John and his colleagues have spent the past several years working to fill in those details. They've been collecting ocean samples and developing their analytical techniques for quantifying different natural isotopes of iron in seawater, which is one means of tracking the origins of the dissolved metal.
Iron finds its way into seawater from a variety of sources. The ratio of the stable natural isotopes iron-56 and iron-54 from these sources can differ from the ratio in the earth's crust because a number of chemical processes change the ratio by favoring the release of one of the two isotopes. The processes controlling release of iron from distinct sources vary, and so different sources can have characteristic iron-56/iron-54 ratios. Tiny variations in this ratio in seawater samples thus provide insight into the origin of the iron found there.
For example, one source is sediments from the ocean's floor, from which iron is typically released into the ocean under very low-oxygen (anoxic) conditions, and release of 'light' iron-54 is favored. Another source is dust from the atmosphere, from which Fe is typically released into the ocean with processes favoring 'heavy' iron-56. Using this information, the researchers were able to establish, for the first time, where dissolved iron in seawater had originated.
John and postdoctoral associate Tim Conway have developed a high-throughput means of purifying seawater samples and determining the iron-56/iron-54 ratio, a method capable of handling the nearly 600 samples they collected in a high-resolution transect of the north Atlantic Ocean on a GEOTRACES cruise.
From those samples, they were able to show in a paper published in the journal Nature that the largest source of iron in the north Atlantic, somewhere between 70 and 90 percent, comes from dust that blows in from the Sahara desert.
The results are helping define a very poorly understood but essential comp
|Contact: Steven Powell|
University of South Carolina