Anthropogenic origin of salt crusts on sandstone sculptures of Prague's Charles Bridge (Czech Republic):: Evidence of mineralogy and stable isotope geochemistry

A prestigious Czech national monument - the medieval Charles Bridge in the historic city centre of Prague - is enclosed with sculptures carved from local sandstones. The stone exhibits varying weathering phenomena that are manifested mostly by surface blackening and local exocrust formation. Influences of the original stone composition and of atmospheric pollution on the formation of exocrust were evaluated using detailed mineralogical study and geochemistry of stable isotopes. The studied exocrusts are gypsum-dominated. Sulphur and oxygen isotope data (delta(34)S sulphate values range from +3.3 to +7.5 parts per thousand CDT with average +4.8 parts per thousand CDT from 15 measurements; delta(18)O values range from +6.4 to +9.7 parts per thousand SMOW with average +8.3 parts per thousand SMOW from 8 measurements), on sulphate, and absence of suitable sulphur source in the stone itself, strongly support anthropogenic atmospheric source of sulphur. Presence of other minor sulphates (of zinc, ammonium, magnesium, and potassium) can be explained by complex restoration and conservation treatments of the sculptures in the past. The sculptural stone - quartz sandstone derived from Cretaceous sedimentary cover of the Bohemian Massif - is deficient in both calcium and other elements forming the sulphate crusts. The results of this study show that some chemicals used for the restoration of the stone sculptures can exacerbate stone weathering especially when applied in heavily polluted urban areas.