It’s elemental. Iron spewed from hydrothermal vents associated with active underwater volcanoes may be fertilising pockets of highly productive waters in the Southern Ocean.
To find out, an international contingent of scientists spent 51 days aboard Australia’s new Marine National Facility, Investigator, mapping the seafloor and sampling the phytoplankton-rich waters on the central Kerguelen Plateau, around Heard and McDonald islands.
According to Voyage Chief Scientist, Professor Mike Coffin, a marine geophysicist with the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS), the voyage* aimed to test the hypothesis of a link between active volcanoes on the seafloor and the availability of the ‘trace element’ iron for phytoplankton growth.
“Active underwater volcanoes drive the circulation of iron-rich fluids that are emitted from the seafloor at hydrothermal vents, and we suspect this iron may be critical to the growth of phytoplankton blooms,” Professor Coffin said
“Phytoplankton are the foundation of life in Southern Ocean ecosystems. They’re a food source for other marine life, they supply approximately half the oxygen in Earth’s atmosphere, and phytoplankton blooms affect the concentrations of carbon, nitrogen, silicon and sulphur in the ocean, which in turn influence the Earth’s climate system.”
Iron biogeochemistry expert, Associate Professor Andrew Bowie of the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) and IMAS, said ocean colour satellite images regularly revealed intense phytoplankton blooms on the central Kerguelen Plateau, compared to surrounding waters. Previous voyages to the region have also detected significantly elevated iron concentrations.
“Generally the Southern Ocean is anaemic, with iron concentrations so small it would be like trying to find a pinhead in 1000 Olympic sized swimming pools,” A/Prof Bowie said.
“But in the waters around Heard and McDonald islands, we’ve detected iron concentrations 10 to 20 fold greater than in deeper offshore waters, with nearby waters on the plateau showing significant increases in phytoplankton biomass.
“While we can’t say categorically where that iron comes from, we think it is partly being delivered by hydrothermal vents in the waters around the islands, and partly by volcanic ash from the erupting Big Ben volcano on Heard Island, mixing with active glaciers and dispersing in the surrounding ocean.”
To look for hydrothermal vents and map the seafloor, the team employed the Investigator’s continuously operating water column imaging, seafloor mapping, and sub-seafloor profiling acoustic systems, as well as a ‘remotely operated towed vehicle’ that measured seawater properties.
Together these instruments located more than 100 ‘acoustic plumes’, indicative of hydrothermal vents and/or other seafloor emissions such as methane bubbles, as the ship circumnavigated the islands several times.
“We mapped around 7000 square kilometres on the central Kerguelen plateau and several hundred square kilometres around the islands,” Professor Coffin said.
“We were surprised by the number of volcanic features on the seafloor around the McDonald Islands and the relative paucity of these features around Heard Island. This may be because the McDonald Islands are younger than Heard Island, so volcanoes are continuing to pop up everywhere, while most of the volcanic activity around Heard Island has coalesced around Big Ben.”
When the weather conditions permitted, the team sampled the water column and seafloor around the hydrothermal vents for iron and other trace elements, nutrients, suspended volcanic particles, volcanic sediment and rocks, phytoplankton, microbes, and various ocean properties.
Among the sampling workhorses were eight ‘in situ pumps’, used to collect and measure particles in the water column. The pumps were lowered to different depths and left to pump and filter hundreds of litres of seawater over several hours.
“Around the islands the filters came up full of black volcanic material that had been resuspended in the water column,” A/Prof Bowie said.
“When we added some of these particles to water collected outside the volcanic region, we detected an increase in biological activity.”
A/Prof Bowie’s team also deployed a trace metal rosette (TMR) at 40 sampling stations. This rosette of a dozen 12 litre bottles collects seawater for iron and other trace element analysis at different depths in the water column. Because any iron contamination will affect the samples, the TMR uses only plastic, titanium and aluminium in its construction, and is deployed, retrieved and stored under clean conditions that prevent contamination from the ship, atmosphere or human operators.
“The TMR samples will allow us to see how the iron concentration changes through the water column, which can help us to see how the iron may be delivered through volcanic ash, from glaciers on the islands, or through hydrothermal vents on the seafloor,” A/Prof Bowie said.
Laboratory tests for the ‘speciation’ of iron will provide information about its origin. Hydrothermal vents create an acidic, low oxygen environment, which results in a form of iron known as ferrous iron (or Fe2+), while regular, oxygenated seawater contains more ferric iron (Fe3+).
“Typically only one or two per cent of total dissolved iron in the Southern Ocean exists as ferrous iron, but around the islands it’s up to 30 per cent,” A/Prof Bowie said.
Samplers set up in the ship’s aerosol lab located in the bow of Investigator will also allow the team to assess atmospheric iron transport — including from a rarely seen eruption of Big Ben during the voyage. The volcanic plume could have been transported thousands of kilometres away but it could also have a local effect. The aerosol samples will help tease this out and constrain possible sources of iron coming from above with what’s coming from below.
Over the next 12 months the team will analyse their water, sediment and rock samples, and the vast amount of underway data collected on the voyage, to look for the “smoking gun” that links the underwater volcanoes in the region to the iron-rich waters that support the growth of phytoplankton.
“If we can prove that hydrothermally-derived iron exerts controls over the dynamics of phytoplankton blooms, it will be the first demonstration of a link between dynamic solid Earth processes, specifically hotspot volcanism, and biological processes,” Professor Coffin said.
Australian Antarctic Division
*The ‘Heard Earth-Ocean-Biosphere Interactions’ voyage was part of a broader collaborative program of research that included the ‘K-Axis’ voyage between the Antarctic continent and southern Kerguelen Plateau on the Aurora Australis, and other complementary research in the region onboard French, Japanese and American vessels.
The voyage included scientists and students from IMAS, ACE CRC, University of Tasmania, Australian National University, CSIRO, University of New South Wales, Microbial Oceanography Laboratory (LOMIC, France), European Institute for Marine Studies, and the Scripps Institution of Oceanography, University of California, San Diego. Four artists were also involved. The voyage was funded through Australia’s Marine National Facility, the Australian Research Council, the Australian Antarctic Science program grant (AAS #4338), and the Australian Council for the Arts.