Were there two Borrobol Tephras during the early Lateglacial period: implications for tephrochronology?

Abstract

A series of AMS radiocarbon measurements of terrestrial plant macrofossil remains are presented from a lake sequence in southernmost Sweden in an attempt to constrain the age of the Borrobol Tephra. This recently identified ash layer has been frequently suggested 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 (GI-1). Two different methods are adopted in order to derive a calendar age estimate for this event. An age of ca 13,900 Cariaco varve yrs BP is derived from a visual wiggle match of the age series to the Cariaco Basin data-set. The second approach is based on Bayesian probability analysis and constrains the age of the Borrobol Tephra in southern Sweden to between 13,800–14,450 and 13,667–14,331 cal yrs BP (95% confidence), using the Cariaco and Lake Suigetsu records as calibration data-sets, respectively. These new age-depth models, together with the pollen stratigraphy, suggest that the Borrobol Tephra as found in southern Sweden falls within the late Older Dryas/GI-1d or the early Allerød/GI-1c, whereas other European sequences indicate that this tephra falls close to the GS-2/GI-1 transition. Whether or not this indicates that the different occurrences of the BT in northern Europe are coeval is discussed and the implications that arise for the application of tephrochronology during this period are outlined.