Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand

Eight basaltic lavas from the Auckland volcanic field, New Zealand, record three distinct sets of excursional geomagnetic field directions and low paleointensities, however the timing and therefore paleomagnetic significance of these records have been poorly understood. Radiocarbon, K–Ar, and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka, a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites. Forty 40Ar/39Ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods, at 39.1 ± 4.1 and 31.6 ± 1.8 ka, coincident with 40Ar/39Ar, astrochronologic, and 14C ages determined for the Laschamp and Mono Lake excursions, respectively. Samples from a lava flow associated with a third cluster of virtual geomagnetic poles (VGPs) are complicated by low concentrations of radiogenic argon in the presence of excess argon, and thus yield discordant age spectra and an imprecise age of 26.6 ± 8.1 ka. Our findings indicate that the Laschamp and Mono Lake excursions, until recently identified unequivocally and isotopically dated only in the Northern Hemisphere, were globally synchronized at 40 ± 1 and 32 ± 1 ka. However, the VGPs of lavas that record the Laschamp excursion in New Zealand and France are inconsistent with a simple clockwise looping geometry inferred from VGP paths obtained in eight marine sediment cores spanning the Laschamp excursion. We suggest that the differences in VGPs recorded at the various sites are significant and may point to non-axial dipole components and lower mantle control on transitional fields during short-lived excursions.