Abstract. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic
Circumpolar Current of the Southern Ocean. Iron emanating from its shelf
system fuels large phytoplankton blooms downstream of the island, but the
actual supply mechanisms are unclear. To address this, we present an
inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore
waters, and the water column in the vicinity of South Georgia, alongside data
on zooplankton-mediated Fe cycling processes, and provide estimates of the
relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments,
modified by authigenic Fe precipitation, were the main particulate Fe source
to shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for
shelf sediments and suspended particles in the water column. Less than 1 %
of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe
and therefore potentially available to organisms. Pore waters formed the
primary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1.
However, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical
diffusive and advective mixing. Other trace metal sources to surface waters
included glacial flour released by melting glaciers and via zooplankton
egestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface
mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1
released directly by the krill. The faecal pellets released by krill included
seafloor-derived lithogenic and authigenic material and settled algal debris,
in addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km
downstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep
winter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was
provided through recycling of biogenically stored Fe following luxury Fe
uptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to
be retained in the surface mixed layer of waters downstream of South Georgia
through continuous recycling and biological uptake, supplying the large
downstream phytoplankton blooms.
Review of the manuscript «Variability and stability of anthropogenic CO2 in Antarctic Bottom Waters observed in the Indian sector of the Southern Ocean, 1978–2018”, by L. Mahieu, et al., Ocean Science Discussion, OS-2020-37
