DATING THE QUATERNARY OLD CROW TEPHRA VIA ZIRCON U-TH-PB-HE GEOCHRONOMETERS

Tephra deposition creates a geologically instantaneous time-stratigraphic link between geographically disparate locations. Establishing the age of a tephra horizon in a sedimentary section can be incredibly powerful, as it can provide absolute and relative time not only to that section, but to all those in which the tephra is found or can be correlated to. In sediment older than the reliable range of radiocarbon dating (> ~50 ka), minerals found within tephra are often the only means by which a section can be directly dated. Application of uranium-based geochronometers such as zircon (U-Th)/He, U-Th, and/or U-Pb to date Quaternary tephra deposits has increased commensurate with advances in dating protocols and instrument sensitivity.

To explore the limits of radiometric dating of Quaternary tephra, we apply the (U-Th)/He, U-Th, and U-Pb methods to zircon crystals from two samples of the Old Crow tephra, the most extensive Quaternary tephra deposit in Alaska and the western Yukon. Our zircon (U-Th)/He, U-Pb, and U-Th dates from one of the Old Crow samples, which is reworked within loess, are identical to each other within uncertainty. Further, they are also identical (within uncertainty) to U-Th and U-Pb dates from the other Old Crow sample, which are derived from zircons that were isolated out of primary pumiceous pyroclasts in the most source-proximal Old Crow sample known. The mean of these dates (~207 ka) is older than previous age estimates by ~ 80 ka, placing Old Crow eruption within MIS7, not MIS5. The accordance of dates from three distinct chronometers on zircons from two disjunct Old Crow samples indicate improved accuracy of the eruption age and validates the dating and age calculation protocols used, signaling a bright future for application of U-based methods to date Quaternary tephra.