Variations of carbon remineralisation in the Southern Ocean illustrated by the BAxs proxy

We present water column profiles of excess particulate Ba (Baxs, 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 Baxs contents appear related to phytoplankton derived particles. Below, in the twilight zone, mesopelagic Baxs 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 Baxs 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 Baxs accumulations were larger, also show this latitudinal trend. In contrast, as observed also for deep particulate organic carbon fluxes, the flux of Baxs 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 234Th 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. © 2004 Elsevier Ltd. All rights reserved.