Sulfur and iron analyses of marine archaeological wood in shipwrecks from the Baltic Sea and Scandinavian waters Yvonne Fors a, b , Farideh Jalilehvand c , Emiliana Damian Risberg a , Charlotte Björdal b , Ebba Phillips d , Magnus Sandström a, * a Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden b Department of Conservation, University of Gothenburg, SE-413 20 Göteborg, Sweden c Department of Chemistry, University of Calgary, Alberta T2N 1N4, Canada d Studio Västsvensk Konservering, Gamlestadsvägen 2-4, B2, SE-415 02 Göteborg, Sweden article info Article history: Received 23 December 2011 Received in revised form 5 March 2012 Accepted 10 March 2012 Keywords: Marine archaeological wood Shipwrecks Baltic Sea Sulfur Iron XANES Archaeometry Analyses abstract Analyses of marine archaeological wood from shipwrecks in the Baltic Sea area, Kronan, Riksnyckeln, Tattran, the Puck Bay Boat and the Ghost wreck, and at the Scandinavian West coast, the Göta wreck, Stora Sofia and the Viking shipwrecks of Skuldelev, show accumulation of sulfur compounds. The penetration profiles of sulfur and iron into the wood and the speciation of characteristic sulfur groups were evaluated by combining X-ray spectroscopic analyses, in particular S K-edge XANES (X-ray absorption near edge structure) and X-ray fluorescence, with ESCA and elemental analyses. The combined analyses support the hypothesis that hydrogen sulfide produced by sulfate-reducing bacteria reacts and accumulates at low iron concentration mainly as organically bound sulfur, which as in previous studies was found by X-ray spectro-microscopy to accumulate in lignin-rich parts of the wood cell walls. The presence of iron(II) ions from corroding iron promotes formation of pyrite and other iron(II) sulfides, which easily oxidise in aerobic conditions with high humidity. No significant differences in sulfur and iron accumulation were found in wood from shipwrecks in the east coast brackish water and the west coast seawater. Sediments from three wreck sites, the Göta wreck, Stora Sofia and Kronan, were analyzed to a depth of a few decimeters and showed especially at the Stora Sofia high sulfur concentrations, exceeding 3 mass%. S K-edge XANES analyses of the sediments showed mainly reduced forms of sulfur, in particular pyrite and iron(II) sulfides together with elemental sulfur. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Structurally intact wooden shipwrecks on the seabed constitute time capsules carrying important archaeological information. The Baltic Sea in particular is known for many such archaeological finds due to its favorable conditions for preserving organic material. The major reasons are primarily the low salinity of the brackish water that so far has prevented the notorious marine borer the great shipworm Teredo navalis, to make serious inroads into the Baltic, in combination with low water temperatures, darkness and low oxygen levels at the seabed (Pearson, 1987; Rönnby, 2001). However, even for a shipwreck appearing well-preserved in situ, the state of the waterlogged wood is important for its long-term preservation after salvage (Fors, 2008). The condition of the timbers, that is the degree and type of deterioration and contami- nation of the wood, depends on the local environment in the wreck site, e.g. sediment burial (MacLeod and Kenna, 1991; Pournou et al., 2001). Even at the low oxygen levels occurring at many marine seabeds or in sediments, bacterial activity may cause degradation and chemical contamination of the waterlogged wood (Kim et al., 1996; Björdal et al., 2000; Nilsson and Björdal, 2008; Fors et al., 2008). Sulfate-reducing bacteria generate hydrogen sulfide at the seabed by oxidizing simple organic molecules, but cannot degrade large biopolymers. However, erosion bacteria degrading water- logged wood (Fig. 1) leave behind organic debris that can be metabolized in the wood by scavenging sulfate-reducing bacteria (Fors et al., 2008). Sulfur accumulation then occurs via competing reactions with the hydrogen sulfide. Organic sulfur compounds, mostly thiols, form within lignin-rich parts of the exposed wood microstructure, while particles of inorganic iron(II) sulfides and pyrite develop in the presence of soluble Fe(II) ions from corroding iron (Sandström et al., 2005; Fors et al., 2008). * Corresponding author. Tel.: þ46 8 161252; fax: þ46 8 152187. E-mail address: magnus.sandstrom@mmk.su.se (M. Sandström). Contents lists available at SciVerse ScienceDirect Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas 0305-4403/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2012.03.006 Journal of Archaeological Science 39 (2012) 2521e2532