Correlating Aleutian Tephra Layers Using Trace Element Ratios

Abstract:

Analyzing both major and trace element concentrations in tephra layers is valuable for stratigraphy and correlation. With trace element chemistry we can distinguish tephras with similar major element concentrations. Using tephra chemistry data from three marine sediment cores from the Umnak Plateau in the southeast Bering Sea and three lake sediment cores from Sanak Island in the eastern Aleutians, we correlated tephra layers between the cores. Within these cores, we have analyzed up to 50 glass shards in each tephra layer for major elements using Electron Microprobe Analysis and for trace elements using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. From 100 tephra layers, we analyzed ~3,200 glass shards for their major element compositions and ~900 shards for their trace element compositions to date. Identified compositions are comparable to the USGS Aleutian Arc Volcanic Whole Rocks database. For each tephra layer, the major element data showed either clustered data points or a range of compositions from basalt or basaltic andesite to andesite or rhyolite. This linear variability in major element chemistry indicates that magmatic evolution occurred during an eruption. We made initial tephra matches based on major element chemistry and guided by uncalibrated radiocarbon ages measured in the marine sediment core HLY02-02-51JPC and published radiocarbon ages for the lake cores. We subsequently matched tephras between the marine sediment cores, between the lake sediment cores, and between marine and lake sediment cores. There are many tephras that do not appear in both the marine and lake settings, revealing smaller, proximal eruptions. Clustered major and minor element compositions are typically used for tephrochronology. Tephra layers with linear variability in composition are difficult to distinguish from one another. In order to match these samples, we use trace element ratios. Some trace element ratios can be diagnostic of particular eruptions because they remain constant even across a range of element concentrations. With the use of these ratios, we can find more tephra matches than were previously possible using solely clustered tephra compositions.