Tracking Ocean Iron

OF ALL THE trace metals in the ocean, iron is the most important for sustaining life. That's because many marine organisms living in surface waters use enzymes that require dissolved iron for vital processes such as photosynthesis. But dissolved iron is scarce in surface waters because it is constantly converted into particles that sink into the deep sea and settle into sediments.

For decades, oceanographers thought atmospheric mineral dust was the major source of dissolved iron in surface waters. Now a surprising and controversial finding indicates that hydrothermal vents deep in the ocean may also be a significant source of that iron.

Edward A. Boyle, a chemical oceanographer from Massachusetts Institute of Technology, gathered the evidence that led to the discovery during a research expedition in April 2007 at a research station seven days away by boat from the coast of Fiji. Data about iron in the ocean are sparse overall, and not a single measurement from below 1,000 meters in the South Pacific Ocean exists in the scientific literature. Boyle set out to profile iron in that area.

Why isn't there any data from deep in the South Pacific? For one reason, because the area is geographically remote, few research cruises visit the region. In addition, sample contamination has been a historically notorious and limiting problem for oceanographers examining iron anywhere. Even a tiny amount of iron, perhaps a small flake of rust from a large ship, can completely compromise a seawater sample, Boyle explains.

He helped pioneer clean sampling devices such as the moored in situ trace element serial sampler (MITESS). He modified it into an overboard sampler tethered to the ship and used it to gather water samples in a vertical water column between 1,000 and 5,000 meters below the surface of the South Pacific.

The metal-free plastic sampling device is a colony of independently operated 500-mL modules. Before the device is lowered into the ocean, each module is cleaned and sealed. All of this preparation is done inside a plastic clean lab aboard the ship to avoid contamination. Researchers use a motor to open the bottles after they have reached the appropriate sampling depth and to close them before pulling the device to the next ocean depth where the next round of sampling takes place. Back on deck, researchers whisk the device into the clean lab where insoluble iron particles are filtered out and the water, now only with its dissolved iron content, is stored.

In a clean facility in his land lab at MIT, Boyle spiked his samples with known amounts of ⁵⁴Fe, a rare natural isotope, and used inductively coupled plasma mass spectrometry to measure the ratio of scarce ⁵⁴Fe to abundant ⁵⁶Fe in the samples. From that ratio, he calculated how much of the natural ⁵⁶Fe was present in the seawater sample and plotted those data against ocean depth.

To Boyle's surprise, the resulting iron profile didn't look like iron profiles from other parts of the deep ocean. He expected the nanomolar iron concentration to be roughly constant at depths greater than 1,000 meters, mirroring iron profiles from the South Atlantic and North Pacific. Instead, the concentration of iron in the South Pacific samples more than doubled at about 2,000 meters relative to the other depths he had sampled.

When Boyle presented his results at a symposium at Woods Hole Oceanographic Institution last year, William J. Jenkins, a marine scientist there, recognized the shape of the curve. It followed the same pattern as ³He/⁴He profiles, which Jenkins knew to be signatures of iron emerging from hydrothermal vents between 2,000 and 3,000 meters in the ocean—roughly the same depth where the concentration of iron peaked in Boyle's iron profile.

Iron-rich sediments were found around hydrothermal vents years ago, says Kenneth W. Bruland, a chemical oceanographer at the University of California, Santa Cruz. But, he adds, everyone thought that the iron coming from the vents was insoluble because it seemed to precipitate quickly into the sediments on the ocean floor. Boyle and Jenkins have results that indicate twice as much dissolved iron exists in the deep waters of the South Pacific than previously thought and that the metal is coming from hydrothermal vents, Bruland says.

Boyle had not measured ³He during his expedition, so Jenkins dug up data from other researchers' previous trips to that region of the South Pacific. They plotted the helium and iron data against one another and got a nearly perfect straight line, which they say suggests that a significant amount of dissolved iron in the deep South Pacific Ocean originates from hydrothermal vents.

Quantifying the amount of iron coming from the vents is tricky because even a small addition of dissolved iron can change how much dissolved iron the water will hold. Boyle and Jenkins calculate that if as little as 1% of the iron that emerges from the vents remains dissolved rather than mineralizing, it could change the total dissolved iron concentration of the region by up to 50%. If even 10% of that dissolved iron made it to the surface of the ocean, it would be significant for growth of surface-dwelling organisms, they say.

Jenkins presented the iron profile at Goldschmidt, an international geochemistry meeting named after a chemist considered to be the founder of modern geochemistry, held in July in Vancouver, British Columbia. Catherine Jeandel, a marine geochemist at the National Center for Scientific Research, in France, who presided over the session, says the results from the South Pacific open the door to further investigation of dissolved iron from hydrothermal vents. If the relationship that Boyle and Jenkins found between iron and ³He is verified elsewhere and the hydrothermal iron can reach the surface waters, it should be significant for the iron cycle, she says.

ONLY ONE OTHER research group—led by a former postdoc in Boyle's lab—has suggested the possibility that hydrothermal vents are a significant source of dissolved iron in the deep ocean, and those findings have not been published to date. But a few researchers in the marine chemistry community raise questions about Boyle's and Jenkins' conclusion that hydrothermal vents are the source of the iron. These researchers, in anonymous review comments on a paper submitted to Geophysical Research Letters by Boyle and Jenkins, suggest that the iron could instead originate from bacterial transformation of iron-containing organic material that falls from the surface ocean.

These other researchers point out that another measurement, called apparent oxygen utilization (AOU), has a similar profile to the iron profile that Boyle's team observed. AOU is the amount of oxygen used by bacteria in deep water, which can, for example, provide more information about bacteria-induced mineralization of various nutrients such as iron.

At the Goldschmidt conference, Jenkins countered by showing that AOU nutrient data don't give the same strikingly linear relationship with iron as ³He data do.

To fully answer the questions surrounding the iron profile, Boyle says that he would like to return to the area to map out the distribution of iron in more detail, noting also that "cruises to the South Pacific are fairly rare." Most likely, he will pursue the mapping as part of an international study of the world's oceans called Geotraces that has recently started out to sea after a decade of planning (see page 57).

As part of the Geotraces' scientific plan, researchers will sample the world's oceans to chart distributions of a large number of trace elements and isotopes. Boyle and Jenkins, who are both members of the Geotraces Scientific Steering Committee, say a better understanding of iron is a key goal of the research program.