Oil-attacking microbes make magnetite.
Everyone knows what the globe looks libe today. But over the earth's history, continents have moved all over, changing the face of the planet considerably. Paleomagnetists reconstruct maps of past continental configurations by observing how the orientations of magnetic fields in rocks differ from the earth's field today. The preserved fields, acquired by rocks as they formed, had been aligned with the earth's magnetic field before continental motions skewed them.
In the last few years, however, paleomagnetists have realized that the measured magnetic field of a rock is not necessarily the same field acquired by the rock when it formed. The addition of a secondary magnetic field to sedimentary layers long after those layers were deposited is more the rule than the exception for Paleozoic-aged rocks, dating from about 200 million to 600 million years ago, says Chad McCabe at Louisiana State University (LSU) in Baton Rouge. McCabe and others hope to understand what causes this secondary magnetization in order to make paleomagnetic reconstructions more accurate.
One possible cause of secondary magnetization is microbes. In work with LSU geochemist Roger Sassen, McCabe has discovered 1- to 150-micron-sized spheres of a magnetic mineral called magnetite at about six sites containing bitumen, a solid hydrocarbon that forms when microbes attack crude oil. Because the shape and texture of the spheres are often associated with biological processes, the researchers think that the magnetite formed as a by-product of biodegradation.
McCabe has found magnetite spheres in limestones too, but it's not clear that those spheres formed in the same way. Scientists have suggested that secondary magnetization in general is caused by chemical and thermal processes, but these have not been satisfactorily demonstrated.
The recent finds have implications for oil exploration as well as for paleomagnetism. According to McCabe, some petroleum geologists have suggested that oil reservoirs are associated with magnetic fields that are different from the expected field of the earth. "Our work could explain that phenomenon," says McCabe, since oil often seeps out of reservoirs and moves toward the surface, where microbes live. If McCabe and Sassen's interpretation of their findings is correct, then it suggests that looking for such magnetic anomalies might be a good way to prospect for oil. It would also give geologists a tool for studying the past migration paths of oil.
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|Date:||Nov 29, 1986|
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