Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems

• Published in: Climate of the past Vol. 16; no. 1; pp. 17 - 27
• Main Authors: Uemura, Ryu, Kina, Yudai, Shen, Chuan-Chou, Omine, Kanako
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 07-01-2020; Copernicus Publications
• Subjects: Analysis; Calcite; Calcite crystals; Calcium carbonate; Carbonate minerals; Carbonates; Climate; Composition effects; Evaluation; Fluid inclusions; Fractionation; Heat exchange; Heating; Hydroclimate; Hydrogen; Hydrogen isotopes; Isotope composition; Isotope fractionation; Isotope ratios; Isotopes; Laboratories; Oxygen; Oxygen isotope ratio; Oxygen isotopes; Paleotemperatures; Proxy; Quaternary; Quaternary climates; Rain; Rain water; Rainwater; Temperature changes; Water
• ISSN: 1814-9332; 1814-9324; 1814-9332
• DOI: 10.5194/cp-16-17-2020

The oxygen and hydrogen isotopic compositions of water in fluid
inclusions in speleothems are important hydroclimate proxies because they
provide information on the isotopic compositions of rainwater in the past.
Moreover, because isotopic differences between fluid inclusion water and the
host calcite provide information on the past isotopic fractionation factor,
they are also useful for quantitative estimation of past temperature
changes. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between the
water and the host carbonate. Thus, it is necessary to estimate the bias
caused by this postdepositional effect for precise reconstruction of
paleotemperatures. Here, we evaluate the isotopic exchange reaction
between inclusion water and host calcite based on a laboratory experiment
involving a natural stalagmite. Multiple stalagmite samples cut from the
same depth interval were heated at 105 ∘C in the laboratory from
0 to 80 h. Then, the isotopic compositions of the inclusion water were
measured. In the 105 ∘C heating experiments, the
δ18Ofi values increased from the initial value by
0.7 ‰ and then remained stable after ca. 20 h. The
hydrogen isotope ratio of water showed no trend in response to the heating
experiments, suggesting that the hydrogen isotopic composition of fluid
inclusion water effectively reflects the composition of past drip water. We
then evaluated the process behind the observed isotopic variations using a
partial equilibration model. The experimental results are best explained
by the assumption that a thin CaCO3 layer surrounding the inclusion
reacted with the water. The amount of CaCO3 that reacted with the water
is equivalent to 2 % of the water inclusions in molar terms. These results
suggest that the magnitude of the isotopic exchange effect has a minor
influence on paleotemperature estimates for Quaternary climate
reconstructions.