Pollenstratigraphische, vegetations- und klimageschichtliche Gliederung des „Bölling-Alleröd-Komplexes“ in Schleswig-Holstein und ihre Bedeutung für die Spätglazial-Stratigraphie in benachbarten Gebieten

Abstract

Pollen diagrams from the Late Weichselian period of Schleswig-Holstein can certainly be correlated and subdivided by means of characteristic changes in the frequency of Hippophaë, Helianthemum, Juniperus, Empetrum, Populus , and others (Figs. 2, 3). In contrast to previous assumptions, it now becomes apparent that the Late Weichselian vegetational development in all of Schleswig-Holstein has proceeded in a very uniform way. The discrepancies between the different diagrams, being considerable in part, mainly derive from differences in the degree of resolution and from different completeness of the underlying series of sediments. In Schleswig-Holstein, the first Late Weichselian climatic amelioration leads to shrub formations with Hippophaë, Betula nana, and Juniperus; Betula pubescens immigrates but remains very scarce ( Hippophaë-Betula nana pollen assemblage zone, see Fig. 2). A transient cooling results in a decrease of shrubs inclusively Betula nana while the Poaceae and Helianthemum increase overproportionally ( Helianthemum-Betula nana-paz ). A second and rather fast improvement of climate leads to a renewed increase of Juniperus and Hippophaë and, above all, to a rapid expansion of Betula pubescens to forests ( Juniperus-B. nana-B. pubescens -paz). Comparisons with diagrams from the Danish Islands together with the litho- and biostratigraphical records from the Alleröd type locality near Copenhagen (H artz & M ilthers 1901) make it quite plain that the afforestation by birches during this pollen assemblage zone marks the beginning of the Alleröd. (According to this, it appears obvious that the preceding Hippophaë-B. nana - and Helianthemum-B. nana -paz are to correlate with the Bölling pollen zone and the Older Dryas, respectively. However, this assumption should nevertheless be wrong; see below.) The stratigraphy of the Alleröd in Schleswig-Holstein is more complicated than has been assumed previously: Diagrams of a high resolution consistently show 3 more or less pronounced Betula minima, 2 Populus maxima, and 3 Juniperus maxima (Fig. 2). The interpretation of these facts points to (at least) 2 cooler phases during the Alleröd: the first being shortlived but distinct at the level of the 1st Betula minimum, and the second one at the level of the Populus minimum and 3rd Betula minimum. Pinus expanded in 2 phases; the main expansion obviously took place when the climate was already deteriorating at the end of the Alleröd. Bio- and lithostratigraphical comparisons of the pre-Alleröd and Alleröd subdivisions observed in Schleswig-Holstein and the adjacent countries (Fig. 3, 4) lead to the following results: (1) The development of the Alleröd in Schleswig-Holstein and the Danish Islands corresponds largely. Differences seem to be pretended again by different resolution and completeness of the diagrams. On the other hand, the diagrams from Lake Bølling/Middle Jutland and the vegetational development in Schleswig-Holstein and the Danish Islands can only be correlated in a satisfying way when the Bölling pollen zone, which was described from this locality by I versen (1942) as being older than the Alleröd, is considered to represent the first part of the Alleröd. Iversens Older Dryas should correspond to the 1st Betula minimum of the diagrams from the Danish Islands and Schleswig-Holstein. According to this, the Hippophaë-Helianthemum -pollen assemblage zone of Schleswig-Holstein, being certainly older than the Alleröd, most probably can not be connected with I versens Bölling pollen zone. — The cold phase of Alleröd-b, as recorded in older diagrams from Denmark, corresponds to the 2nd and 3rd Betula minimum and to the pollen assemblage zones 10—12 of the Alleröd subdivision for Schleswig-Holstein, respectively (Fig. 2). (2) The sections of South Swedish diagrams which are classified as Bölling belong to the Alleröd. But they probably are contemporaneous with I versens Bölling pollen zone. (3) In diagrams from the Netherlands and Lower Saxony, those sections which are considered to represent the Bölling and Older Dryas, corresponds in biostratigraphy and age to the Hippophaë-B. nana - and Helianthemum-B. nana -pollen assemblage zone of Schleswig-Holstein. Also in The Netherlands and in the lowland of Lower Saxony the climatic amelioration before the Alleröd culminated in shrub formations with only very limited presence of tree birches. (4) The sections of diagrams from Great Britain, attributed to the Bölling, belong to the Alleröd; the "Older Dryas" of these diagrams corresponds to the 2nd and 3rd Betula minimum and to the Alleröd-b, respectively, of the Schleswig-Holsteinian and the (older) Danish diagrams. Hence, in diagrams from Schleswig-Holstein, from The Netherlands and the lowland of Lower Saxony (but possibly also in some diagrams from the Danish Islands) there is evidence for a warming phase prior to the Alleröd. On the other hand, in diagrams from Southern Sweden and Great Britain (and most probably from the Lake Bølling, too), the sections which are classified as Bölling belong to the early Alleröd. The conclusions concerning the course of temperature during pre- and early Alleröd do not agree with assumptions which derive from measurements of oxygen isotopes concentrations in other regions. In contrast to this, the warming during the late Alleröd (more accurately: at the beginning of the Populus-Betula pubescens -pollen assemblage zone II; Fig. 2) seems also to be recorded by the δ 18 O-curves.