Using correlated tephras to refine radiocarbon-based age models, upper and lower Whitshed Lakes, south-central Alaska

Tephra deposits correlated between nearby lakes provide the opportunity to improve age estimates of the sediment sequences, even if the ages of the tephras are previously unknown. We explore this potential using cryptotephras and visible tephra deposits in sediment cores from Upper and Lower Whitshed Lakes near Cordova, Alaska. Each tephra was described in terms of visual stratigraphy and shard morphology, and the major-oxide glass geochemistry was analyzed. Independent age models were developed for the cores using radiocarbon ages and profiles of short-lived radioisotopes for the near-surface sediments. Four tephras were correlated between the two lakes based on the magnitude and spacing of magnetic susceptibility peaks and glass major-oxide geochemistry. These correlations confirm agreement of the age models because the independently modeled confidence intervals overlap for each correlated tephra. The stratigraphic correlations were subsequently used to improve the age models by extracting the subset of possible age-model iterations that produce similar ages for each of the four correlated tephras at the two lakes. The iterations that agree within 25 years for each correlated tephra were used to create tephra-matched age models for both lakes, which narrowed the width of the 95% confidence intervals of the age models by 3% overall and reduced the uncertainty in age estimates of the correlated tephras by 34% on average. This synchronization technique may be useful in other studies that have multiple independently dated records with confident stratigraphic correlations.