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Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO2 degassing. More recently, studies have shown that mass-47 and mass-48 CO2 from acid digested carbonate minerals (Δ47 and Δ48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO2, temperature) for (dis)equilibrium using Δ47-Δ48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ47-Δ48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ47-predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothemsamples, we find Δ47-Δ48 values consistent with CO2 degassing effects, however, the majority of sampleswith KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopicbuffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ47 and Δ48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ47 and Δ48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest averagedepartures from equilibrium (ΔΔ47 = − 0.032 ± 0.007, ΔΔ48 = − 0.104 ± 0.035, where ΔΔi is the measured value – the equilibrium value). This study shows how the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects.
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Delineating kinetic and equilibrium isotope effects in a central Sierra Nevada stalagmite using both the dual clumped isotopes (47 and 48) of carbonate and fluid inclusion stable isotopes
Stalagmites are an important archive of terrestrial climate information. However, there remains questions about the ability of stalagmites to form in oxygen isotopic equilibrium and thus record, in a simple manner, the oxygen isotopic composition and temperature of formational fluids. Recent studies have suggested that the combined application of 48 and 47 carbonate clumped isotope measurements can quantify the extent of kinetic isotope fractionation in stalagmites and thus used to correct for these kinetic isotope effects and solve for the original formation temperatures. Here we measure the 47 and 48 values from 16 different samples of the same stalagmite from central California that spans the deglaciation (11 to 20 kya). Each sample is replicated three to five times. We find that based on these measurements the extent of kinetic fractionation present in the carbonate from this stalagmite is minimal. The temperature calculated from 47 in this stalagmite ranges from 11.6 to 19.9 °C, in agreement with regional reconstructions of temperatures from 47 values of lake carbonates. In contrast, previously published the 18O and 2H values of inclusion fluids (Wortham et al., 2022) from this stalagmite suggest periods of increasing kinetic fractionation of the water isotopes at 13 and 15 ka. These periods have been previously interpreted to be times of a reduction in effective moisture regionally. We suggest by this comparison that the use of both water isotopes and the dual clumped isotope system in stalagmites can aide the interpretation of where kinetic fractionation occurs in the hydrologic and carbonate precipitation system in caves. We will discuss the work’s implications for paleoclimate records from stalagmites and other terrestrial systems in seasonally dry and Mediterranean regions.
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- Award ID(s):
- 1804262
- PAR ID:
- 10492528
- Publisher / Repository:
- The Geological Society of America
- Date Published:
- Journal Name:
- Abstracts Geological Society of America
- ISSN:
- 0435-3986
- Format(s):
- Medium: X
- Location:
- Portland
- Sponsoring Org:
- National Science Foundation
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