Stable isotopes as tracers of copper corrosion in the atmosphere and soil environment: a geographic study on brochantite and malachite formations on historic and archaeological material using δ18O, δ13C and δ2H

A refined methodology using light stable isotopes in precipitation to determine the formation location of copper minerals is tested, for the eventual use on archaeological bronzes that corroded in the soil environment. To do this, δ18O in historic brochantite and archaeological malachite is used to determine whether the region in which these common minerals formed may be identified when the δ18O is a function of the δ18O in meteoric water and temperature. The waters in each phase of the hydrologic cycle have distinct isotopic compositions and can yield information on climate or water provenance. The possibility that this δ18O study can be supplemented by including the hydroxyl-δ2H in brochantite or δ13C in malachite is explored as sub-studies. This study shows no significant difference between the δ18OSO4 in brochantite, and the δ18OSO4 variances are about equal among the cities considered. Furthermore, there is no statistically significant correlation between δ18OSO4 or hydroxyl-2H and the δ18O in precipitation, precipitation amount, pH, surface temperature, latitude, or age of deposit. About 50% of brochantite’s δ18OSO4 is sourced from local meteoric water. Of the three SO2 → SO4 pathways that contribute oxygen in brochantite, oxygen percentages can be calculated for two pathways at a time. The malachite data shows significant difference between the geographic groups surveyed, but does not suggest a statistically significant relationship between the δ18O or δ13C in malachite and the δ18O in local precipitation or surface temperature. A significant relationship is apparent between δ18O and δ13C with some clustering within sample sites. Therefore, a comprehensive study into the δ13C of malachite would be worthwhile. Climate-focused research also shows potential. This work can be used to advise on authenticity issues or guide in provenance studies when choosing between suspected sites. Further refinement is necessary before it may be adopted for cases without suspected formation sites.