Reconciling discrepant chronologies for the geomagnetic excursion in the Mono Basin,California: Insights from new 40Ar/39Ar dating experiments and a revised relative paleointensity correlation

Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by ¹⁴C and ⁴⁰Ar/³⁹Ar geochronology to have occurred within the last 50 ka. However, ¹⁴C and ⁴⁰Ar/³⁹Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New ⁴⁰Ar/³⁹Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous ¹⁴C and ⁴⁰Ar/³⁹Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined ⁴⁰Ar/³⁹Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than ¹⁴C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.