Radium isotopes to investigate the water mass pathways on the Kerguelen Plateau (Southern Ocean)

Natural iron fertilization promotes the phytoplankton bloom that takes place on the Kerguelen Plateau in the Indian sector of the Southern Ocean. We measured 228Ra (T1/2=5.75 yr) and 226Ra (T1/2=1602 yr) in waters above the Kerguelen Plateau in order to provide information on the water-mass pathways, which in turn could help elucidate the mechanisms controlling iron fertilization in that area. 228Ra activities are extremely low in this region, being in most cases <0.15 dpm/100 kg (<2.5×10−18 g/kg). Station A3 (520 m depth), located on the plateau in the middle of the bloom area, also displays such low values but with higher 228Ra activities (and higher 228Ra/226Ra ratios) in the upper 150 m. Such a pattern suggests the presence of a water mass that recently interacted with sediments and therefore has been advected onto the Kerguelen Plateau. Elevated 228Ra activities were found in shallow waters in the vicinity of Heard Island, south of the Kerguelen Plateau. Contact of the water masses with the shallow lithogenic sediments likely explains these high 228Ra activities. When combined with physical observations, these results suggest that the water mass advected onto the plateau originates from the south of the Kerguelen Plateau. This northward advection might represent a supply of dissolved and/or particulate Fe for the observed phytoplankton bloom.Using the 228Ra profile obtained at station A3, we could estimate the vertical eddy diffusivity (Kz) on the Kerguelen Plateau. Our Kz estimate (1.5×10−4±1.3×10−4 m2/s) compares well with that reported by Park et al. [2008a. Internal tides and vertical mixing over the Kerguelen Plateau. Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.027] based on CTD/LADCP data. By combining this Kz estimate with the gradient of dissolved Fe observed at A3 [Blain, S., Sarthou, G., Laan, P., 2008. Distribution of dissolved iron during the natural iron fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean). Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.028], we calculate a vertical Fe flux of 1.0–14.3 nmol/m2 d.