Geoarchaeological studies on Roman time harbour sediments in Cologne — comparison of different OSL dating techniques

Abstract

Due to the construction of a new North-South subway in Cologne, Roman time harbour sediments were exposed and were sampled for luminescence dating. A very good independent age control was given by the precise knowledge of the chronology of Roman activity and by radiocarbon ages of charcoal samples. Hence, different methodological approaches within luminescence dating were applied for Holocene heterogeneously bleached fluvial samples and were compared to the known ages. For one sample, optically stimulated luminescence (OSL) dating was applied to coarsegrained quartz using a single aliquot regenerative-dose (SAR) protocol. After D e -measurements, different statistical approaches were tested (i.e. arithmetic mean, median, minimum age model, finite mixture model, leading edge method and the Fuchs and Lang approach). It is demonstrated that the Fuchs and Lang approach along with the leading edge method yielded the best matching OSL ages with respect to the known ages. For the other sample which showed feldspar contamination within the quartz signal, the post-IR blue stimulated luminescence (double SAR protocol) was measured in three different ways to calculate the D e -value: with continuous wave (CW) stimulation with an IR-bleach at 50°C and at 225°C for 100 s prior to the OSL, and pulsed OSL (POSL). It was demonstrated that the IR-stimulation at 225°C has very good potential to remove the feldspar signal contribution as well as pulsed OSL, but the former might deplete parts of the quartz OSL signal.

Pulsed IRSL: A stable and fast bleaching luminescence signal from feldspar for dating Quaternary sediments

“…Sample SW-II was taken from the Roman time harbour sediment in Cologne, Germany (Fig. 1), excavated by the construction works of the subway (Lauer et al, 2011a). The fluvial sediment at the former harbour of the Rhein contains broken ceramics and bricks from Roman time, and the harbour was known to have been used in the early first century (Dietmar and Trier, 2006).…”

Optically stimulated luminescence dating of young fluvial deposits of the Middle Elbe River Flood Plains using different age models

“…This is the opposite of the observation in Arnold et al (2009) where the leading edge model gave ages which are older than from minimum age models. In the study of Lauer et al (2011) fluvial sediments from a Roman harbor in Cologne (Germany) were investigated and different statistical approaches were used to calculate the OSL age. The method from Fuchs and Lang (2001) and the leading edge model from Lepper and McKeever (2002) gave results which are consistent with the expected historical age, whereas the result from the MAM-3 underestimates the expected historical age.…”

Fluvial response to climatic and anthropogenic forcing in the Moselle drainage basin (NE France) during historical periods: evidence from OSL dating

“…It is likely that the lower sandy layer (unit S2a), which exhibits cross-bedding features, relates to a braided system which developed locally in the first half of the 18th century. However, more research, and in particular the recognition of the facilities of the harbour, is required to confirm this hypothesis and to provide a better understanding of the relationships between fluvial evolution and past human occupation, as was the case for the harbour of Cologne (Lauer et al, 2011). The low thickness of the section demonstrates that the Liegeot channel was not the main one of the Moselle.…”

“…As the Roman harbour of the city is located a few kilometres downstream, this incision may be explained by a human-induced headward erosion wave following the dredging in the harbour area. However, more research, and in particular the recognition of the facilities of the harbour, is required to confirm this hypothesis and to provide a better understanding of the relationships between fluvial evolution and past human occupation, as was the case for the harbour of Cologne (Lauer et al, 2011). In the latter area, OSL dating was performed on sandy flood deposits that were embedded in fine-grained material hosting charcoal and Roman artefacts like broken bricks and wood pieces, which provided an excellent independent age control (by dendrochronology and radiocarbon dating).…”

“…After exclusion of the aliquots which did not fit the rejection criteria, the equivalent dose was calculated using different parameters which are commonly applied to fluvial sediments (Table III; Rodnight et al, 2006;Lauer et al, 2011): the weighted mean, the Central Age Model, and the Minimum Age Model (Galbraith et al, 1999;Arnold et al, 2007). The selection of the model was based on the skewness and overdispersion parameters (Table III) and with the equivalent dose distributions on radial plots following Thomas et al (2005).…”