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

Abstract

Contiguous 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.

Tephrochronological dating of paleoearthquakes in active volcanic arcs: A case of the Eastern Volcanic Front on the Kamchatka Peninsula (northwest Pacific)

“…observed those in the same section. Recently, medium-K KHG tephra was found in Greenland ice where it was dated at 7872 ± 50 a BP (Cook et al, 2018). This tephra matches KHG samples collected in the northern part of the CKD, northnortheast of the volcano (Fig.…”

“…At that time, we were not able to differentiate the dispersal areas of these supposed three tephras and never observed those in the same section. Recently, medium‐K KHG tephra was found in Greenland ice where it was dated at 7872 ± 50 a bp (Cook et al ., ). This tephra matches KHG samples collected in the northern part of the CKD, north‐northeast of the volcano (Fig.…”

Glacier Peak and mid‐Lateglacial Katla cryptotephras in Scotland: potential new intercontinental and marine‐terrestrial correlations

“…The ice‐core ages of both layers overlap the terrestrial age for the Borrobol Tephra (14.19–14.00 cal ka bp ; Bronk Ramsey et al , ) and, in the case of the younger layer, the age of the Penifiler Tephra (14.06–13.81 cal ka bp ; Bronk Ramsey et al , ). However, Cook et al () note the absence of any Borrobol‐type layer in the overlying Interstadial GI‐1d ice equating to the Penifiler Tephra age. This suggests that this layer was not deposited in the Greenland ice‐core stratigraphy.…”

“…This suggests that this layer was not deposited in the Greenland ice‐core stratigraphy. Cook et al () speculate that terrestrial occurrences of the Borrobol Tephra in Northern Europe are either a merging of this closely spaced GI‐1e tephra couplet (e.g. due to slow sediment accumulation rates), or that only one of these two Greenland layers was deposited in Europe.…”

“…They occur singly or together in a number of terrestrial sequences in northern Europe (summarised by Lind et al , , and references therein). The recent detection of Borrobol‐type cryptotephras in early Interstadial Greenland ice (Cook et al , ) marks a possible further correlation with one or more of these layers in this valuable high‐precision record. We identify an approximate stratigraphic position for the Scottish Glacier Peak set G layer just above the Penifiler Tephra, which is consistent with the relative radiocarbon ages of these tephras.…”