THE INFLUENCE OF SOIL MOISTURE, TEMPERATURE AND OXYGEN ON THE OXIC DECAY OF ORGANIC ARCHAEOLOGICAL DEPOSITS* J. HOLLESEN 1,2 † and H. MATTHIESEN 1 1 Department of Conservation and Natural Sciences, National Museum of Denmark, I.C. Modewegsvej, Brede, 2800 Lyngby, Denmark 2 Center for Permafrost (CENPERM), Department of Geosciences and Natural Resource Management (IGN), University of Copenhagen. Øster Voldgade 10, 1350 Copenhagen K, Denmark The sensitivity of organic-rich archaeological layers at Bryggen in Bergen, Norway, to changes in soil temperatures, water contents and oxygen concentrations is investigated. This is done by linking measurements of oxic decay at varying temperatures and water contents with on-site monitoring data using a one-pool decomposition model. The results show that the model can be used to elucidate the current in situ decay and to evaluate where and when the decay takes place. Future investigations need to include long-term incubation experiments and decay studies at zero or very low oxygen contents in order to improve the robustness of predictions. KEYWORDS: IN SITU PRESERVATION, DECAY RATE, ENVIRONMENTAL MONITORING, DECOMPOSITION MODEL, BRYGGEN, NORWAY INTRODUCTION The policy of in situ preservation has been politically galvanized by the Valletta Convention (1992), as an ethical and sensible way of preserving archaeological remains. However, this policy implies that the archaeological deposits lie safely or can be protected with no, or only insignifi- cant, decay processes under way. The strategy for preserving organic remains is often to keep the remains in an oxygen-free environment below the groundwater table. But maintaining a high groundwater table can be very difficult, especially in urban areas where drainage and a reduced infiltration capacity influence the soil-water content (Holden et al. 2006; de Beer et al. 2008; Reed and Martens 2008). There have been numerous discussions on the effect of urban devel- opment on the preservation of archaeological layers (Davis et al. 2001; Kenward and Hall 2004; Reed and Martens 2008; Loska and Christensson 2012; Martens et al. 2012; Sidell 2012), but so far an important knowledge gap exists with regard to the sensitivity of organic-rich archaeolo- gical layers to changes in the soil environment on a quantitative scale. At Bryggen, in the old part of the city of Bergen in Norway, the buildings are currently settling by up to 6–8 mm year −1 (Christensson et al. 2008). These high settling rates are considered a direct consequence of a lowered groundwater table that has led to physical compression of the soil layers due to reduced pore-water pressure, increased the availability of oxygen in the soil and increased the decay of the organic archaeological deposits. However, investigations from natural soils (Fang and Moncrieff 2001; Elberling 2003) have shown that the oxic decay of soil organic carbon is also influenced by soil temperatures and often increases exponentially with increasing temperature. With soil temperatures in some areas of Bryggen being >5°C higher than the mean *Received 1 November 2013; accepted 3 February 2014 †Corresponding author: email joergen.hollesen@natmus.dk Archaeometry ••, •• (2014) ••–•• doi: 10.1111/arcm.12094 © 2014 University of Oxford