Distal volcanic ash layers in the Lateglacial Interstadial (GI-1): problems of stratigraphic discrimination

The newly-detected Icelandic Penifiler Tephra occurs in the mid-Interstadial sediments of a number of Scottish Lateglacial sequences. It is rhyolitic in composition and possesses a chemistry which is similar to the Borrobol Tephra of early Lateglacial Interstadial age, which also occurs in a number of these same sequences. Such chemical similarity may account for some of the confusion regarding the age and stratigraphic position of Borrobol-like ash layers occurring elsewhere. However, where the Borrobol Tephra has been identified in these sequences it consistently exhibits a diffuse distribution accompanied in some cases by stratigraphic bimodality. A number of sedimentological and taphonomic factors are considered here in order to account for this distribution. One possibility is that these distributions are produced by taphonomic factors. Another possibility is that the Borrobol Tephra may not be the product of a single Icelandic eruption, but of two events closely spaced in time. In at least two of the sequences investigated in this study, basaltic shards were found in association with the Penifiler and Borrobol ash layers, suggesting either a basaltic phase associated with these eruptions, or coincident eruptions from a separate basaltic volcanic centre. Such basaltic populations may strengthen correlations with basaltic 2 ashes recently detected in the NGRIP ice-core. The detection of the new Penifiler Tephra contributes to the regional tephrostratigrapic framework, and provides an additional isochron for assessing the synchroneity of palaeoenvironmental changes during the Interstadial. The true stratigraphic nature and, therefore, age of the Borrobol Tephra, however, is persistently difficult to resolve making its use as an isochron more problematic