An official website of the United States government
Abstract Since the last glacial period, North America has experienced dramatic changes in regional climate, including the collapse of ice sheets and changes in precipitation. We use clumped isotope (∆47) thermometry and carbonate δ18O measurements of glacial and deglacial pedogenic carbonates from the Palouse Loess to provide constraints on hydroclimate changes in the Pacific Northwest. We also employ analysis of climate model simulations to help us further provide constraints on the hydroclimate changes in the Pacific Northwest. The coldest clumped isotope soil temperaturesT(47) (13.5 ± 1.9°C to 17.1 ± 1.7°C) occurred ∼34,000–23,000 years ago. Using a soil‐to‐air temperature transfer function, we estimate Last Glacial Maximum (LGM) mean annual air temperatures of ∼−5.5°C and warmest average monthly temperatures (i.e., mean summer air temperatures) of ∼4.4°C. These data indicate a regional warming of 16.4 ± 2.6°C from the LGM to the modern temperatures of 10.9°C, which was about 2.5–3 times the global average. Proxy data provide locality constraints on the boundary of the cooler anticyclone induced by LGM ice sheets, and the warmer cyclone in the Eastern Pacific Ocean. Climate model analysis suggests regional amplification of temperature anomalies is due to the proximal location of the study area to the Laurentide Ice Sheet margin and the impact of the glacial anticyclone on the region, as well as local albedo. Isotope‐enabled model experiments indicate variations in water δ18O largely reflect atmospheric circulation changes and enhanced rainout upstream that brings more depleted vapor to the region during the LGM.
more »
« less
This content will become publicly available on February 19, 2026
Timing of Pedogenic Carbonate Formation in Fine‐Grained Soils: Decoupled T(Δ 47 ) and δ 18 O w Seasonal Bias
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.
more »
« less
- Award ID(s):
- 2023385
- PAR ID:
- 10576608
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 40
- Issue:
- 2
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Ancient greenhouse periods are useful analogs for predicting effects of anthropogenic climate change on regional and global temperature and precipitation patterns. A paucity of terrestrial data from polar regions during warm episodes challenges our understanding of polar climate responses to natural/anthropogenic change and therefore our ability to predict future changes in precipitation. Ellesmere and Axel Heiberg Islands in the Canadian Arctic preserve terrestrial deposits spanning the late Paleocene to middle Eocene (59–45 Ma). Here we expand on existing regional sedimentology and paleontology through the addition of stable (δ13C, δ18O) and clumped (Δ47) isotope analyses on palustrine carbonates. δ13C isotope values range from −4.6 to +12.3‰ (VPDB), and δ18O isotope values range from −23.1 to −15.2‰ (VPDB). Both carbon and oxygen isotope averages decrease with increasing diagenetic alteration. Unusually enriched carbon isotope (δ13C) values suggest that analyzed carbonates experienced repeated dissolution‐precipitation enrichment cycles, potentially caused by seasonal fluctuations in water availability resulting in summer carbonate dissolution followed by winter carbonate re‐precipitation. Stable isotopes suggest some degree of precipitation seasonality or reduction in winter water availability in the Canadian Arctic during the Paleogene. Clumped (Δ47) temperature estimates range from 52 to 121°C and indicate low temperature solid‐state reordering of micritic samples and diagenetic recrystallization in sparry samples. Average temperatures agree with vitrinite reflectance data for Eureka Sound Group and underlying sediments, highlighting structural complexity across the region. Broadly, combined stable and clumped isotope data from carbonates in complex systems are effective for describing both paleoclimatic and post‐burial conditions.more » « less
-
Abstract Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post‐depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturationin situ, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ18O, δ13C, and Δ47values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth‐based changes of porewater carbonate chemistry to microbial community composition, in situ carbon cycling (using δ13C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ13C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite—each with a different origin. Authigenic micrite is interpreted to have precipitated in situ from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ13Ccarb) and clumped isotope values (Δ47) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments.more » « less
-
Abstract Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47and Δ47values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47(I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale.more » « less
-
Abstract Faunal analog reconstructions suggest that Last Interglacial (MIS 5e) sea surface temperatures were cooler around Bermuda and in the Caribbean than modern climate. Here we describe new and revised clumped isotope measurements ofCittarium picafossil shells supporting previous findings of cooler than modern temperatures in Bermuda during the Last Interglacial. We resolve temperature and δ18Owdifferences between two closely located and apparently coeval sites described in Winkelstern et al. (2017),https://doi.org/10.1002/2016pa003014through reprocessing raw isotopic data with the updated Brand/IUPAC parameters. New subannual‐resolution clumped isotope data reveal large variations in δ18Owout of phase with seasonal temperature changes (i.e., lower δ18Owvalues in winter). Supported by modern δ18Owmeasurements identifying similar processes occurring today, we suggest past variations in coastal δ18Owwere driven by seasonally variable freshwater discharge from a subterranean aquifer beneath the island. Taken together, our results emphasize the importance of δ18Owin controlling carbonate δ18O, and suggest that typical assumptions of constant δ18Owshould be made cautiously in nearshore settings and can contribute to less accurate reconstructions of paleotemperature.more » « less
