Multiple Brunhes Chron Excursions Recorded in the Eifel Volcanic Field

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 to 45° N and 30 to 50° E (over Iraq). Paleointensities of 37 lavas reveal that 9 transitionally magnetized and four normally magnetized lavas are <30 μT and have VADMs < 3.8 x 10^{22} Am2 [Schnepp and Hradetzky, 1994; JGR]. Until now, the ages of these lava flows have been known only from imprecise 40Ar/39Ar data that implied acquisition of magnetic remanence between about 460 and 660 ka. Thus, they have been interpreted to record a single, but poorly defined, excursion. To incorporate these paleofield data into the global high-resolution Geomagnetic Instability Time Scale (GITS), we have determined precise ages of groundmass from 11 transitionally magnetized lava flows using the 40Ar/39Ar incremental heating method. New unspiked K-Ar age determinations from two samples are indistinguishable from their 40Ar/39Ar ages. The age determinations fall into five groups at 722 ± 38, 626 ± 24, 578 ± 8, 555 ± 4, and 528 ±16 ka (2σ analytical uncertainties, relative to 1.194 Ma Alder Creek sanidine). The group of 4 flows with an age of 578 ±8 ka correspond in age with lava flow sequences on Tahiti and La Palma that record the Big Lost excursion at 579 ±6 ka. The six other lavas erupted from 626 ka onward correlate with a group of second-order paleointensity lows in SINT-800 centered on the broad low during which the Big Lost excursion occurred. These lava flows thus record snapshots of the behavior of the total vector field experienced at this site during one of the most complex periods of geodynamo instability of the Brunhes Chron. Our findings suggest that four temporally distinctive excursions are recorded between 626 and 528 ka and that each weakening of the geodynamo during this period revealed a non-dipole field which consistently produced VGPs over Iraq.