On the accuracy of lichenometric dates: an assessment based on the 'Little Ice Age' moraine sequence of Nigardsbreen, southern Norway

Abstract

Families of lichenometric dating curves are constructed using four historically-dated moraine ridges on the glacier foreland of Nigardsbreen (Jostedalsbreen ice cap), southern Norway. A further 20 'Little Ice Age' moraine ridges are dated in terms of median-predicted dates derived from the families of curves. Four approaches are used to assess the accuracy of the median-predicted dates: (1) the extent to which the dates form a temporal sequence; (2) the extent to which they agree with local independent evidence of glacier variations; (3) the extent to which dates agree when the moraine sequences on the two sides of the glacier foreland are dated separately; and (4) the extent to which they agree when section Rhizocarpon is differentiated from lichens of the Rhizocarpon subgenus as a whole. It is concluded that under conditions favourable for lichenometric dating, median-predicted dates have attained an accuracy of ~10% on the 'Little Ice Age' timescale. Realistic error limits are considered to be of the order of ±20 years for the oldest moraines and ±5 years for the youngest. Previous dates for the Nigardsbreen moraine sequence derived from single lichenometric dating curves may be ~5% less accurate. It is suggested that the range of predicted dates obtained from families of lichenometric dating curves can be used as a conservative surrogate for accuracy.