Glacial and hydrothermal sources of dissolved iron(II) in Southern Ocean waters surrounding Heard and McDonald Islands

Thomas Holmes, Institute for Marine and Antarctic Studies, Hobart, TAS, Australia; Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC), Hobart, Australia, Kathrin Wuttig, Antarctic Climate & Ecosystems Cooperative Research Centre (ACE CRC), University of Tasmania, Hobart, TAS, Australia, Zanna Chase, University of Tasmania, Institute for Marine and Antarctic Studies, Hobart, TAS, Australia, Christina Schallenberg, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS, Australia, Pier van der Merwe, Antarctic Gateway Partnership, University of Tasmania, Hobart, Australia, Ashley Townsend, University of Tasmania, Central Science Laboratory, Hobart, TAS, Australia and Andrew R Bowie, University of Tasmania, Hobart, TAS, Australia
Abstract:
The Southern Ocean is the largest body of water in which iron (Fe) limits the growth of phytoplankton. However, a phytoplankton bloom on the order of thousands of square kilometres forms each spring-summer in the Indian sector of the Southern Ocean, both above and to the east of the Kerguelen Plateau. The central region of the Kerguelen Plateau is an active volcanic hotspot, hosting the subaerial volcanically active islands, Heard and McDonald (HIMI), the former of which is largely covered by glaciers. The specific sources and processes governing the supply of Fe from HIMI to the region were relatively unknown. Using Fe redox measurements, we show here that each of the adjacent islands are strong sources of dissolved Fe (DFe) and Fe(II) (DFe(II)), though controlled by different supply mechanisms.

At Heard Island, the greatest DFe(II) concentrations (max 0.57 nmol L-1) were detected north of the island, and an inverse correlation of DFe(II) concentrations with salinity suggests the origin is from a sea-terminating glacier on the island. At McDonald Islands, the greatest DFe(II) concentrations (max 1.01 nmol L-1) were detected east of the island which, based on DFe(II) profiles from five targeted stations, appears likely to originate from shallow diffuse hydrothermalism. The elevated concentrations of DFe(II) around HIMI, a highly labile and bioavailable form of Fe, may increase Fe availability for biota in the region. High DFe(II) concentrations may also indicate slower oxidation kinetics in the region, which has implications for transport of Fe away from the islands to the broader northern Kerguelen Plateau where the annual plankton bloom is strongest.