Timing of glacial - non-glacial stages in Finland: An exploratory analysis of the OSL data

The Optically Stimulated Luminescence (OSL) age database of Finland was established, and it includes all of the published OSL age results from different sediment sequences in Finland. The OSL database includes ~180 published OSL ages ranging from 235,000 years to 300 years; that is, from the Middle Saalian interstadial to the present. Two statistical clustering methods, K-means and model-based clustering with the package mclust, were used to analyze the internal structure of the assembled OSL data. The results of these analyses were also compared to the established Northwest European (Fennoscandian) chronostratigraphical stages. When the data were analyzed by the K-means method, the “right” number of clusters (K) was seven. The model-based clustering method (K = five) created bigger clusters for the youngest and the oldest ages compared to the K-means clusters. Both methods show that the ages followed the increasing trend from the youngest to the oldest, and the standard error of ages was constantly increasing except in the age group 70–115 ka, where the standard errors were exceptionally high. Seven clusters obtained from the age data corresponded relatively well with the number of stratigraphically established interglacials and interstadials in the late Middle and Late Pleistocene in Fennoscandia. The exceptionally large standard error of ages in the Early Weichselian age group 70–115 ka might result from mixing of heterogeneous and poorly or partly bleached mineral material from both Eemian and Early Weichselian sediment layers. The OSL-dated Middle Saalian interstadial sediments in the data support a strong stadial and interstadial variation also before the Late Pleistocene.