Using cryptotephra layers to understand volcanic ash clouds

The aim of this thesis is to evaluate the use of microscopic volcanic ash (‘cryptotephra’) layers for providing information on the timing, characteristics and spatial extent of past volcanic ash clouds. The fine ash produced by explosive eruptions can travel long distances and even in low concentrations represents a hazard for aviation. Understanding the frequency and nature of ash clouds is important if economic and social losses are to be mitigated. This thesis is split into two research compartments. Compartment 1 focusses on understanding the limits of tephrochronology and investigates cryptotephra preservation and reworking bias in lakes and peatlands. Compartment 2 focusses on applying cryptotephras as records of ash cloud events. In addition to the objectives which fit into the two research compartments, two overarching objectives were outlined. These objectives focus on conducting new field campaigns in order to fill spatial gaps in existing cryptotephra records through the development of new, high quality tephrostratigraphies. I examine tephra layers from 13 new sites and contribute toward filling spatial gaps in northern European tephra records in northern Sweden, Poland, Wales and Southern England. Three new tephra layers are identified as part of this study, two in northern Europe: a basaltic tephra, CLA-L1 most likely derived from an eruption of the Iceland’s Grímsvötn volcano and SN-1, from the Icelandic Snæfellsjökull volcano - identified for the first time in mainland Europe. Finally, the AUC-1 tephra traced to a probable Ecuadorian source, represents the first discovery of a Holocene cryptotephra in the Amazon basin and highlights the opportunities for extending tephrochronology to tropical peatlands. In this study I present a number of methodological advances which are important for the design of future tephra studies, including: the replicability of tephrostratigraphies from a mid-latitude peatland; differences in lake and peatland records even at sites in close proximity; and the robustness of tephra glass shard geochemistry to acidic conditions and acid extraction. Beyond methodological advances, this study demonstrates how cryptotephra records can be utilised in new ways to compliment proximal records of volcanism; includes the first comprehensive analysis of distal tephra shard size (9500 shards); and presents a new recurrence estimate for the frequency of ash cloud events over northern Europe (44 ± 7 years).