Авторы: Singera,∗, Kenneth A. Hoffmanb,a, Elisabeth Schnepp

2008 г.

Phys.Earth.Planet.Int.

Volcanic records of excursional geomagnetic field behavior, in particular paleointensity estimates, are fragmentary for the Pleistocene. The West Eifel volcanic field is unique in that 12 of 66 measured lava flow sites record Virtual Geomagnetic Poles (VGPs) clustered between 34–45◦N latitude and 30–50◦E longitude over the Middle East and Southeast Asia. Paleointensity determinations reveal that 13 of 37 lavas erupted during times when the ambient field was weak. Specifically, results on nine transitionally magnetized and four normally magnetized lavas indicate paleofield intensities less than 30T associated with virtual dipole moments (VDMs) <4×10²² Am² [Schnepp, E., Hradetzky, H., 1994. Combined paleointensity and ⁴⁰Ar/³⁹Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): evidence for an early Brunhes geomagnetic excursion. J. Geophys. Res 99, 9061–9076]. Until now, the age of these lava flows has been known only from imprecise ⁴⁰Ar/³⁹Ar dating that implied acquisition of magnetic remanence between about 570 and 450 ka. Thus, they have been interpreted to record a single, but poorly defined, excursion in the lower Brunhes Chron. To incorporate these paleofield data into the global high-resolution Geomagnetic Instability Time Scale (GITS), we have determined precise ages of groundmass from 12 excursional and 7 normally magnetized lava flows using the 40Ar/39Ar incremental heating method. Unspiked K–Ar age determinations from two excursional samples are indistinguishable from their 40Ar/39Ar results. Ages of the excursional lava flows fall into five groups at 722±38, 626±24, 578±8, 555±4, and 528±16 ka (±2 analytical uncertainties, relative to 1.194Ma Alder Creek sanidine). The group of three excursional flows with an age of 578±8 ka correspond temporally with lava flow sequences on Tahiti and La Palma that record the Big Lost excursion at 579±6 ka. Eight other excursional lavas erupted from 722 to 528 ka correlate with less well-defined excursions or paleointensity minima differentially expressed in marine sediment records. Our findings that at least four, and perhaps five, temporally distinct excursions are recorded between 722 and 528 ka further suggest that each weakening of the dynamo during this periodwas associated with demise of the axial dipole. The VGPs produced during these excursions are found to be dispersed longitudinally across Eurasia and are remarkably similar to those associated with the calculated south VGPs atWest Eifel for the non-axial dipole (NAD) field during the 20th century. A simple model in which the axial dipole repeatedly collapses leaving behind a residual NAD field that is largely held by stationary sources, probably in the lowermost mantle, provides an explanation for the recurrent excursional behavior during this ∼200 kyr period.

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