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 with 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 at least three excursions are recorded in the northern hemisphere. Thirty-four 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 30.1 ± 4.4 ka, coincident with 40Ar/39Ar and astrochronologic ages determined for the Laschamp and Mono Lake excursions, respectively*. Experiments on lavas associated with a third cluster of virtual geomagnetic poles (VGPs) are complicated by a low concentration of radiogenic argon in the presence of excess argon, yielding discordant age spectra and an imprecise age of 27.7 ± 7.9 ka. VGP clusters from the lavas that we correlate with the Laschamp and Mono Lake excursions fall close to the broad looping VGP pathways obtained from high resolution sediment records of these two excursions. Our findings imply that these excursions, previously identified unequivocally only in the northern hemisphere, were globally synchronized. This supports the hypothesis that during short-lived excursions the geomagnetic field, although weakened, remains largely dipolar. *ages relative to 1.194 Ma ACs and 28.34 Ma TCs standards, reported with 2 σ analytical uncertainties.