Multiple oscillations during the Preboreal as recorded in a calcareous gyttja, Kingbeekdal, The Netherlands

Abstract

Environmental changes, occurring during the Weichselian Lateglacial to Early Holocene transition, are supposed to be caused by rapid climate changes. Vegetation changes that occurred during the Early Holocene show a pattern of increased boreal forest development with dominance of birch (Friesland phase) to a more open vegetation with an increase of predominantly grasses (Rammelbeek phase), subsequently followed by renewed birch and, later on, pine forest development. Based on palynological evidence and botanical macrofossils, the Rammelbeek phase is supposed to have been caused by a change to dry, rather than cold climate. Detailed 14C-dating, using a wiggle-matched AMS- 14C chronology, is used to place the vegetation changes in a time-stratigraphic framework. The Rammelbeek phase can be placed to around 11.3 ka cal BP. A direct correlation to the 11.2 ka event, as recorded in the Greenland ice-cores, is tempting. In this paper we propose an alternative approach by using a common proxy registered in both ice-core and terrestrial records. Based on oxygen isotopes, a Preboreal oscillation (PBO) appears to be present in many Early Holocene lacustrine carbonate-rich records. We used the δ 18O signal of calcareous lake deposits (Kingbeekdal, Southern Netherlands), in which biostratigraphically a Rammelbeek phase is present. The signal in the stable isotopes seems to correlate to the Greenland ice-core records. This exercise shows that the PBO as recorded in the oxygen isotopes occurs not during the palynologically defined Rammelbeek phase but early in the Friesland phase. Moreover, the palynological record of the Kingbeekdal sequence shows, at the level of the δ 18O reversal, a distinct opening of the birch forest and a temporary disappearance of thermophilous taxa, while the light requiring juniper increases. This implies a more complex pattern of climate response registered by the different proxies as previously thought.