Skip to main content
U.S. flag

An official website of the United States government

U–Pb zircon eruption age of the Old Crow tephra and review of extant age constraints

February 27, 2021

Eruption of the Old Crow tephra deposited ~200 km3 of volcanic ash throughout Alaska and the northwestern Yukon (eastern Beringia), providing an isochronous marker across the region on a scale unique in the Pleistocene. The Old Crow tephra represents a critical temporal piercing point used extensively to link geographically disparate stratigraphic sections and the paleo-environmental records they contain. Although the canonical age of the Old Crow suggests eruption during the transition between the glacial and interglacial periods of Marine Isotope Stages (MIS) 5 and 6 at ~125 ka, recent U–Th–Pb and (U–Th)/He zircon dating of the tephra suggests eruption at ~200 ka, within MIS 7. If accurate, this revised eruption age begets significant change to existing models describing the geologic and biotic evolution of Beringia in the Pleistocene. Thus, confidently knowing the age of the tephra is critical to its time-stratigraphic utility and for past and future work in the region where the tephra has been found. With this contribution, we review existing Old Crow age constraints and present an eruption age for the tephra determined via high spatial resolution ion microprobe U–Pb surface analysis on zircon crystals isolated from source-proximal (<500 km from plausible source) pumiceous pyroclasts of the tephra. By dating only glass-mantled crystals isolated from discrete pumice clasts, we limit the potential for sample contamination from exotic crystals and resulting age bias. The young population of dates from this dataset corroborate previous radiometric dates and confirm Old Crow eruption within late MIS 7 at 207 ± 13 ka.

    Publication Year 2021
    Title U–Pb zircon eruption age of the Old Crow tephra and review of extant age constraints
    DOI 10.1016/j.quageo.2021.101168
    Authors Seth D. Burgess, Jorge A. Vazquez, Christopher F. Waythomas, Kristi L. Wallace
    Publication Type Article
    Publication Subtype Journal Article
    Series Title Quaternary Geochronology
    Index ID 70227199
    Record Source USGS Publications Warehouse
    USGS Organization Volcano Science Center