RADIOCARBON DATING THE BORROBOL TEPHRA IN SWEDEN AND THE DETECTION OF NEW EARLY HOLOCENE TEPHRAS

Analyses of two infilled lakes in Blekinge, south east Sweden indicate the presence of at least three tephra horizons of Termination 1 and early Holocene age. Geochemical analyses confirm the presence of the Borrobol Tephra, the Askja Tephra (10,000 ¹⁴C yr BP) and one previously unreported tephra of Icelandic origin. The relatively unknown Askja Tephra and the newly discovered Hässeldalen Tephra are stratigraphically placed at the Younger Dryas/Preboreal transition. The Borrobol Tephra is a recently identified ash layer that has been outlined as a potentially significant marker horizon for assessing the precise timing of the initial warming in different parts of Europe at the start of Greenland Interstadial 1. Until now, the distribution of this tephra was confined to Scotland and the Icelandic plateau. Extending the geographical limits to Scandinavia illustrates that the Borrobol Tephra is far more widespread than previously realised. This poster demonstrates the suitability and success associated with the extraction techniques for tracing micro-tephra horizons in areas distal to volcanic sources.

The age of the Borrobol Tephra, however, is poorly constrained with current radiocarbon age estimates from a site in northern Scotland and a marine record from the Icelandic plateau showing significant differences. New results obtained from a comprehensive radiocarbon dating exercise on terrestrial macrofossils from one of the sequences in south eastern Sweden are presented for the first time, enabling a rigorous underpinning of the age of the Borrobol Tephra. By wiggle matching the series of ages to the Cariaco Basin data-set, using a Bayesian probability approach we derive an age estimate of c. 13.9 Cariaco varve ka BP for this volcanic event which is considerably younger than previous age estimates. According to the Cariaco record, such an age estimate places this tephra within the early Allerød/GI-1c or the Older Dryas/GI-1d. A number of important questions are raised and outlined in relation to these new results, which require further investigation if tephrochronology is to be used successfully as a precise correlation tool during this time period.