A varve-based calibration of the Bridge River tephra fall

A 1 cm thick layer of Bridge River tephra is preserved in a core of varved lake sediments recovered from Hector Lake, Alberta. The varve sequence extends from the core top to well below the tephra layer and has bseen calibrated to calendar years. A varve count yields a calendar age of 2332 BP for the ash layer, with a potential error of about ±50 years. This age is about 30 years younger than the current best radiocarbon-based estimate of the time of the eruption. However, in light of the potential errors in both the varve count and the radiocarbon age, the two ages are not inconsistent. The tephra contains no biotite. In the past, biotite-free Bridge River tephra has been associated with a possible late (ca. 2000 BP) tephra fall, but the tephra from Hector Lake is clearly associated with the main (2300–2400 BP) eruptive event.