Geomagnetic field characteristics in the Paleozoic, based on three very extensive extrusive sequences

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The Earth's magnetic field is known to change annually in strength, orientation and in its location of the magnetic poles, which move on average some 3000 feet per month. These changes are readily observed by comparing the directions in which a compass needle points from year to year. Geographic north and the Earth's rotation axis will stay more or less in place, as magnetic north moves considerably. Thus, the compass-needle direction is typically different from the true ("geographical") north-pointing direction, and is called the geomagnetic declination. Records of the magnetic field from 19th-century logs of navigational parameters and from magnetic observatories have revealed, for instance, a shift of some 1500 km in the location of the magnetic northpole in the last 150 years. This shift may be seen in declination changes in the continental United States up to a degree every five or six years. Such changes are called secular variation, derived from one of the meanings of the latin word seculum as "century". For older times, it turns out that ancient lava flows have recorded the direction of the magnetic field that existed when they solidified and cooled. Thus, rocks formed in the last five million years have recorded the geomagnetic field and reveal secular variation. However, this variation is only sparsely documented for the preceding 250 million years of Earth history, whereas before that time, in the Paleozoic and the Precambrian, secular variation characteristics have not been determined in any reliable and robust ways. The is remedying that situation by characterizing the secular variation in three unusually thick lava sequences in central Asia, with ages of about 500, 370 and 270 million years. The project is also assessing whether an evolutionary trend may exist in the magnitude of the geomagnetic field's secular variation as the Earth's solid inner core has grown steadily at the expense of the liquid outer core where most of the Earth's magnetic field is generated. Because the geomagnetic field generation in the outer core is influenced by the thickness of the outer core layer, secular variation studies may, in turn, reveal geodynamo characteristics. A more technical evaluation of the total geomagnetic field observable at the surface of the Earth describes its structure as constituting a main ("dipole") field plus higher-order fields (e.g., quadrupole, octupole etc.). During the last 550 million years the field consisted predominantly of a dipole field, but secular variation implies that on short time-scales higher-order fields exist and may wax or wane at any moment in time. A well-known application of records of the magnetic field has been to determine the ancient latitudes of continents; the method is called paleomagnetism. In order for a given magnetic direction to be interpreted as originating at a given ancient latitude, one must assume that the averaged magnetic field was purely that of a dipole. This assumption is testable for limited geological times, and here too this project will significantly extend the knowledge about the characteristics of the magnetic field. Lastly, the magnetic field is known to reverse its polarity at irregular intervals, in a process whereby the (magnetic) northpole becomes southpole and vice versa, in an otherwise unchanging, steadily rotating Earth. In a few geological periods, very long intervals of constant polarity are known, and it has been speculated that in these intervals secular variation may be significantly reduced. The proposed research is testing this idea for one of these long polarity intervals ending at about 265 million years ago. In addition to the research goals of this project, the award is providing support for the education and training of a graduate and undergraduate student at the University of Michigan.