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Abstract Historically, clumped isotope thermometry (T(∆47)) of soil carbonates has been interpreted to represent a warm‐season soil temperature based dominantly on coarse‐grained soils. Additionally, T(∆47) allows the calculation of the oxygen isotope composition of soil water (δ18Ow) in the past using the temperature‐dependent fractionation factor between soil water and pedogenic carbonate, but previous work has not measured δ18Owvalues with which to compare to these archives. Here, we present clumped isotope thermometry of modern soil carbonates from three soils in Colorado and Nebraska, USA, that have a fine‐to‐medium grain size, contain clay, and are representative of many carbonate‐bearing paleosols preserved in the rock record. At two of the three sites, Briggsdale, CO and Seibert, CO, T(∆47) overlaps with mean annual soil temperature (MAST), and the calculated δ18Owoverlaps within uncertainty with measured δ18Owat carbonate bearing depths. At the third site, in Oglala National Grassland, NE, mean T(∆47) is 8–11°C warmer than MAST, and the calculated δ18Owhas a significantly higher isotope value than any observations of δ18Ow. At all three sites, even in the fall season, δ18Owvalues at carbonate bearing depths overlap with spring rainfall δ18Ow, and there is little to no evaporative enrichment of δ2Hwand δ18Owvalues. These data challenge long‐held assumptions that all pedogenic carbonate records a warm‐season bias, and that δ18Owat carbonate‐bearing depths is affected by evaporative enrichment.
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Determination of KGa‐1b and SHCa‐1 Δ′ 17 O and δ 18 O via Laser Fluorination of Lithium Fluoride Clay Pellets
ABSTRACT RationaleStable oxygen isotope measurements in silicate clays, such as smectite and kaolinite, provide crucial information for understanding Earth's climate history and environmental changes. Despite a growing interest in the oxygen isotope analysis of silicate clays and clay‐rich sediments, there lacks a consensus on the preparation and standardization of clay mineral samples. To improve the accuracy and interlaboratory comparisons of clay isotope measurements, especially those involving laser fluorination techniques, newly established kaolinite and smectite oxygen isotope standards are much needed. MethodsWe employed conventional nickel bomb fluorination combined with dual‐inlet isotope ratio mass spectrometry to establish precise δ18O and Δ′17O values for leached clay reference materials KGa‐1b and SHCa‐1, a kaolinite and a hectorite/smectite, respectively. We further measured leached KGa‐1b and SHCa‐1 pressed into pellets with a lithium fluoride as a binding agent for the laser fluorination method, allowing us to test the reproducibility between methods and utilize a standard laser chamber drift correction scheme. ResultsThe laser fluorination technique yielded highly precise and reproducible δ18O and Δ′17O measurements for the KGa‐1b and SHCa‐1, aligning with bomb values of δ18O. This confirms the method's reliability and comparability to conventional isotope measurement techniques while also stressing the importance of proper sample preparation and laser chamber drift corrections. ConclusionsThis study demonstrates that laser fluorination is an effective method for accurately measuring the stable oxygen isotope composition of silicate clays or clay‐rich sediments when corrected with known silicate clay standards. These methods offer a valuable methodology for future research and applications that will significantly improve our understanding of past climate and environmental conditions.
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- PAR ID:
- 10661124
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Rapid Communications in Mass Spectrometry
- Volume:
- 39
- Issue:
- 11
- ISSN:
- 0951-4198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/rcm.10021
