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

Abstract

Abstract. 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 post-depositional effect for precise reconstruction of palaeo-temperatures. 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 for 0–80 hours. 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 hours. 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 dripwater. We then evaluated the process behind the observed isotopic variations using a partial equilibration model. The experimental results are best explained when we assumed 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 palaeo-temperature estimates for the Quaternary climate reconstructions.

Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

“…The oxygen isotope exchange that we describe here was already determined experimentally for speleothem calcite by Uemura et al (2019), who found that δ 18 O fi values increased by 0.7‰ after heating at 105 °C for over 80 h. In our study, we report a comparable 0.6‰ increase in δ 18 O fi for speleothem calcite sample after heating at 175 °C for 90 minutes. The isotopic increase of δ 18 O fi in the single-crystal inorganic aragonite sample remained within statistical uncertainty.…”

“…Schematic representation of (a) a fluid inclusion in carbonate (i.e., liquid water trapped in carbonate) and (b) organic-associated internal water in carbonate (i.e., carbonate bound to inter-and intracrystalline organic material). Hypothetically, the gray rim that surrounds the fluid inclusion, the Carbonate Exchange Fraction (CEF), marks the location where (1) oxygen isotope (δ 18 O) exchange by microscopic dissolution-reprecipitation reactions occurs and (2) the clumped-isotope (Δ 47 ) thermometer is reset (modified fromUemura et al, 2019 andNooitgedacht et al, 2021).…”

“…Dry heating (or "roasting") experiments revealed that the transition to calcite requires more time and/or higher temperatures, but is nevertheless accompanied by an oxygen isotopic shift in the carbonate (Milano et al, 2018;Müller, Staudigel, et al, 2017;Staudigel & Swart, 2016). This isotopic shift, which occurs in absence of interaction with an outside medium, is believed to be caused by a set of exchange reactions that occur with internal fluids (Moon et al, 2020;Pederson, Weiss, et al, 2019;Uemura et al, 2019), and/or internal organic material (Li et al, 2020).…”

“…More recently, it was proposed that clumped-isotope resetting in gastropod fossils occurred at very low NOOITGEDACHT ET AL. Hypothetically, the gray rim that surrounds the fluid inclusion, the Carbonate Exchange Fraction (CEF), marks the location where (1) oxygen isotope (δ 18 O) exchange by microscopic dissolution-reprecipitation reactions occurs and (2) the clumped-isotope (Δ 47 ) thermometer is reset (modified from Uemura et al, 2019 andNooitgedacht et al, 2021).…”

An isotope ratio mass spectrometry‐based method for hydrogen isotopic analysis in sub‐microliter volumes of water: Application for multi‐isotope investigations of gases extracted from fluid inclusions

“…In the last two decades several reports have been published where inclusion water was extracted by thermodecrepitation or crushing. [25][26][27][28][29][30][31] However, all the presented devices and techniques are able to study only oxygen and hydrogen from water inclusions,…”