image/svg+xml19 VI/1/2015 INTERDISCIPLINARIA ARCHAEOLOGICA NATURAL SCIENCES IN ARCHAEOLOGY homepage: http://www.iansa.eu Contemporaneousness of Trackway Construction and Environmental Change: a Dendrochronological Study in Northwest-German Mires Inke Achterberg a* , Andreas Bauerochse b , Thomas Giesecke a , Alf Metzler b , Hanns Hubert Leuschner a a Department for Palynology and Climate Dynamics, University of Goettingen, Untere Karspuele 2, 37073 Göttingen, Germany b Lower Saxony State Service of Cultural Heritage, Scharnhorststr. 1, 30175 Hanover, Germany 1. Introduction The northwest-German lowland changed in the millennia following the last glacial: from a periglacial wasteland to a forest-wetland mosaic, a landscape where the expanding mires eventually covered about one third of the area (Behre 2008; Metzler 2006). While the North Sea successively claimed the land between Denmark and England, pushing ground water levels up and maritime conditions further south-east, people bridged the spreading mires by wooden trackways, evidently since the early Neolithic (Metzler 2006). Finds of peat-preserved trackways are frequently reported for northwest Germany ( e.g. Metzler 2006), and also from Ireland (Raftery 1996) and southwest England (Coles and Coles 1992). Whether their construction (Behre 2005; Metzler 2003), or possibly their preservation (Spurk et al. 2002), might be related to environmental changes and climatic fuctuations is debated (Bauerochse 2003; Baillie and Brown 1996). In Ireland, the occurrence of fve “lulls” in trackway construction activity between the Neolithic and Modern Age was found to relate rather to cultural changes than to long-term hydrological variations (Plunkett et al. 2013). The present study, however, is focused rather on the precise alignment of individual constructions with the mostly short (decadal) phases of water table rise in northwest Germany. Indications for increased humidity in trackway layers have been repeatedly described – using pollen and peat analysis ( e.g. Bauerochse 2003; Leuschner et al. 2007). Whether the constructions were actually contemporaneous to or following such environmental change is investigated in this study using dendrochronology. This provides precise dating for both the trackway constructions and the mire water table rise. The latter is possible due to the large dendrochronological record of peat-preserved trees, originating from former mire (and mire-margin) woodland Volume VI ● Issue 1/2015 ● Pages 19–29 *Corresponding author. E-mail: iachter@gwdg.de ARTICLE INFO Article history: Received: 8 th August 2014Accepted: 19 th July 2015 Keywords: wooden trackwaysdendrochronologymireenvironmental changeHolocene climateNeolithic ABSTRACT Tree rings provide not only a precise dating tool, but also contain information on environmental change. The well replicated tree ring record of northwest Germany therefore provides environmental implications with immanent, absolute and precise dating from 6703 BC to 931 AD. This offers the opportunity to investigate the environmental context of archaeological fnds, if they, too, are dated by dendrochronology. We investigated 13 peat-preserved trackways from the Northwest-German lowland between 4629 BC (Neolithic) and 502 AD (Migration Period) for contemporaneousness with water table rise in the landscape. Such environmental change is well refected in the clearly notable die-off phases of trees preserved in the mires. As an environmental proxy, the parameter “ tree die-off rate a-30 ” is introduced: The annual number of tree die-off events is divided by the number of live trees 30 years previously. Overall, the majority of trackway constructions were found to be contemporane-ous to mire water table rise and mire expansion. Possibly, a period of water table rise was a motivation for trackway construction.
image/svg+xmlIANSA 2015 ● VI/1 ● 19–29 Inke Achterberg, Andreas Bauerochse, Thomas Giesecke, Alf Metzler, Hanns Hubert Leuschner: Contemporaneousness of Trackway Construction and Environmental Change: a Dendrochronological Study in Northwest-German Mires 20 in the study area. The tree-ring record consists of 4700 trees, oak ( Quercus spec . ) and pine ( Pinus sylvestris ), from the mires of the northwest-German lowland. The chronologies span from 6703 BC to 931 AD (at various stages described by, for example, Delorme 1983; Leuschner et al. 2002; Eckstein et al. 2011). The peat-preserved trees grew at sites strongly affected by hydrological change ( e.g. Schweingruber 1993; Linderholm 2002; Eckstein 2010). The trees show phases of woodland establishment, growth and collapse (Eckstein et al. 2011). These phases show much synchrony across different sites in the study area (Eckstein et al. 2011), and also with the Netherlands, Ireland (Leuschner 2002) and Southern Sweden (Edvardsson 2011). They therefore qualify as an indicator for environmental change in the region, which ought to be mostly climatically driven (Leuschner 2002). The phases of high tree mortality (die-off phases) have been identifed to indicate mire expansion and mire water table rise. This was evident on the basis of upward growing roots, the composition of peat forming plants and the degree of decomposition (Leuschner et al. 2002; 2007; Eckstein et al. 2009; 2010). Tree die-off phases are a good indicator for mire water table rise (Leuschner et al. 2002; 2007; Eckstein 2009; 2010), whereas tree-ring-width (TRW) has been found to refect hydrological changes not exclusively at central European mire sites (Dauskane et al. 2011). The meteorological implication of mire water table rise and mire expansion varies, but here, we focus on the timing of such landscape-level changes rather than their causes.This study investigates a possible correlation of trackway construction with mire water table rise and mire expansion. In the following, dendrochronological dates for wooden trackways are evaluated for contemporaneity to die-off phases of the peat-preserved trees from the area. 2. Material and methods2.1 The peat-preserved trees of northwest Germany The tree species regarded in this study only include oak ( Quercus spec.) and pine ( Pinus sylvestris ). The woodland remains were preserved in peat, and mostly exposed by peat-harvesting. A total of 2090 oak and 2610 pine trees from northwest-German mires, spanning from 6703 BC to 931 AD, form the environmental record. These remains are only tree stumps and stems; no timbers from human constructions, like the trackways or buildings, are included in the environmental record.The chronology of peat-preserved oaks cross-dates well with the Göttingen chronology of timbers from buildings and other constructions, which reaches back from the present (2009 AD) to 610 BC. The main sections of both (oak and pine) chronologies of peat-preserved tree remains cross-date well with the Göttingen chronology of riverine oak from central and northwest Germany, which covers from 7197 BC to 1136 AD. All sections of the peat-preserved pine chronology are securely cross-dated with the peat-preserved oak chronology.