How iron gets into the North Pacific

Lam and Bishop's recent studies of iron in the North Pacific HNLC region were focused on a region thousands of kilometers farther west. They 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.

Like other HNLC regions, this area has 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 has 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.

Iron from the Ring of Fire

Lam and Bishop again found particulate iron beneath the surface, and again the concentrations peaked at depths between 100 and 200 meters. But these concentrations were six times greater than those they had found in the Eastern Subarctic Pacific. The chemistry was a giveaway: the iron was "reduced," that is, having less oxygen than oxidized samples from the surface of the Earth (oxidized iron is better known as rust). Like material brought up from 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 volcanic 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 dust. As in the Eastern Subarctic Pacific, the particulate iron Lam and Bishop found in the Western Subarctic Pacific -- and by inference the dissolved iron essential to plankton growth -- was concentrated at depths indicating 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 (iron that can fertilize plankton) from both wind-blown dust and continental sources led Lam and Bishop to conclude that a minimum of 55 percent of the bioavailable iron they found at this site -- and probably much more -- comes from the nearby volcanic continental margins.