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

Abstract

Abstract. We report on the zonal variability of mesopelagic particulate organic carbon remineralization and deep carbon transfer potential during the Kerguelen Ocean and Plateau compared Study 2 expedition (KEOPS 2; OctoberNovember 2011) in an area of the polar front supporting recurrent massive blooms from natural Fe fertilization. Mesopelagic carbon remineralization (MR) was assessed using the excess, non-lithogenic particulate barium (Ba xs ) inventories in mesopelagic waters and compared with bacterial production (BP), surface primary production (PP) and export production (EP). Results for this early season study are compared with the results obtained during a previous study (2005; KEOPS 1) for the same area at a later stage of the phytoplankton bloom. Our results reveal the patchiness of the seasonal advancement and of the establishment of remineralization processes between the plateau (A3) and polar front sites during KEOPS 2. For the Kerguelen plateau (A3 site) we observe a similar functioning of the mesopelagic ecosystem during both seasons (spring and summer), with low and rather stable remineralization fluxes in the mesopelagic column (150-400 m). The shallow water column (∼ 500 m), the lateral advection, 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. For deeper stations (> 2000 m) located on the margin, inside a polar front meander, 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 of surface carbon export. However, when considering the upper 800 m, in some cases, the entire flux of exported carbon is remineralized. In the polar front meander, where successive stations form a time series, two successive events of particle transfer were evidenced by remineralization rates: a first mesopelagic and deep transfer from a past bloom before the cruise, and a second transfer expanding at mesopelagic layers during the cruise. Regarding the deep carbon transfer efficiency, it appeared that above the plateau (A3 site) the mesopelagic remineralization was not a major barrier to the transfer of organic matter to the seafloor (close to 500 m). There, the efficiency of carbon transfer to the bottom waters (> 400 m) as assessed by PP, EP and MR fluxes comparisons reached up to 87 % of the carbon exported from the upper 150 m. In contrast, at the deeper locations, mesopelagic remineralization clearly limited the transfer of carbon to depths of > 400 m. For sites at the margin of the plateau (station E-4W) and the polar front (station F-L), mesopelagic remineralization even exceeded Published by Copernicus Publications on behalf of the European Geosciences Union. S. H. M. Jacquet et al.: Early season mesopelagic carbon remineralizationupper 150 m export, resulting in a zero transfer efficiency to depths > 800 m. In the polar front meander (time series), the capacity of the meander to transfer carbon to depth > 800 ...

Untangling biogeochemical processes from the impact of ocean circulation: First insight on the Mediterranean dissolved barium dynamics

“…The upper mesopelagic depletion horizon observed over most of the sections would indicate that regeneration of organic material occurs at these depths as commonly observed in most oceanic basins [Dehairs et al, 1997[Dehairs et al, , 2008Jacquet et al, 2007bJacquet et al, , 2011Jacquet et al, , 2015. The upper mesopelagic depletion horizon observed over most of the sections would indicate that regeneration of organic material occurs at these depths as commonly observed in most oceanic basins [Dehairs et al, 1997[Dehairs et al, , 2008Jacquet et al, 2007bJacquet et al, , 2011Jacquet et al, , 2015.…”

“…Among these biogeochemical tracers, particulate biogenic barium (P_Ba barite-BaSO 4 ) is one of the few proxies used to estimate organic carbon remineralization in the mesopelagic interior (100-1000 m) [Dehairs et al, 1980[Dehairs et al, , 1997[Dehairs et al, , 2008Cardinal et al, 2001Cardinal et al, , 2005Jacquet et al, 2005Jacquet et al, , 2007aJacquet et al, , 2008aJacquet et al, , 2008bJacquet et al, , 2011Jacquet et al, , 2015Stroobants et al, 1991]. Studies have shown that in a global ocean undersaturated with respect to barite [Monnin et al, 1999;Monnin and Cividini, 2006], P_Ba barite mainly precipitates in the mesopelagic zone inside oversaturated biogenic microenvironments during organic matter degradation by marine prokaryotes [Dehairs et al, 1980[Dehairs et al, , 1997Collier and Edmond, 1984;Bertram and Cowen, 1997;Bishop, 1988;Ganeshram et al, 2003].…”

Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes

“…5). The sampling being done during the early stages of bloom development, it is likely that an accumulation of biomass in surface waters could limit the export (Jacquet et al, 2015;Planchon et al, 2015). Indeed, the lowest ThE PN and ThE BSi were recorded at A3-2, E-4W and F-L stations, confirming that only a small fraction of the produced biomass was exported.…”

“…The organic C content of sinking particulate matter is also impacted by remineralization (Jacquet et al, 2015). Using BSi:POC and BSi:PN ratios, the POC and PN degradation relative to BSi is reflected in the increase of ratios with depth (Fig.…”

Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean

“…Low export efficiency suggests that the products of primary production are mainly recycled within the mixed layer by an efficient microbial loop (Sarthou et al, 2008;Obernosterer et al, 2008;Christaki et al, 2014;Malits et al, 2014). The carbon sequestration efficiency induced by iron fertilization remains proportionally relatively low (Jacquet et al, , 2015.…”