Controls on the stable isotopic composition of speleothems, Barbados, West Indies

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Mickler, Patrick John

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Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to most correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothem calcite. This new approach directly compares the δ18O and δ13C values of modern speleothem calcite formations with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. Carbon isotope values for the majority of modern speleothem calcite samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of 1) calcite-water fractionation factors from the literature, 2) measured temperatures, and 3) measured δ13C values of the dissolved inorganic carbon of drip waters. Enrichments and depletions in 13C, relative to equilibrium C isotopic compositions, are also observed. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3- (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3- (aq) reservoir during degassing of 13C-depleted CO2. In contrast to the C isotopic results, most modern speleothem calcites from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip water and are enriched in 18O relative to equilibrium values. δ18O variations of modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3- (aq) and CaCO3 from inferred equilibrium conditions to non-equilibrium conditions, consistent with rapid speleothem calcite growth. In contrast to δ13C, the effects of Rayleigh distillation on the δ18O values of modern calcite are buffered by CO2 hydration and hydroxylation reactions that influence the O isotopic composition of the HCO3- (aq) reservoir. The effects of Rayleigh distillation manifest themselves in samples taken along a growth layer by producing a progressive enrichment away from the growth axis with a constant δ13C vs. δ18O slope. This observation has significance to ancient speleothem studies. A review of the literature has found that 62% of 141 studies show positive δ13C vs. δ18O correlation consistent with our non equilibrium models.