How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit

Highlights

• The tephra layer produced by the 1980 eruption of Mount St Helens was surveyed.

• Isopach maps derived from a)our survey & b)measurements made in 1980 were compared.

• Preservation of the tephra layer was good in undisturbed areas.

• Despite good preservation, our model overestimated the volume of the tephra deposit.

• Our results demonstrate the sensitivity of reconstructions to measurement location.

Abstract

Tephra layers are frequently used to reconstruct past volcanic activity. Inferences made from tephra layers rely on the assumption that the preserved tephra layer is representative of the initial deposit. However, a great deal can happen to tephra after it is deposited; thus, tephra layer taphonomy is a crucial but poorly understood process. The overall goal of this research was to gain greater insight into the taphonomy of terrestrial tephra layers, specifically the extent to which deposit thickness is altered over time, with implications for tephra volume estimation. We approached this by a)conducting a new survey of the tephra layer from the recent, well-studied eruption of Mount St Helens on May 18th, 1980 (MSH1980); b)modelling the tephra layer thickness using a mathematical technique and c)comparing our results with an equivalent model based on measurements taken immediately after the eruption. In this way, we aimed to quantify any losses and transformations that have occurred. During our study, we collected measurements of tephra layer thickness from 86 locations ranging from <20 to >600 km from the vent. Geochemical analysis was used to identify tephra of uncertain origin. Our results indicated that the extant tephra layer at undisturbed sites was representative of the original deposit: overall, preservation in these locations (in terms of thickness, stratigraphy and geochemistry) had been remarkably good. However, the isopach maps generated from our measurements diverged from isopachs produced in the same way, but derived from the original survey data. Furthermore, our estimates of the quantity of tephra produced during the eruption greatly exceeded previous estimates of the fallout volume. In our study, inaccuracies in the modelled fallout arose from issues of sampling strategy, rather than taphonomy. Our results demonstrate the sensitivity of volcanological reconstructions to measurement location, and the great importance of reliably measured low/zero values in reconstructing tephra deposits.

Introduction

Tephra layers are frequently used to reconstruct records of past volcanic activity, and to infer the characteristics of the eruptions that produced them (Bonadonna and Houghton, 2005; Carey and Sparks, 1986; Pyle, 1989). This research is necessary to expand the short and rather patchy record of volcanic eruptions based on historical accounts, and is fundamental to understanding volcanic processes (Bonadonna etal., 2015; Bonadonna etal., 1998; Carey and Sparks, 1986). However, inferences made from tephra layers rely on the assumption that the preserved tephra layer is representative of the initial deposit. A great deal can happen to tephra after deposition (e.g.,Dugmore etal., 2020), and recent research has demonstrated that environmental conditions (notably vegetation cover) play a key role in determining how much of the deposit is preserved in situ (Blong etal., 2017; Buckland etal., 2020; Cutler etal., 2016a; Cutler etal., 2016b; Dugmore etal., 2018). In terrestrial settings, tephra deposits can be re-worked by the elements and slope processes, sometimes for a period of years (e.g.,Fontijn etal., 2014; Liu etal., 2014; Panebianco etal., 2017; Smith etal., 2019; Wilson etal., 2011). Once interred, tephra layers may be altered by bioturbation, soil processes (e.g.,eluviation) and geochemical transformation. Thus, even apparently well-preserved tephra layers may not record the characteristics of the fresh deposit faithfully: taphonomy matters. Despite the implications for volcanogenic reconstruction, the ways that tephra deposits are preserved (or not) are poorly understood. Our research targets this knowledge gap, aiming to gain greater insight into the taphonomy of terrestrial tephra layers, and, by extension, an improved understanding of past volcanism.

Our strategy was to survey a buried tephra layer produced by a recent, well-studied eruption, and to compare our data with measurements taken soon after deposition, thereby calibrating any losses and transformations that occurred during the nearly 40-year intervening period. We applied techniques that would typically be used in the absence of contemporary records of tephra deposition. Hence, we collected a substantial number of widely dispersed measurements, selected survey locations with high preservation potential, and deployed an established mathematical modelling technique. We focussed our study on the tephra layer produced by the May 18th, 1980 eruption of Mount St Helens (hereafter, MSH1980). The MSH1980 tephra layer was particularly suited to a study of this type because the US Geological Survey (USGS) measured it within 2–3 days of the eruption (recording deposit thickness, bulk density and mass loading) before it had been substantially reworked (Sarna-Wojcicki etal., 1981). The 1980 survey of the fresh deposit collected over 200 measurements of tephra thickness, extending >600 km from the vent (Sarna-Wojcicki etal., 1981). This rich dataset gives an unparalleled record of baseline conditions, and facilitates the production of detailed isopach maps and volume estimates (Engwell etal., 2015).

We conducted an initial survey of the MSH1980 layer in 2015 (Cutler etal., 2018), measuring tephra thickness, stratigraphy and mass loading. We found that the preservation of the layer was remarkably good in certain settings. The overall characteristics of the original deposit were retained in the tephra layer (i.e.,systematic thinning along and across the main plume axis and a marked secondary thickening ~300 km from the vent). Grain size characteristics of the initial deposit (originally established by Carey and Sigurdsson, 1982; Durant etal., 2009; Eychenne etal., 2015) were also preserved. However, our 2015 samples were concentrated in two areas, one proximal (20–40 km) and one distal (~300 km) to Mount St Helens. We therefore conducted further field measurements in 2018, focussing on deposit thickness measurements and aiming to extend the range and density of our sampling. In particular, we wanted to collect more data from a)areas towards the edge of the tephra deposit b)points along the tephra deposit's main axis, and c)areas under-sampled in the original (1980) survey. Our objective was to construct isopach maps of the MSH1980 layer using a mathematical modelling technique, and to compare these outputs with a map generated using the same technique, but based on survey data from 1980.

Constructing isopach maps from terrestrial tephra layers is a standard procedure in volcanology (Cioni etal., 2015; Klawonn etal., 2014a; Klawonn etal., 2014b; Thorarinsson, 1954; Walker and Croasdale, 1971). The key difference with our project is that we constructed isopachs from both the original (1980) measurements, and the extant tephra layer, using the same methodology, including mathematical technique, assumptions and parameters. This allowed us to compare quantitatively the ‘before-and-after’ isopach maps, identify spatial variability in preservation, and quantify how well a tephra layer can represent the fresh fallout. We anticipated that our reconstruction of the fallout based on the extant tephra layer would substantially underestimate the volume of tephra produced (in terms of both bulk and dense rock equivalent (DRE) volume), due to the winnowing of fine material and the erosion/compaction of thin deposits on the margins of the fallout zone.