Glacial and hydrothermal sources of dissolved iron(II) in Southern Ocean
waters surrounding Heard and McDonald Islands
Abstract
The Southern Ocean is the largest region in which iron limits the growth
of phytoplankton. However, a phytoplankton bloom thousands of square
kilometres in area 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 hosts the volcanically active
islands, Heard and McDonald (HIMI), the former of which is largely
covered by glaciers. The sources and processes governing supply of iron
from HIMI to the region are relatively unknown. In the austral summer of
2016, the first voyage to focus on biogeochemical cycling in the HIMI
region was undertaken (GEOTRACES process study GIpr05). Using iron redox
measurements, we show here that each of the adjacent islands are strong
sources of dissolved iron(II) (DFe(II)), though controlled by different
supply mechanisms.
At Heard Island, the greatest DFe(II) concentrations (max 0.57 nmol L)
were detected north of the island. 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) were detected east of the
islands which, based on DFe(II) profiles from five targeted stations,
appears likely to originate from shallow diffuse hydrothermalism.
Elevated DFe(II) around HIMI may increase Fe availability for biota and
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.