Mesopelagic organic carbon remineralization in the Kerguelen Plateau region tracked by biogenic particulate Ba

International audienceWe report on the distribution of excess, non-lithogenic particulate barium (Ba), a proxy for twilight-zone remineralization of organic matter (OM), in the Kerguelen Plateau area during the late austral summer of 2005. This study was part of a broader investigation focusing on natural iron fertilization controlling the characteristic and recurrent phytoplankton blooms in the region. The Plateau area contrasted strongly with the off-shelf HNLC area. Plateau stations had quite high excess of Baxs contents (up to 5000 pM) in surface waters (upper 150 m) that coincided with enhanced phytoplankton biomass. However, it had a lower mesopelagic (125–450 m) excess of biogenic Ba contents when compared to the off-shelf stations. Particulate excess Ba in the twilight zone (125–450 m depth layer) proved to be significantly set by the vertical distribution of bacterial activity, with higher particulate Ba contents in situations where significant bacterial activity extended deeper in the water column. These observations are in agreement with the role of excess biogenic particulate Ba as a proxy of twilight-zone OM remineralization. Using a transfer function established during earlier work, we calculated organic carbon remineralization rates and weighed these against other carbon fluxes (primary production, bacterial carbon demand), including carbon export from the 125-m horizon (234Th method). Off-shelf HNLC stations had a larger fraction of the organic carbon production exported and subsequently mineralized in the mesopelagic waters (125–450 m). Plateau stations had a smaller fraction of produced carbon exported, but this fraction appeared less prone to remineralization in the twilight zone and therefore had potential to reach the shallow (not, vert, similar500 m) seafloor or to be exported off-shelf. Differences in trophic structure and in composition of the diatom community would explain the variability in twilight-zone remineralization processes and in transfer efficiency through the mesopelagic zone in the Kerguelen Plateau area