Soluble salt sources in medieval porous limestone sculptures: A multi-isotope (N, O, S) approach

Abstract

International audienceThe sources and mechanisms of soluble salt uptake by porous limestone and the associated degradation patterns were investigated for the life-sized 15th century "entombment of Christ" sculpture group located in Pont-à-Mousson, France, using a multi-isotope approach on sulfates (delta34S and delta18O) and nitrates (delta15N and delta18O). The sculpture group, near the border of the Moselle River, is within the potential reach of capillary rise from the alluvial aquifer. Chemical analyses show a vertical zonation of soluble salts with a predominance of sulphates in the lower parts of the statues where crumbling and blistering prevail, and higher concentrations of nitrates and chloride in the high parts affected by powdering and efflorescence. Isotope fingerprints of sulphates suggest a triple origin: (1) the lower parts are dominated by capillary rise of dissolved sulphate from the Moselle water with characteristic Keuper evaporite signatures that progressively decreases with height; (2) in the higher parts affected by powdering the impact of atmospheric sulphur becomes detectable; and (3) locally, plaster reparations impact the neighbouring limestone through dissolution and re-precipitation of gypsum. Nitrogen and oxygen isotopes suggest an organic origin of nitrates in all samples. N isotope signatures are compatible with those measured in the alluvial aquifer of the Moselle River further downstream. This indicates contamination by sewage or organic fertilizers. Significant isotopic contrasts are observed between the different degradation features depending on the height and suggest historical changes of nitrate sources

Investigations on historical monuments’ deterioration through chemical and isotopic analyses: an Italian case study

“…Moreover, we also detected the formation of the same salts, with a similar isotopic delta value (8.6 ± 0.2‰), inside the ancient Palazzo Fruscione which is located on the north side of S. Pietro a Corte. Values of δ 15 N consistent with the degradation of organic substances have been previously reported in other ancient architectural sites: in the "entombment of Christ" sculpture group located in Pont-à-Mousson, France (Kloppmann et al 2014), the nitrogen stable isotope ratio ranges from + 7.3 to + 9.5‰ suggesting an organic origin of nitrates, derived from animal waste contamination of the alluvial aquifer; the nitrate efflorescences on the sculptures of Burgos Cathedral, Spain (Gázquez et al 2015), present a value δ 15 N of +15.4‰ related to the anthropic pollution of the groundwater caused by farming and sewage leaks.…”

“…In the field of cultural heritage, stable isotope ratio was successfully utilized to discover the origin of gypsum-rich black crusts developed on granites (Rivas et al 2014 ) and sulphates and nitrate efflorescences in sandstones from monuments (Schweigstillová et al 2009 ; Schleicher and Recio Hernández 2010 ) and porous limestone sculptures (Kloppmann et al 2014 ; Gázquez et al 2015 ).…”

Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ<sup>33</sup>S reservoir of sulfate aerosols?

“…Anthropogenic sulfur was found to be the major source contributing to monument degradation in several localities compared to marine or volcanic sulfate sources (Longinelli and Bartelloni, 1978;Montana et al, 2008Montana et al, , 2012Torfs et al, 1997). Sulfates from the host rock, i.e., plaster, mortar, or oxidized pyrite (defined as intrinsic in the literature; Klemm and Siedel, 2002;Kloppmann et al, 2011;Kramar et al, 2011;Vallet et al, 2006), and sulfates from aquifer rising by capillarity (Kloppmann et al, 2014) were also identified as sulfur sources in black crusts. Black crusts being sometimes the host of microbial activity (Gaylarde et al, 2007;Sáiz-Jiménez, 1995;Scheerer et al, 2009;Schiavon, 2002;Tiano, 2002), other studies investigated the role of bacteria in gypsum formation through sulfate reduction and/or SO 2 oxidation (Tiano, 2002;Tiano et al, 1975).…”

Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative ∆<sup>33</sup>S-reservoir of sulfate aerosols?

“…. Intrinsic sulfates, that are plaster, mortar or oxidized pyrite (Klemm and Siedel, 2002;Kloppmann et al, 2011;Kramar et al, 2011;Vallet et al, 2006) and sulfates from aquifer rising by capillarity (Kloppmann et al, 2014) were also identified as sulfur sources in black crusts. Black crusts being sometimes the host of microbial activity (Gaylarde et al, 2007;Sáiz-Jiménez, 1995;Scheerer et al, 2009;Schiavon, 2002;Tiano, 2002), other studies investigated the role of bacteria in gypsum 85 formation through sulfate reduction and/or SO 2 oxidation (Tiano, 2002;Tiano et al, 1975).…”