Variations in tephra stratigraphy created by small‐scale surface features in sub‐polar landscapes

Polly I J Thompson,Anthony J Newton,Nick A Cutler,Andrew J Dugmore,Richard T Streeter

doi:10.1111/bor.12557

Polly I J Thompson, Anthony J Newton + Show 3 more

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https://doi.org/10.1111/bor.12557

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Abstract

We explore the effect small‐scale surface features have on influencing the morphology and grain‐size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub‐polar landscapes. Icelandic thúfur, small cryogenic earth mounds, are used to assess how and why the morphology and GSD of tephra layers vary over such formations. Through measurement of tephra layer thickness and GSD, Hekla 1947 and Grímsvötn 2011 tephra layers are analysed. Results indicate that such microtopographic features do indeed alter the form of tephra deposits and therefore the tephra layer that is preserved in the stratigraphy. Tephra thickness is significantly greater in hollows than on the thúfur crests. There is greater variation in tephra thickness measurements from thúfur in comparison to control measurements from a surface where thúfur are absent. Thúfur crests contain larger grain sizes than hollows, for both H1947 and G2011 tephras; however this was only statistically significant for the G2011 tephra. Such morphological patterns are thought to arise from an interplay of tephra characteristics, altered topography from the thúfur formations and earth surface processes operating at the sites. This study provides insight into the potential of tephra layer morphology and internal structures as indicators of Quaternary landforms and processes. Additionally, it provides important context for the appropriate sampling of tephra layers to infer volcanological processes, as the characteristics of preserved layers do not necessarily reflect those of the original fall‐out.

Highlights

We explore the effect small-scale surface features have on influencing the morphology and grain-size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub-polar landscapes
At Site B, only four thufur crests contained a measurable layer of Hekla 1947 (H1947); overlying soil was measured on every crest where sufficient tephra grains were present to define the surface in 1947
Our results show that the characteristics of the tephra layers formed by H1947 and Grımsvo€tn 2011 (G2011) tephra deposits are influenced by microtopographic variations created by thufur formations

Summary

Introduction

We explore the effect small-scale surface features have on influencing the morphology and grain-size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub-polar landscapes. Thufur are a common cryogenic landform and are found globally across high-latitude areas such as Iceland, Greenland, Russia and Canada, as well as in upland areas at lower latitudes that contain enough soil for them to form, such as Dartmoor in southwestern England (Grab 2005) Such small-scale (width, height and/or length ~20– 250 cm) topographic features are important landforms for inferring current and past earth surface processes and land management. It is important to understand how small-scale surface features alter tephra layers because in addition to forming chronological markers, they are used to infer the parameters of past eruptions (Lowe 2011) Both approaches make assumptions about the tephra.

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