Source identification of sulphate forming salts on sandstones from monuments in Salamanca, Spain—a stable isotope approach

Background, aim and scope Salt efflorescences markedly contribute to the alteration and deterioration of building material, in this case the Villamayor Sandstone of the facades in the Old Town of Salamanca, Spain (United Nations Educational, Scientific and Cultural Organization world cultural heritage site). A better understanding of the mechanisms of salt formation and the involved elements would allow more precise measures in monument conservation. The magnesium which is required for the salt precipitation originates from selective processes of hydrolysis. The source of sulphate, however, is presently not as clear. Identifying the source of the sulphur was the main goal of this research. Isotope ratio measurement of δ³⁴S and δ¹⁸O was used to clarify the origins of Mg sulphate salts. Materials and methods A total of 56 Mg sulphate samples were collected in two different seasons (July and November 2005) from monuments of the Old Town of Salamanca. These sampled salt efflorescences were analysed for δ³⁴S and δ¹⁸O by mass spectrometry. A ‘dual-inlet' type by VG Isotech was used for δ³⁴S and continuous flow type Isoprime by GV Instruments for δ¹⁸O. Samples were measured in triplicates and standard material was analysed for quality control. Results δ³⁴S values range between 3.6‰ and 15.4‰ with a median value of 10.2‰ for the July samples and of 10.1‰ for November samples. The results of the sulphur ratios hint towards a bimodal distribution (with modes at δ³⁴S = 6‰ and 12‰) for winter samples, which is less obvious during summer. δ¹⁸O values range from 7.1‰ to 41.1‰. However, most values range from 7.1‰ to 20.8‰, whereas only few summer samples show outliers towards higher δ¹⁸O values. The median δ¹⁸O value for July samples is 15.5‰ and for November samples 14.6‰. Discussion The isotopic ratios of the analysed sulphate samples were compared with values of possible source materials. Sulphur sources in the case of Salamanca are barites from the Villamayor Sandstone itself, sea spray, sulphides from regional rocks, biogenic sulphur (soil, avian excreta), as well as sulphur from anthropogenic sources such as building materials (especially mortar) or traffic exhaust. Salamanca is a representative site for non-industrial cities with no heavy industry and thus, there are no significant SO₂ emissions from industry. Conclusions Based on the measured isotopic ratios, it was ascertained that more than one sole sulphur source is present. However, based on additional information about the source material and possible transport ways, some sources could be excluded whereas others only played a minor role. Finally, there is strong indication that the main sulphur source is atmospheric pollution and the exhaust emissions from vehicles in particular, while mortar as building material also contributes to a minor extent. The δ¹⁸O values support this hypothesis. Moreover, the reported δ¹⁸O values are a strong indicator of the secondary nature of the Mg sulphates. Isotope ratio measurement and especially the combined use of δ³⁴S and δ¹⁸O values have proven to be a good instrument in clarifying the origin of salt efflorescences on buildings. Recommendations and perspectives Further studies should investigate more closely the isotopic composition of atmospheric aerosols in Salamanca in order to get a more detailed knowledge about the main sulphur sources, as well as to quantify the relation between the isotopic values and the amount and mineralogical form of the salts.