Middens, currents and shorelines: complex depositional processes of waterlogged prehistoric lakeside settlements based on the example of Zurich-Parkhaus Opéra (Switzerland)

Circumalpine lakeside settlements have been declared Unesco world heritage in 2011. Their importance is mainly due to waterlogged preservation of organic material and hence the outstanding potential of these sites for performing detailed archaeological studies of prehistoric societies. However, the details of the taphonomic processes (depositional environment, development of anoxia, lateral or vertical displacement of objects, etc.) have rarely been studied. Consequently, interpretations based on find distributions or comparisons of find densities remain difficult. Zurich-Parkhaus Opéra is a large-scale excavation of waterlogged Neolithic settlement deposits. Eight settlement phases of the late fourth and early third millennium BC were documented and dated using dendrochronology as well as six settlement layers, two of which showed excellent organic preservation. Based on a large number of sediment samples we conducted a multidisciplinary study in taphonomic processes influencing these layers. Our results indicate that a multi-indicator approach can provide detailed information on formation processes of waterlogged cultural layers. We found that 1) aquatic invertebrate remains and geotechnical calculations gave evidence for continuous shallow water conditions and eutrophic/anoxic deposition environment during occupation of the site. 2) Position and distribution of finds and loam patches indicate that disposal of household waste was focused on middens, which were still intact. 3) High variability in sediment contents (both spatially and in terms of state of preservation) is due to different factors such as oxygen depletion, deposition rate, erosion and enrichment of different materials, while all factors can affect each other resulting in highly complex formation processes.