Like most living things, microscopic marine plants need iron and other minerals to live and grow. On land, soil provides a ubiquitous source of minerals, but how do essential nutrients get into vast watery stretches of the open ocean?
The question has long mystified oceanographers. According to one theory, large swirling currents, called eddies, pump nutrients from the depths up toward the sunlit surface, giving phytoplankton the ingredients they need to flourish. But a larger source of iron may be dust storms, which blow huge quantities of mineral-rich soil particles (called Aeolian dust) out to sea, particularly from desert regions in Africa and Asia.
Until now, scientists investigating the latter theory have been stymied by an inability to measure when, where, and how much dust falls into the oceans, said Ed Sholkovitz, a marine geochemist at Woods Hole Oceanographic Institution (WHOI). They have had to rely on dust samples collected on islands.
To remedy this situation, Sholkovitz teamed with three WHOI engineers—Geoff Allsup, David Hosom, and Mike Purcell—who collectively call themselves the “Dust Busters.” They designed a device, mounted atop a moored buoy, that collects wind-blown particles in the open ocean. Unlike islands, which aren’t portable, buoys can be placed in scientifically strategic locales.
The heart of their device is a motorized carousel that rotates 24 dust-collecting filters under a small intake opening. As air passes through the opening, the filters catch samples of windborne particles, which are chemically analyzed back at WHOI.
The particle sampler had to be rugged and watertight to survive months at sea. Software ran the device automatically, coordinating it with meteorological instruments on the buoy to pause sampling when high waves or winds might clog the filters with sea spray.
The Dust Busters collaborated with Peter Sedwick, a marine geochemist at the Bermuda Biological Station for Research, to test their particle sampler from May to September 2004 on a buoy moored off Bermuda. The scientists said the instrument successfully captured dust throughout the summer, including pulses of brown-yellow particles from two large African dust storms in June and August. They reported their findings in an article in press in the journal Deep-Sea Research.
The instrument’s initial success persuaded the National Science Foundation to fund redeployment of an improved particle sampler in 2007 and 2008. With a grant from the WHOI Access to the Sea Program, Sheri White and Norman Farr, engineers in WHOI’s Applied Ocean Physics and Engineering Department, were enlisted to develop a sensor that can measure and monitor color changes in captured dust (indicating its source) in real time. New two-way communications will let scientists modify sampling procedures in response to natural events.
While the particle sampler monitors the supply side of the question, Sholkovitz’s collaborators, Sedwick and Thomas Church from the University of Delaware, will focus on the demand side: How is iron from continental dust chemically transformed and released in seawater to catalyze blooms of phytoplankton at the base of the marine food chain?
The research will help unravel a fundamental process that fuels life in the ocean.