The urban sediments of Karlsplatz, Vienna (Austria) as a candidate Auxiliary Boundary Stratotype Section and Point for the Anthropocene series

Anthropogenic strata form the layered urban archive in the underground of large cities. In a transdisciplinary project involving geosciences, isotope physics and urban archaeology, we looked for artificial isotopes and anthropogenic trace metals. The tested archaeological site Karlsplatz is situated in the city of Vienna (Austria). Archaeology and historical data set provide age constraints around 1922, post-1945 and at 1959. A layer on top of the WW2 rubble that covers foundations of a 1922 building post-dates 1945, and pre-dates the levelling of the artificial park ground in 1959. The fine-grained sediment matrix of these layers is mixed with backfilled soil material. Samples were analysed for trace elements such as lead, copper and zinc, and prepared for chemical separation of actinides analysed by Accelerator Mass Spectrometry (AMS). Several artificial radionuclides, including the plutonium and uranium isotopes 239Pu, 240Pu and 236U, were found in the post-1945 layer, and the 240Pu/239Pu isotope ratio clearly points to the presence of atmospheric atomic bomb fallout material of the 1950s. Thus, the bomb-spike can be identified and used as a primary stratigraphic marker even in coarse urban anthropogenic sediments, exemplifying the correlation potential of these radionuclide markers and demarking a correlative auxiliary stratotype section and point.