Sources of iron in the Ross Sea Polynya in early summer

• Vertical diffusive DFe transport from seafloor sediments was the main DFe source to the phytoplankton bloom in the Ross Sea.
• This vertical transport was positively related to DFe near the sediment and vertical turbulent eddy diffusivity (Kz).
• The greatest transport of DFe were observed above banks and sediments with a bottom nepheloid layer.
• In general, sea ice was not an important DFe source for phytoplankton, except in one location in the marginal sea ice zone.

Dissolved Fe (DFe) was measured in the Ross Sea Polynya (RSP), Antarctica, during a GEOTRACES cruise between 20 December 2013 and 5 January 2014. DFe was measured over the full water column with special emphasis on samples near the seafloor. In the upper mixed layer, DFe was very low everywhere (< 0.10 nM). DFe increased with depth to values between 0.60 and 2.76 nM near the seafloor. The highest DFE concentrations were found at stations where a bottom nepheloid layer (BNL) was present (28 out of 32 stations). Deep DFe was lower (0.24–0.38 nM) at stations with no BNL. The main DFe supply to the upper mixed layer was vertical diffusive transport from the seafloor sediments, with a mean flux of 3.3 × 10− 8 mol DFe m− 2 day− 1. DFe fluxes showed large spatial variability of three orders of magnitude and were positively correlated to DFe concentrations near the sediment and vertical turbulent eddy diffusivity (Kz) and negatively correlated to water depth. The greatest fluxes were observed above the shallow banks such as Ross and Pennell Banks, and sediments with a BNL. We studied the horizontal diffusive transport from Franklin Island as an example of horizontal DFe transport from landmasses. No DFe transport was detected in the upper 100 m of the water column, probably due to uptake by phytoplankton. However, at 200 and 300 m depth, the DFe transport at distances between 50 and 100 km from Franklin Island was as large as the mean diffusive upward transport, indicating the potential importance of landmasses as a local source of DFe. Conversely, no horizontal transport of DFe from banks was detected. In addition, the Ross Ice Shelf (RIS) was a negligible source of DFe. Only the Ice Shelf Water (ISW), a water mass formed under the RIS, contained slightly elevated DFe (0.18–0.26 nM) compared to the surrounding waters. However, this elevated DFe did not reach into the RSP. Icebergs were not encountered and were not evaluated as a DFe source. Overall, we conclude that DFe from the seafloor and land mass sediments are the main DFe sources of DFe that support phytoplankton in the upper mixed layer of the Ross Sea Polynya in early summer.