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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This content will become publicly available on May 31, 2024
Frontiers of Carbonate Clumped Isotope Thermometry
Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δiis a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δivalues measured by mass spectrometry (Δ47, Δ48) or laser spectroscopy (Δ638) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δivalues that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals. ▪ Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments. ▪ Mature analytical methods enable carbonate clumped Δ47, Δ48, and Δ638measurements to address diverse questions in geological and biological sciences. ▪ These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.
more » « less- Award ID(s):
- 2040716
- NSF-PAR ID:
- 10488201
- Publisher / Repository:
- Annual Reviews
- Date Published:
- Journal Name:
- Annual Review of Earth and Planetary Sciences
- Volume:
- 51
- Issue:
- 1
- ISSN:
- 0084-6597
- Page Range / eLocation ID:
- 611 to 641
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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