Using x-ray fluorescence to identify Mazama ash in the Lake Bonneville basin,Utah,USA

Abstract

Generalizable solutions are needed for identifying volcanic tephra in sedimentary records. We used x-ray fluorescence (XRF) analysis first to detect volcanic tephra material, followed by electron microprobe analysis to identify the tephra as Mazama ash (7682–7584 cal yr BP) in three sediment cores from the Lake Bonneville basin. We compare these records from varying environmental settings to evaluate tephra presence in the mid-Holocene. We suggest the preservation of tephras such as Mazama ash is tied to the environmental conditions in which they are deposited. Environments with standing water such as lakes or stable wetland springs are more likely to preserve identifiable ash material, while settings susceptible to alternating wet and dry conditions may retain only a trace elemental signature of a tephra due to physical erosion. We propose the development of tephra indices based on unique geochemistry is a useful tool for detection and identification of tephra material in sedimentary records. Here, we develop a Mazama Tephra Index (MTI), specific to Mazama ash in Lake Bonneville basin sediments: MTI = (Y + Zr + Nb)/Rb. We tested the applicability of the MTI and were successful in XRF detection of Mazama ash in all three records.