Revisiting the Borrobol Tephra
The study of palaeoclimates, especially the rapid climate changes during the Last Glacial-Interglacial Transition (LGIT ca. 15-8 ka BP), requires precise dating as well as the ability to link climate records over large regional areas. As has recently been demonstrated by the Eyjafjallajokull eruption, volcanic ash (tephra) has the possibility of a widespread geographical distribution which for geological purposes is instantaneous. Tephrochronology, the study of volcanic ash as a geochronological method, has over the past few decades developed methods to identify distal cryptotephras, tephras invisible to the naked eye. This has expanded the number of maker horizons as well as the areas in which they can be found. The studies presented here expand the geographical extent of one marker horizon, the Hasseldalen Tephra, in Southern Sweden. At the same time, some of the problems of the method are examined. The method builds on horizons being distinct and geochemically fingerprinted. The Borrobol Tephra has been found in Scotland and Sweden but with widely differing dates leading to the possibility of there actually being two geochemically identical horizons in close stratigraphic proximity. The present studies provide new results from the original type-site as well as reanalysis of the geochemistry of shards from the first Swedish findings. The major element analysis used here reflects improvements in Electron Probe Microanalysis (EPMA) settings which provides more robust data with slightly altered values from those previously published. These altered values must be taken into account when making inter-site comparisons. Trace element data for the Borrobol type-site and for the two original Swedish sites are also published. Results remain inconclusive as no differences were found between the Swedish and Scottish horizons. Even some problems with confusing the Hasseldalen Tephra with another horizon, the Askja-S Tephra, in Swedish sites are considered.