Marine magnetite made by bacteria.

Magnetite, the iron oxide mineral used in the earliest compasses, becomes imprinted with the earth's magnetic field after it has been heated to high temperatures. This is why geologists interested in the past orientation of the geomagnetic field typically study rocks that were heated up or churned out by volcanoes. Yet geologists also find small magnetite particles in marine sediments that have never been subjected to the requisite high temperatures. "No one has really come up with a good way to explain how this particular component of magnetite gets there," observes John F. Stolz.

Some scientists have suggested that this magnetite is produced by marine invertebrates called chitons, which grind down their magnetite-containing teeth when eating algae embedded in limestone. But Stolz and others have more recently come to believe that the main source of the magnetite in marine sediments is bacteria. Such "magnetotactic" bacteria, able to orient themselves in the geomagnetic field by building an internal compass or chain of magnetic particles, have been found in beach sands and lakes (SN: 4/26/80, p. 267). Now Stolz, Shih-Bin R. Chang and Joseph L. Kirschvink at Caltech in Pasadena, Calif., have discovered live magnetotactic bacteria and their magnetite chains in the open sea, from sediment cores taken 598 meters underwater in the Santa Barbara basin. Stolz's conclusion that the magnetite grains in these sediments were made by bacteria is based on the grains' distinctive, highly regular shape and their size, 50 to 150 nanometers in diameter. This is the optimum size for recording the earth's magnetic field, says Stolz. The alignment of smaller grains would be upset by thermal vibrations, and larger grains would have a smaller net magnetic moment.

Scientists think the magnetotactic bacteria may use the geomagnetic field, which tends to point into or out of the earth, to find their way toward the oxygen-poor sediment layers in which they like to live. If so, traces of magnetotactic bacteria in the rock record might be used as indicators of past oxygen depletion. Stolz next plans to look for magnetotactic bacteria in Pacific cores taken 6,000 meters underwater. Their presence at such depths could indicate that these bacteria play an important role not only in producing magnetic marine sediments but also in the planet's iron cycle. For now at least, says Stolz, "in the marine environments we've looked at, [the bacteria] are almost the sole source of the ultrafine-grain magnetite in marine sediments today."