First identification and characterization of Borrobol-type tephra in the Greenland ice cores: new deposits and improved age estimates.

Eliza Cook,Esther R Guðmundsdóttir,Siwan M Davies,Peter M Abbott,Nicholas J G Pearce

doi:10.1002/jqs.3016

Abstract

ABSTRACTContiguous sampling of ice spanning key intervals of the deglaciation from the Greenland ice cores of NGRIP, GRIP and NEEM has revealed three new silicic cryptotephra deposits that are geochemically similar to the well‐known Borrobol Tephra (BT). The BT is complex and confounded by the younger closely timed and compositionally similar Penifiler Tephra (PT). Two of the deposits found in the ice are in Greenland Interstadial 1e (GI‐1e) and an older deposit is found in Greenland Stadial 2.1 (GS‐2.1). Until now, the BT was confined to GI‐1‐equivalent lacustrine sequences in the British Isles, Sweden and Germany, and our discovery in Greenland ice extends its distribution and geochemical composition. However, the two cryptotephras that fall within GI‐1e ice cannot be separated on the basis of geochemistry and are dated to 14358 ± 177 a b2k and 14252 ± 173 a b2k, just 106 ± 3 years apart. The older deposit is consistent with BT age estimates derived from Scottish sites, while the younger deposit overlaps with both BT and PT age estimates. We suggest that either the BT in Northern European terrestrial sequences represents an amalgamation of tephra from both of the GI‐1e events identified in the ice‐cores or that it relates to just one of the ice‐core events. A firm correlation cannot be established at present due to their strong geochemical similarities. The older tephra horizon, found within all three ice‐cores and dated to 17326 ± 319 a b2k, can be correlated to a known layer within marine sediment cores from the North Iceland Shelf (ca. 17179‐16754 cal a BP). Despite showing similarities to the BT, this deposit can be distinguished on the basis of lower CaO and TiO2 and is a valuable new tie‐point that could eventually be used in high‐resolution marine records to compare the climate signals from the ocean and atmosphere.

Highlights

Tephrochronology has long been established as a tool that exploits ash deposits with unique geochemical fingerprints to precisely correlate a diverse range of palaeoarchives from widely separated localities (e.g. Lowe, 2011)
The Greenland Ice Core Chronology 2005 (GICC05) multi-core (NGRIP, DYE-3, GRIP) timescale was constructed by counting annual layers back from 2000 AD (b2k) using multiple parameters (e.g. d18O, calcium ions) and uncertainty is based on a maximum counting error (MCE) of ambiguous layers, equivalent to 2s, where cumulative errors increase with depth (Andersen et al, 2006; Rasmussen et al, 2006)
Other stratigraphically significant cryptotephras were identified within these sampling windows (Fig. 2), some of which were used by Seierstad et al (2014) for the timescale transfer of GICC05 to GRIP; this work focuses only on the Borrobol-type horizons and the full tephrostratigraphy will be reported elsewhere

Summary

Introduction

Tephrochronology has long been established as a tool that exploits ash deposits with unique geochemical fingerprints to precisely correlate a diverse range of palaeoarchives from widely separated localities (e.g. Lowe, 2011). With a new pollen stratigraphy and age estimates, Davies et al (2004) showed that the horizon identified in H€asseldala port is associated with Older Dryas sediments (probably analogous to the shortlived GI-1d cold event in Greenland). This discovery was inconsistent with the Scottish occurrences that were associated with older Lateglacial interstadial sediments (analogous to the warmer GI-1e) and prompted Davies et al (2004) to propose that two tephras with identical geochemistry were deposited during GI-1.

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Discussion

Conclusion