Laser-ablation ICP-MS zircon U-Pb ages for key Pliocene-Pleistocene tephra beds in unglaciated Yukon and Alaska

Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for K-40-40Ar (or 40Ar/39Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma -mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (< 1 Ma) tephra. Here, we demon-strate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2 sigma uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and 40Ar/39Ar ages: Biederman tephra (178 +/- 17 ka), HP tephra (680 +/- 47 ka), Gold Run tephra (688 +/- 44 ka), Flat Creek tephra (708 +/- 43 ka), PA tephra (1.92 +/- 0.06 Ma), Quartz Creek tephra (2.62 +/- 0.08 Ma), Lost Chicken tephra (3.14 +/- 0.07 Ma), and GI tephra (542 +/- 64 ka). We also present newly revised glass fission-track and 40Ar/39Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for 230Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other strati -graphically important tephra in the region.

Automated summary

Tephrochronology is an effective method for correlating and dating Quaternary deposits. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating is a reliable technique for determining the ages of key tephra beds in the Yukon and Alaska regions.