Condensation corrosion alters the oxygen and carbon isotope ratios of speleothem and limestone surfaces

Abstract

Condensation corrosion is a natural process which enhances the chemical weathering of limestone cave chambers and speleothems. We evaluated the use of carbonate tablets for detecting condensation corrosion in Glowworm Cave, New Zealand, using local limestone and speleothem as experimental substrates (herein tablets). Evidence for condensation corrosion was assessed via three methods: gravimetric (mass wasting), microscopic (surface pitting, recrystallization) and isotopic (δ13C and δ18O changes). Our results show little evidence of tablet mass loss throughout a 6-month deployment period. However, SEM imaging and isotope analysis (δ13C and δ18O) of the upper ~50 μm layer of the tablets, suggest that condensation corrosion operates in the cave, especially in sectors affected by large diurnal microclimate variations.Most notably, condensation water altered the tablet surface δ13C and δ18O values. Small, positive shifts in surface δ13C and δ18O values are considered to reflect pure dissolution (where dissolution favours the removal of lighter isotopologues). In contrast, tablets that exhibited large positive shifts in δ13C in tandem with large negative shifts in δ18O values, are interpreted as showing calcite recrystallization and the inheritance of higher DIC δ13C values (13C fractionation by CO2 degassing), lighter water δ18O values and/or kinetic fractionation of δ18O. This study therefore demonstrates that stable isotopes could be applied to detect paleoclimatic episodes of condensation corrosion in speleothems.