Early spring mesopelagic carbon remineralization and transfer efficiency in the naturally iron-fertilized Kerguelen area

[摘要] We report on the zonal variability of mesopelagic particulate organic carbonremineralization and deep carbon transfer potential during the KerguelenOcean and Plateau compared Study 2 expedition (KEOPS 2;October–November 2011) in an area of the polar front supporting recurrentmassive blooms from natural Fe fertilization. Mesopelagic carbonremineralization (MR) was assessed using the excess, non-lithogenicparticulate barium (Ba_xs) inventories in mesopelagic waters andcompared with bacterial production (BP), surface primary production (PP) andexport production (EP). Results for this early season study are compared withtheresults obtained during a previous study (2005; KEOPS 1) for the same area ata later stage of the phytoplankton bloom. Our results reveal the patchinessof the seasonal advancement and of the establishment of remineralizationprocesses between the plateau (A3) and polar front sites during KEOPS 2. For theKerguelen plateau (A3 site) we observe a similar functioning of themesopelagic ecosystem during both seasons (spring and summer), with low andrather stable remineralization fluxes in the mesopelagic column(150–400 m). The shallow water column (~500 m), the lateraladvection, the zooplankton grazing pressure and the pulsed nature of the particulate organic carbon(POC)transfer at A3 seem to drive the extent of MR processes on the plateau. Fordeeper stations (>2000 m) located on the margin, inside a polar frontmeander, as well as in the vicinity of the polar front, east of Kerguelen,remineralization in the upper 400 m in general represents a larger part ofsurface carbon export. However, when considering the upper 800 m, in some cases,the entire flux of exported carbon is remineralized. In the polar frontmeander, where successive stations form a time series, two successive eventsof particle transfer were evidenced by remineralization rates: a firstmesopelagic and deep transfer from a past bloom before the cruise, and asecond transfer expanding at mesopelagic layers during the cruise. Regardingthe deep carbon transfer efficiency, it appeared that above the plateau (A3site) the mesopelagic remineralization was not a major barrier to thetransfer of organic matter to the seafloor (close to 500 m). There, theefficiency of carbon transfer to the bottom waters (>400 m) as assessedby PP, EP and MR fluxes comparisons reached up to 87% of the carbonexported from the upper 150 m. In contrast, at the deeper locations,mesopelagic remineralization clearly limited the transfer of carbon to depthsof >400 m. For sites at the margin of the plateau (station E-4W) and thepolar front (station F-L), mesopelagic remineralization even exceeded upper150 m export, resulting in a zero transfer efficiency to depths >800 m.In the polar front meander (time series), the capacity of the meander totransfer carbon to depth >800 m was highly variable (0 to 73%).The highest carbon transfer efficiencies in the meander are furthermorecoupled to intense and complete deep (>800 m) remineralization,resulting again in a near-zero, deep (>2000 m) carbon sequestrationefficiency there.

[学科分类] 地球化学与岩石