When is a tephra not a tephra layer? Issues in working with Holocene cryptotephra in Ireland and Greenland

• Main Authors: Plunkett, Gill, Pilcher, Jonathan, Sigl, Michael, Coyle McClung, Lisa, Hall, Valerie
• Format: Conference Object
• Language: English
• Published: 2014
• Subjects: Coulter; Garry; Greenland; Pilcher; Greenland ice cores; Iceland; North Greenland; Alaska
• Online Access: https://pure.qub.ac.uk/en/publications/when-is-a-tephra-not-a-tephra-layer-issues-in-working-with-holocene-cryptotephra-in-ireland-and-greenland(dc480d12-2141-4028-a127-fa45ce870f9e).html

Since the early 1990s, research on tephras in northwest Europe has demonstrated the value of tephrochronology as a means of dating and inter-correlating palaeoenvironmental archives. Irish bogs have been shown to be a particularly rich source of Holocene cryptotephras (Hall & Pilcher 2002; Swindles et al. 2012; Lawson et al. 2012). Here, the majority of tephras whose provenances have been reliably established derive from Iceland, yet the origins of a substantial number of tephra layers remain unknown; many more potential marker horizons have not been reported because of low shard concentration and/or difficulties in obtaining large enough, homogenous geochemical datasets (Figure 1). Recent research on Late Holocene Greenland ice cores has highlighted the presence of tephras whose sources are predominantly non-Icelandic; recognized provenances include Alaska and China (Coulter et al. 2012; Sun et al. 2014), and potentially the Cascades and Italy, but again, many tephras remain uncorrelated with eruptive events and/or volcanic systems. This paper considers some of the issues that currently hinder tephrochronology from achieving its maximum potential in distal locations. Firstly, low shard concentrations in both peatlands and ice cores are often dismissed as “background” or detrital material, and may not be considered worthy of the time and effort needed to prepare them for geochemical analysis. Secondly, fine grained shards (< 50 microns) can be problematic and time-consuming to analyze, impeding the collection of large datasets. Finally, inaccessibility of comparative datasets can prevent the linkage of tephras to source – arguably not essential for the use of a tephra as an isochron provided it is distinctive, but critical if volcanic histories and impacts are to be investigated. We illustrate these issues with reference to case studies drawn primarily from Garry Bog (northeast Northern Ireland), a site with one of the best resolved tephrostratigraphies in Ireland, and NEEM-2011-S1 (north Greenland) where targeted volcanic events have been examined. We propose that the robustness of sparse results can be aided by multiple analyses on individual shards (shard size permitting) and the replication of the tephrostratigraphies at multiple sites. We argue that sparse, distal tephra horizons can be informative on a number of fronts, such as contributing to the improved dating of volcanic events, the assessment of extra-regional volcanic impacts and to volcanic histories, and are thus worthy of reporting to the wider tephra community.