Underwater volcanoes contribute to chemical cycle that sustains life

Underwater volcanoes are responsible for an unusual chemical cycle which helps to sustain life, according to a discovery by an international research group operating in the Pacific.

Post-doctoral fellow Dr Sarah Nicholas of NUI Galway's earth and ocean sciences was one of the participants in the project, which discovered a geochemical mechanism that helps to sustain life in remote parts of the Pacific.

For years scientists have known that the Atlantic ocean is more productive – as in that it makes more biomass – than the Pacific. Nicholas says this is due to a combination of factors, including oceanic currents and freshwater discharges from rivers that provide nutrients from the land to marine waters.

These nutrients are used by phytoplankton – tiny floating plants – for photosynthesis, by which all plants convert sunlight and CO2 into sugar.

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Scientists estimate that phytoplankton are responsible for about half of all photosynthesis on earth, and iron is a necessary nutrient.

A research expedition to the eastern Pacific which Nicholas participated in has discovered that that iron generated from underwater volcano vents is a key generator of productivity far from terrestrial sources and over very large areas.

The findings, recently published in scientific journal Nature Geoscience, record how hydrothermal iron persists for much longer and at greater distances from volcanic sources than previously calculated.

Nicholas was one of 35 other scientists on board the US research vessel Thompson, which set off from Ecuador in November 2013 as part of the Geotraces project.

The nine-week expedition aimed to research trace elements emitted from a large hydrothermal vent or underwater volcano in an area known as the southeast Pacific rise – a chain of underwater volcanoes along the boundary of tectonic plates in the Pacific Ocean.

Hydrothermal water

Scientists on the cruise sampled water and particles across the chain of volcanoes. Then they followed a plume of hydrothermal water, which was enriched in dissolved gases, metals, and particles, as the plume flowed west, away from the vent.

“Previously it was thought that iron from hydrothermal vents was not an important part of the marine iron budget, the iron that is available for biogeochemical processes in the ocean,” says Nicholas.

“It had been thought that hydrothermal iron quickly transformed into solid iron-oxide –rust – and fell to the ocean floor, leaving the water and becoming unavailable for chemical reactions and/or as a nutrient for phytoplankton.”

However, the scientists were surprised to be able to measure elevated levels of dissolved and solid iron as far as 4,300km from the underwater volcano.

“In this study we used synchrotron spectroscopy and microscopy, and iron isotope measurements to examine the iron-bearing particles,” says Nicholas.

She describes a synchrotron as a “very large particle accelerator that produces powerful X-rays by accelerating electrons in a circular ring”.

“The synchrotron in this particular study has a ring about 120m in diameter, and that’s one of the smaller ones. The synchrotron at the European synchroton radiation facility in Grenoble [France] has a ring approximately 300m in diameter.

“From these measurements we found that particles that persisted in the plume were closely bound with low-density organic matter [ligands] that slow the sinking of the particle, like a life jacket made of glue.

Iron isotopes

“That discovery in itself was exciting because it keeps the iron in the water,” says Nicholas. “What made it even more interesting were the iron isotope measurements on the dissolved and particulate iron” – isotopes being atoms of the same element that have slightly different weights.

“By measuring the ratios of the different iron isotopes in particles and water collected along the length of the plume [4,300km], we found that chemical and/or biological reactions were occurring that moved iron back and forth from the water to the particles, and back again.

“This exchange seems to have been made possible by the organic matter associated with the iron.”

She says given its role as an essential micronutrient for photosynthesis, the process that allows hydrothermal iron to persist may explain how phytoplankton survive in remote parts of the Pacific, where productivity would have been expected to be much lower.

Prof Peter Croot of NUI Galway’s earth and ocean sciences says the research is “another example of how little we know about the processes occurring in the deep ocean, and their impact on marine biogeochemistry”.

He said the discovery that a significant portion of the iron coming from the hydrothermal plume sinks more slowly than expected –and can be transported hundreds of kilometres – overturns earlier ideas about iron cycling in the ocean.

The Geotraces project: studying trace elements in the marine environment

When Dr Nicholas signed up for a PhD at the University of Minnesota her focus was thousands of miles from the sea. Her subject was arsenic in glacial aquifers. However, it was while working on the 10-year Geotraces project that she became interested in marine geochemistry, and ultimately moved with her husband and two children to NUI Galway.

Geotraces is a 10-year collaboration which relies on funding from international research institutions in Europe, the US, Canada, Japan and India. It is well over halfway through a programme of some 46 cruises.

The aim is to study all major ocean basins and thereby improve understanding of biogeochemical cycles and large-scale distribution of trace elements and their isotopes in the marine environment.

As the collaboration explains, trace elements serve as regulators of biological processes in the ocean, including marine ecosystem dynamics and the carbon cycle. Yet knowledge of the marine biogeochemical cycles of these essential micronutrients and their sensitivity to changing environmental conditions is “surprisingly incomplete”.

Human activity has had a significant impact, with increased discharge of harmful elements into the ocean. Part of the Geotraces brief is to improve prediction of the transport and fate of contaminants.

Nicholas was able to continue her post-doctoral fellowship at NUI Galway with the support of the Irish Centre for Research in Applied Geosciences, which is funded by Science Foundation Ireland.