Stable carbon and oxygen isotope investigation in historical lime mortar and plaster – Results from field and experimental study

Abstract

Abstract Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ 13 C matrix and δ 18 O matrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ 18 O matrix = 0.61 · δ 13 C matrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO 2 into alkaline Ca(OH) 2 solutions shows a similar relationship, δ 18 O calcite = 0.67 · δ 13 C calcite − 6.4 (VPDB). Both relationships indicate that the 13 C/ 12 C and 18 O/ 16 O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO 2 from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO 2 ( δ 13 C matrix ≈ −25‰ and δ 18 O matrix ≈ −20‰). As calcite formation continued the remaining gaseous CO 2 is subsequently enriched in 13 C and 18 O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H 2 O, e.g. evaporation, the source of CO 2 , e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for 14 C dating.