HR: 14:50h
AN: U42A-05 INVITED     [PDF]
TI: What can Paleomagnetism Tell us About the Next Reversal?
AU: * Coe, R S
EM: rcoe@es.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AB: Our sole source of evidence about geomagnetic polarity reversals is the paleomagnetism of rocks. Observations show that the stable magnetization acquired by igneous and sedimentary rocks when they formed possesses reversed polarity almost as frequently as the normal polarity of the field today. Averaged over the past 20 Myr the dipole field has reversed almost five times per million years, though at irregular intervals. The last one occurred 0.78 Myr ago, giving rise to the speculation that we are overdue for a reversal. What happens during a reversal? Geologically speaking, the field takes very little time to reverse, 1 to 10 kyr as generally inferred from sedimentary records, and the various rock magnetic recorders are imperfect. Thus, paleomagnetic records are always incomplete and give only lower bounds on how rapidly changing and complex the behavior of the reversing field may have been. What we know with greatest certainty is that the field intensity is much reduced during a reversal, usually falling as low as 10-20% of the average value it had during full polarity. In most other regards, reversal records exhibit great variety. The intensity decrease may lead or accompany the departure of direction from full polarity, and its recovery may accompany or lag reestablishment of full polarity. Although the change in field direction might be relatively simple and progressive during some reversals, several of the highest resolution records suggest complex behavior with episodes of rapid directional oscillations. It seems reasonable that the boundary conditions imposed by the lowermost mantle on the core could confer some regularity on the transitional field morphology, but whether they do and to what degree is debated. The transitional field is almost certainly much more multipolar in character than is the full polarity field, but the equatorial dipole may nonetheless exert considerable influence on directional systematics at the earth's surface during some reversals. Prominent non-dipole field features during a transition could cause records of the same reversal at different places to to start and finish at different times, making the global duration of a reversal significantly longer than the local duration. Increasingly accurate radiometric dating of transitional lava flows is beginning to suggest this to be true for some reversals, and sophisticated geodynamo simulations also exhibit reversals that vary greatly from one to another and possess many of the above-described characteristics. Is the field starting to reverse? Today we are indeed witnessing a pronounced decrease in field strength. From a broad maximum about 40% higher than today, intensity experiments on archeological materials of fired clay and on young lava flows show that the dipole field has been weakening monotonically for the past 2 kyr. Direct measurements during the modern era confirm that the field is diminishing and indicate that the decay is accelerating: from 5.5% per century from 1850-1950, to 6.8% per century from 1945-1995, to 8.0% per century from 1965 to 1995. However, paleomagnetic records show that the geomagnetic intensity has dropped dramatically many times in the past and recovered without reversing, so that an actual reversal this time is far from certain. The more important societal question is whether the field intensity will continue to diminish down to very low values comparable to the transitional field, or instead turn around uneventfully. Since the last reversal we know that it has decreased to very low values quite a number of times, often accompanied by large excursions of field direction. The environmental effects of such an excursion would likely be very similar to that of a successful reversal.
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: U
MN: 2003 Fall Meeting