Iron Speciation in Proximity to an Active Volcanic Hotspot, Kerguelen Plateau, Southern Ocean

Low iron (Fe) concentrations limit biological productivity in over a third of the world's oceans. The Southern Ocean (SO) is the largest of these regions of Fe deficiency. However, within the Indian sector of the SO lays an oasis of relatively Fe rich waters overlaying the Kerguelen Plateau. At the southern part of the central Kerguelen plateau is an active volcanic hotspot, hosting two volcanically active islands, Heard and MacDonald (HIMI), the former of which is largely covered by glaciers. Waters in the region are subject to an intense mixing regime, caused by shallow bathymetry and the location of the plateau in the path of strong currents associated with the polar front. Fed by the Fe rich waters formed on the plateau, a plankton bloom on the order of thousands of square kilometres forms in the lee of the plateau annually each summer. Here we present dissolved iron (dFe), dissolved iron(II) (dFe(II)) and hydrogen peroxide (H2O2) data from waters surrounding HIMI on the central Kerguelen Plateau collected on RV Investigator voyage IN2016_V01 in early 2016. Analysis was performed at sea using three separate flow injection - chemiluminescence instruments. Two broadly zonal transects revealed that dFe was present in limiting concentrations (<0.2 nM) in surface waters over the central Kerguelen plateau in late summer. In shallow (<200m) coastal waters around HIMI, dFe was present in non-limiting concentrations (1 - 3 nM). Dissolved Fe(II) was found at elevated concentrations (0.6 nM) in several regions close to observed acoustic plumes emanating from seafloor vents close to the island and accounted for up to 30% of the total dFe. We also observed an intense mixing regime homogenising short-lived Fe(II) and H2O2 species throughout the water column in shallow areas of the plateau. Our results revealed varying Fe speciation patterns between the islands indicating multiple Fe sources around HIMI, including reducing hydrothermal, sedimentary and glacial sources.