineralisation further south. Mesopelagic carbon remineralisation is higher in summer than in spring as also observed on deep sediment traps carbon fluxes. Our findings are supported by 234 Th and N-uptake proxies from the same transect. This study expands the utility of Ba as an indicator of biogeochemical processes in the twilight zone and supports its usability as a paleoceanographic proxy for deep C export. r 2004 Elsevier Ltd. All rights reserved. Keywords: Barium; Carbon remineralisation; Carbon export; Twilight zone; Particles; Phytoplankton; Southern Ocean ARTICLE IN PRESS www.elsevier.com/locate/dsr 0967-0637/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.10.002 Ã Corresponding author. Tel.: +3227695439; fax: +3227695432. E-mail address: damien.cardinal@africamuseum.be (D. Cardinal).