Preliminary geomagnetic paleointensities from Long Valley Caldera, California

Abstract

Paleointensities are being determined in order to characterize geomagnetic intensity variations during Quaternary time. Because the geomagnetic field acts as a shield against cosmic radiation, its intensity directly affects the production rate of radiocarbon (^C) in the upper atmosphere. Coe and others (1978) showed that the variation in dipole moment during Holocene time is nearly proportional to the difference between ^C and tree-ring age. The ^C timescale is currently well calibrated to about 9000 years B.P., and tentatively to about 13,000 years B.P., by comparison with tree-ring and glacial-varve chronologies (Stuiver and others, 1986). However, a comparison of ages determined by the l^C and 234u_230rh methods on coral samples off the island of Barbados (Bard and others, 1990) showed that U-Th ages were consistently older than ^4C ages in material older than about 9,000 years. The discrepancy is so large (3,500 years at 25,000 years B.P.) that Bard and others consider that it can only be due to a significant decrease in geomagnetic intensity during the last glacial period. Dipole field intensity has a similar modulating effect on the production of other cosmogenic nuclides that are used for geochronologic studies and will also affect their calibration curves. Mankinen and Champion (1993a,£) recently established broad trends in geomagnetic paleointensity for latest Pleistocene and Holocene time on Hawaii. The current study is the beginning of an effort to establish a similar record for western North America. Data from North America and Hawaii will eventually be averaged with other data from globally distributed locations to eliminate the effects on nondipole variations so that a true picture of the global (dipole) field can be obtained. The dipole variations can then be used to model accurately the production rate of cosmogenic nuclides as has been attempted by Mazaud and others (1991, 1994) using some relative paleointensity records from marine sediment cores.