Analyses of sulfur and iron in marine-archaeological wood

Abstract

Following the discovery of high acidity caused by oxidation of reduced sulfur compounds accumulated in the timbers of the Swedish 17 th century warship Vasa, we have analysed cores from several shipwrecks worldwide with a variety of techniques.To obtain detailed profiles of how the total sulfur and iron concentrations vary with depth in the wood, line scans along the cores with an x-ray fluorescence technique is introduced, using condensed and focused high intensity x-rays and CCD detection of the excited x-ray fluorescence.At intervals along the cores sulfur K-edge X-ray near edge absorption spectroscopy (XANES) provides speciation of the sulfur compounds, while multielement analyses with x-ray photoelectron spectroscopy (ESCA) is used to follow the variation of sulfur in reduced and oxidised forms, the uniform penetration of borate, and the distribution of carbon in CH 2 and CO species as an indicator of cellulose degradation.Scanning electron microscopy (SEM), combined with energy dispersive x-ray spectroscopy (EDS), reveals the sulfur and iron distribution in the cell walls and in particles in the wood.The sulfur species originate from hydrogen sulfide, which is produced in anoxic conditions from sulfate ions in seawater by sulfatereducing bacteria.The hydrogen sulfide probably reacts with active groups in ligninrich parts of the wood forming organosulfur species, or with iron ions to iron sulfides and elemental sulfur.High accumulation of such reduced sulfur compounds in the wood seems to be coupled to the bacterial degradation of the cellulose, and may result in appreciable sulfur concentrations in waterlogged wood from shipwrecks.The iron and sulfur profiles sometimes follow each other for the Vasa, indicating formation of iron-sulfur compounds, but not for the Mary Rose.The Bremen Cog, which was preserved in river water, has low sulfur and iron concentration in samples from the hull timbers.Implications of the results for modified conservation procedures are discussed.

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