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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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Seasonal Hydroclimate Recorded in High Resolution δ 18 O Profiles Across Pinus palustris Growth Rings
Abstract Rainfall amount and intensity are increasing under anthropogenic climate change, but many instrument records span less than a century. The oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) reflects local source water, climate, and tree physiology. The patterns of δ18Ocellwithin tree‐rings has the potential to extend pre‐instrument climate records with subannual resolution, but the influences on intra‐ring δ18Ocellprofiles are unexplored in many settings. In this study, high‐resolution δ18Ocellprofiles were analyzed on three longleaf pine trees growing in a native savanna in Louisiana, United States. The time series covers a wide range of rainfall conditions from 2001 to 2008 C.E. with a total of 421 δ18Ocellanalyses. The δ18Ocellvalues for individual years are well correlated with each other both within and between trees (r = 0.71–0.78). We used principal components analysis andk‐means clustering to differentiate δ18Ocellprofiles into two groupings: symmetrical δ18Ocellprofiles versus asymmetrical profiles that have depressed latewood δ18Ocellvalues. The slope of latewood δ18Ocellprofiles and mean δ18Ocellvalues of latewood tissue correlate with total June‐November precipitation. We hypothesize that poorly drained soils in the study area mediate the influence of any individual storm event: in dry years,18O‐depleted signals from convective storms are moderated by subsequent evaporative enrichment of standing water, whereas in wet years, increased humidity and frequent re‐supply of18O‐depleted water overrides evaporative enrichment effects, resulting in low δ18Ocellof latewood. These results suggest that δ18Ocellproxies for tropical storm occurrence need to account for soil conditions at the site of tree growth.
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- Award ID(s):
- 1903601
- PAR ID:
- 10448290
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 126
- Issue:
- 12
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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