Most oceanographers have assumed that, in the areas of the world's oceans known as High Nutrient, Low Chlorophyll (HNLC) regions, the iron needed to fertilize infrequent plankton blooms comes almost entirely from wind-blown dust.
Image Credit: Siyapath/Shutterstock.com
Researchers from the Earth Sciences Division of the Department of Energy's Lawrence Berkeley National Laboratory have proven that such a belief may not apply at least in the North Pacific.
In a past issue of Geophysical Research Letters, it was reported that the key source of iron in the Western North Pacific is not dust but the volcanic continental margins of the Kamchatka Peninsula and the Kuril Islands.
Can iron affect climate change?
Understanding the origins, transport mechanisms, and the fate of naturally occurring iron in high-nutrient, low-chlorophyll surface waters is important in calculating climate change. For example, artificial iron-fertilization schemes, although based on inadequately tested assumptions, hope to reduce greenhouse gases by stimulating plankton blooms to suck carbon dioxide from the atmosphere and store it in the oceans.
For instance, it is iron that enables phytoplanktons to use nitrate; without it, the plants are denied access to often substantial nitrogen sources in HNLC regions, of which the Subarctic North Pacific belongs and is perceived to be one of three major subarctic regions in the world.
Two recognized natural sources of iron
The atmospheric dust and the upwelling from below are believed to be the two recognized natural sources of iron in the open ocean. In the North Pacific, however, researchers found that there is perhaps a third source—the continental margins. This discovery had led researchers to call on the revision of the ocean carbon cycle.
The wind-blown-dust theory of iron fertilization had no direct evidentiary support until researcher Jim Bishop made the first observation of dust in action. In the spring of 2001, two robotic Carbon Explorer floats recorded the rapid growth of phytoplankton in the upper layers of the North Pacific Ocean after a passing storm had deposited iron-rich dust from the Gobi Desert. The Carbon Explorers had been designed by Bishop with colleagues at the Scripps Institution of Oceanography; their measurements, radioed back to him by satellite, marked the first time that wind-blown terrestrial dust had been recorded fertilizing the growth of aquatic plant life.
Bishop discovered that the plankton blooms that the two Carbon Explorers saw lasted only two weeks, raising questions on the actual importance of the transport mechanism. Further doubts arose when, beginning in 2001, another researcher, Lam, analyzed samples that Bishop had collected five years earlier well out in the Eastern North Pacific. In February 1996, a rare break in the winter weather had allowed Bishop to deploy a Multiple Unit Large Volume Filtration System (MULVFS), an array of collectors lowered over the side of the research vessel by cable. What MULVFS brought back were samples that contained iron plus evidence of a vigorous plankton bloom in the middle of the Eastern Subarctic Pacific, during the cold, dark days of midwinter.
There was no evidence that dust storms could have carried terrestrial iron to the North Pacific that February, nor was the chemistry of the iron in the characteristic of the sample of iron from upwelling or past deposited dust. As the source of the iron, only the continental margins were logical.
Like other HNLC regions, this area had low biomass compared to what might be expected for such nutrient-rich waters, although it does have higher biological productivity than the Eastern Subarctic Pacific. It also had higher iron concentrations, traditionally explained by its proximity to sources of Asian dust storms, which deposit three times as much dust in these waters as in the Eastern North Pacific.
Lam and Bishop published their studies in 2006, concluding that the iron had indeed come from the continental margins of the Aleutian Islands, 900 kilometers northwest of the site where the midwinter plankton bloom had been found. Iron particles and soluble iron had been carried there along with a layer of denser water roughly 100 to 150 meters deep (the pycnocline), and the iron had been stirred up by storms that made it available to near-surface plankton in the dead of winter.
This study of iron in the North Pacific HNLC region was focused on a region thousands of kilometers further west. The researchers used samples they collected with MULVFS in the late summer of 2005, during a VERTIGO project cruise led by scientists at the Woods Hole Oceanographic Institution (VERTIGO stands for Vertical Transport in the Global Ocean). The cruise concentrated on a site in the Western Subarctic Pacific that was hundreds of kilometers south of the Kamchatka Peninsula and east of the Kuril Islands.
Iron from the ring of fire
Whilst further studying the pacific, particularly the Ring of Fire, Lam, and Bishop again found particulate iron beneath the surface, and similarly, the concentrations peaked at depths between 100 and 200 meters. The difference, however, was that the concentrations were six times greater than what was found in the Eastern Subarctic Pacific. The iron was "reduced” – having less oxygen than oxidized samples from the surface of the Earth. Similar to material brought up from the earth's mantle, many of these iron-rich samples had not been weathered; in fact, they were characteristic of basalts found in the continental shelves of the Kurils and Kamchatka, part of the Pacific's Ring of Fire.
Iron must be dissolved to be accessible to phytoplankton, and the reduced iron in volcanic silicates from island-arc sediments may dissolve more readily than iron in the dust. As in the Eastern Subarctic Pacific, the particulate iron found in the Western Subarctic Pacific—and by inference the dissolved iron essential to plankton growth—was concentrated at depths indicating that it had traveled from the ocean edges along the pycnocline. Upwelling or vertical mixing would make concentrations of continental iron at these depths readily available to plankton.
Conservative estimates of bioavailable iron from both wind-blown dust and continental sources led the researchers to conclude that a minimum of 55 percent of the bioavailable iron found at the site comes from the nearby volcanic continental margins. Lam and Bishop's findings have implications far beyond correcting estimates of the iron budget in HNLCs.
This research was funded by the U.S. Department of Energy's Office of Science, Biological and Environmental Research Program, and by the Woods Hole Oceanographic Institution, the Richard B. Sellars Endowed Research Fund, and the Andrew W. Mellon Foundation. Portions of the work were carried out at DOE's Advanced Light Source at Berkeley Lab and DOE's Stanford Synchrotron Radiation Laboratory.
Lawrence Berkeley National Laboratory