Evidence for a Waalian thermomer pollen record from the research borehole Heidelberg UniNord, Upper Rhine Graben, Baden-Württemberg

A pollen record from the borehole Heidelberg UniNord, in the northern Upper Rhine Graben shows evidence for one of the rare Waalian thermomers. The record may not comprise the complete thermomer it certainly refl ects a sort of succession, initiated by sequences with high values of pine and spruce, followed by a dominance of pine, hemlock-spruce and spruce and fi nished by a dominance of hemlockspruce, oak and hornbeam becomes visible. Pollen percentage values for Tsuga reach 20 % indicating that this taxon is an important forest element. There is a continuous presence of Carya, Pterocarya, Eucommia, Celtis and Ostrya-type. Neighbouring pollen records reveal similar patterns. The pollen diagram is closely related with the Waalian profi les Leerdam and Eindhoven from the Netherlands. [Nachweis der Waal-Warmzeit anhand pollenanalytischer Untersuchungen an der Forschungsbohrung Heidelberg UniNord, Oberrheingraben, Baden-Württemberg] Kurzfassung: Eine Pollensukzession aus der Bohrung Heidelberg UniNord im Oberrheingraben kann mit der bisher selten nachgewiesenen Waal-Warmzeit verknüpft werden. Das wohl nicht vollständig erfasste Thermomer refl ektiert eine angedeutete Sukzession, die von einer basalen Fichten-Kiefernzeit über einen Abschnitt mit Kiefer, Hemlocktanne und Fichte, hin zu einer abschließenden Waldzeit verläuft, in der Hemlocktanne (Tsuga), Eiche und Hainbuche dominieren. Tsuga erreicht im Gesamtdiagramm Werte bis zu 20 % und repräsentiert ein wichtiges Waldelement. Ständig vorhanden sind in geringen Anteilen ferner Carya, Pterocarya, Celtis, Eucommia und Ostrya-Typ. In Nachbarbohrungen deutet sich dasselbe Muster an. Das Pollendiagramm wird korreliert mit den Waal-Profi len aus Leerdam und Eindhoven (Niederlande).


Introduction
In order to develop a better understanding of the Pleistocene geological processes associated with the evolution of the Upper Rhine Graben (URG), a scientifi c core drilling has been brought down at the campus of Heidelberg University. In this location, one of the thickest successions of continental Quaternary deposits has been expected according to various pre-site surveys (BUNESS, GABRIEL & ELLWANGER 2008). For technical reasons, a fi rst borehole (Heidelberg UniNord1) has been cored down to 190 m in 2006, followed by a second borehole (UniNord2) down to 500 m in 2008. For a fi rst summary of geological results, cf. ELLWANGER et al. (2008); for an outline of the Heidelberg project cf. GABRIEL et al. (2008). In this paper, only results from the fi rst borehole (0 -190 m; UniNord1) have been evaluated. According to ELLWANGER et al. (2008), only about one third of the succession consists of fi ne-grained i.e. silt-dominated deposits. Out of these, only three intervals, altogether amounting to less than fi ve meters of sediment core, turned out to be suitable for a detailed pollen analysis (57 -58 m; 81 -87 m and 180 -181.70 m). The remaining 65 m of silt-dominated sediments were poor in pollen-content. This may be due to oxidation and differential preservation of the pollen, e.g. concentration of thick-walled grains such as conifers and Tilia, but also of poor core quality. The biostratigraphic review maybe summarized as follows: (1) At 57 -58 m and 82 -85 m depth a pinespruce forest indicating cool climate; (2) At 81 -82 m and at 87 m depth a middle Pleistocene pollen spectrum without Fagus indicates an interglacial environment is indicated; (3) At 180 -181.70 m depth some very pollen rich samples were found in peaty sediments with more or less silt. The higher pollen percentage values of the genus Tsuga (Hemlock spruce) indicates a temperate "interglacial" resembling an early Pleistocene age. This study is focused on the interval (3).

Methods
Peaty samples were taken at 10 cm intervals. In total, 17 pollen samples were analysed. Samples were subjected to HF (70 %) treatment followed by ultrasonic sieving (mesh size 6 x 8 μm) then acetolysis. With the exception of the two oldest samples, arboreal pollen counts of at least 500 were attained. The basis for representation on the pollen diagram was the total pollen sum (all pollen types excluding aquatics and ferns). The pollen diagram (Fig. 2) was prepared using the plotting program Tilia (GRIMM 1991). Non-arboreal taxa that were only encountered in few samples and in low numbers are not shown on the diagram. Values below 1 % are also represented (symbol "•").

Results
The pollen diagram ( Fig. 1)  Evidence for a Waalian thermomer pollen record types are of less importance and mostly below 10 %, with the exception of two samples with Filipendula (up to 30 %!) and an unknown, very small Liliaceae-type. Both elements represent the infl uence of local swampy vegetation.

Discussion
According to the implied succession of the signifi cant arboreal taxa, the pollen diagram ( Fig. 1) has been divided in three pollen assemblage zones. There are, however, no taxa, which are immigrating in the course of the sequence. All are present right from the beginning. Complete sequences of the immigration of tree species during an interglacial cycle (e.g. shrubs -birch -pine -broad leaf trees -climax zone -conifers) is well known from the Holocene, from the Eemian, and from middle Pleistocene thermomers (Holsteinian, Cromerian). These characteristics are also reported from Bavelian sequences (ZAGWIJN & DE JONG 1984). However, as indicated above, this type of succession is not recorded in the Waalian thermomer, the older Pleistocene (Tiglian thermomers) and the Reuverian.
The decrease of arboreal taxa during the Pleistocene is known from various studies. Pterocarya is ultimately recorded at the end of the Holsteinian (e.g. BEAULIEU & MONJUVENT 1985).
Eucommia and Celtis barely occupy interglacial I and II of the Cromerian-Complex, e.g. HOMANN & LEPPER (1994), HAHNE (1996a,b). VAN DER HAMMEN et al. (1971)  Tsuga was the fi rst taxon recognised as an element of the Older Pleistocene (e.g. by VAN DER HAMMEN et al., 1971). According to ZAGWIJN (1957ZAGWIJN ( , 1960 and HAHNE (1996a,b), this taxon is present in low levels in the Reuverian and in the Pretiglian as well as in the Tiglian A, B and C. KNIPPING (2004) (Fig. 1). All those are frequent in the Reuverian and, with the exception of Sciadopitys, very sparsely during Tiglian (ZAG- WIJN 1960WIJN , 1963WIJN , 1989. Some very first results of the lower parts of the Heidelberg UniNord borehole confi rm this trend , Fig. 3).

Conclusions
The pollen diagram (Fig. 1)  Pleistocene character is evident whereas a succession of the expansion of various trees due to differences in re-immigration emphasizes a strong resemblance with thermomers younger than Tiglian and Waalian. The pollen diagram (Fig. 1) shows signifi cant similarities with the Leerdam-Profi le (Fig. 2) which is the type locality for Waalian (ZAGWIJN & DE JONG 1984). The similarity is not restricted only to the spectra of the arboreal pollen types, but also in their values (with the exception of Picea, which in Leerdam has a smaller representation). Different with Leerdam is also the missing of indications of systematic suc- In Kronau, about 20 km south of Heidelberg, another borehole revealed cores of peaty sediments at depths around 85 m. Here several pollen samples reveal nearly identical Waalian pollen assemblages as in Heidelberg. ZAGWIJN (1989) noted that the Waalian generally has the character of an interglacial complex consisting of an interstadial situated between two thermomers. A similar sequence is recorded by MENKE (1976) for the "Tornesch thermomer" (Lieth series) situated in Schleswig-Holstein which is also assigned to the Waalian. Although it reveals a domination of Ericales (pointing toward Atlantic heaths) apart from Tsuga all Older Pleistocene elements are present. Running analysis on the cores of the UniNord2 borehole at Heidelberg reveal 26 m underneath the peaty Waalian sediments again typical Waalian pollen sequences which possibly could assigned to the older Waalian thermomer. The intermediate sediments are fl uvial gravels, which, unfortunately, are poor of pollen so that their climatic character is still uncertain. Even a separation within either the upper or the lower thermomer by fl uvial gravels, without any difference in climate, may be considered. The question remains why there are such great similarities in the forest vegetation between the Upper Rhine Graben and the Netherlands, whereas from Lieth (Schleswig-Holstein, Fig.2) a quite different forest vegetation and maritim infl uenced different climatic conditions has been reported by MENKE (1969MENKE ( , 1976. In our view, this indicates uniform climate conditions along the Rhine, and different conditions further east. Today the Rhenish Schist highlands act as a barrier between the Netherlands and the Upper Rhine Graben. In the Waalian stage, prior to their major uplift, the Rhenish Schist was an area of some low hills. In this scenario, there was an almost fl at landscape from Heidelberg to the Netherlands, with uniform forest vegetation and climate conditions (Fig. 2). However, K.-E BEHRE (Wilhelmshaven) has expressed another view (pers. comm.). The Neth-erlands situated in the lower Rhine area, were an excellent sediment and pollen catching area, including reworked pollen from further south.
Their pollen records refl ect the complete upper Rhine vegetation combined with the regional elements. On the other hand, the Lieth series (MENKE 1976) refl ects the regional vegetation of Holstein, maybe including input from the Baltic river system (Fig. 2). The chronostratigraphic age of the Waalian amounts to 1. By now, we also know that the thickness of the Quaternary at the UniNord location will amount to more than 500 m (well exceeding the originally assumed thickness of ~ 400 m). The Reuverian is expected well below 500 m.