‘Cosmogenic 10 Be ages on the Pomeranian Moraine, Poland’: Comments

Be cosmogenic nuclide dating of the Pomeranian end moraine system is taken as an indication of a ‘Bølling age’ for the last major glacial invasion into the western Baltic basin and the northern Polish lowland (Rinterknecht et al. 2005). This reasoning is incompatible with other well-established chronologies on Late Weichselian ice-marginal stages and the migration of Lateglacial biota. Rinterknecht et al. (2005) set up a scenario where an ice stream moving through the Baltic reached 200 300 m above the present sea level to give the Pomeranian end moraines in northern Poland and Germany their final shape at c. 14.8 thousand years (kyr) ago. This would imply a glacier thickness in the western Baltic basin of at least this magnitude at onset of the Bølling. Despite published evidence indicating that the deglaciation in southern Scandinavia had been proceeding for several millennia, the Pomeranian ice advance should accordingly have commenced retreat, while subarctic shrub tundra and open birch forests existed in larger parts of the southern circum-Baltic region and active glacier ice was confined to the south Swedish uplands. Because the focus of the article aims solely at introducing ‘the first direct dating of the southern margin of the Scandinavian Ice Sheet in Poland’, we find ourselves compelled to expose a regional perspective on the authors’ conclusions. Cosmogenic nuclide dating has been applied to boulders from the surface of the Pomeranian End Moraines in northern Poland. Eight dates from sites in the Odra Oder Lobe in western Poland (Fig. 1) yielded ages ranging from 18 to 11 Be kyr, with a mean of c. 14.39/0.8 Be kyr, while 19 dates from the Mazury and Suwalki lobes in northeastern Poland taken together give 15.09/0.5 Be kyr. The average age of 14.89/0.4 Be kyr is used as evidence to imply that the entire western Baltic basin was occupied by an ice stream at the onset of the Lateglacial warming that characterizes the Bølling period. This climatostratigraphic zone (Iversen 1954, 1973) is referred to as the Bølling Chronozone (B/13.0 12.0 C kyr; Mangerud et al. 1974), and corresponds to the GI-1e event in the Greenland ice cores 14.7 14.0 cal. kyr ago (Björck et al. 1998). Interpretation of these young cosmogenic exposure ages has wide implications for the understanding of the glacial and biogeographic development during the deglaciation of the last Scandinavian Ice Sheet in the Baltic Basin. However, we find it distressing that the authors neglect affluent evidence which is out of line with their conclusions. For example, Iversen (1954, 1973), Berglund (1979), Usinger (1985) and Stankowska & Stankowski (1988) have long since shown that pioneer biota had migrated into the region during the Oldest Dryas and that subarctic shrub tundra was established during the subsequent climatic amelioration of the Bølling period. The application of a new and promising dating method, the strengths and weaknesses of which are not fully acknowledged (Walker 2005), needs careful handling, and conclusions on for example the age of glacial morphological features must be tested against other independent dating methods. We therefore find it surprising that Rinterknecht et al. (2005) avoid discussing their results and conclusions in the light of previous works on the age of moraine formation and deglaciation dynamics in the southern Baltic region. The authors disregard the consequences such a new age estimate has for reconstructing the biotic and palaeogeographic setting during the Lateglacial period in the region. In addition, they fail to discuss a possible subsequent revision of the already established timing of the Lateglacial development. The latter is based on numerous radiocarbon-dated plant remains, pollenstratigraphic correlations, the Swedish varve-chronology and palaeomagnetic records; data that are readily available from the scientific literature published in recent decades. With this note, we take the liberty of providing readers of Boreas with input to such a discussion.