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1. Carbon and carbon-13 in the preindustrial and glacial ocean

   https://doi.org/10.1371/journal.pclm.0000434  

   Schmittner, Andreas; Fillman, Nathaniel J (July 2024, PLOS Climate)

   Carré, Matthieu (Ed.)

   Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope¹³C in the ocean are not well understood. Models typically do not decompose C and¹³C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ¹³C_DIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ¹³C_DIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ¹³C_DICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ¹³C_DIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ¹³C_DIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ¹³C_DIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ¹³C_DICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO₂. Spatial distributions of the δ¹³C_DICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors. 

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2. Large herbivore δ ¹⁸ O as a proxy for aridity in the South African winter and year-round rainfall zone

   https://doi.org/10.1017/qua.2024.21  

   Luyt, Julie; Faith, J Tyler; Sealy, Judith (November 2024, Quaternary Research)

   Abstract This study explores patterning in δ¹⁸O values of tooth enamel in contemporary African herbivores from mainly C₃-dominated ecosystems. Evapotranspiration causes plants to lose H₂¹⁶O to a greater extent than H₂¹⁸O, leaving leaves enriched in¹⁸O. In eastern Africa, ES species (evaporation-sensitive species: those obtaining water from food) tend to have more positive δ¹⁸O_enamelvalues than EI species (evaporation-insensitive species: those heavily dependent on drinking water); the magnitude of the difference increases with increasing aridity. We find the same pattern applies in the winter and year-round rainfall region of southern Africa, allowing us to use δ¹⁸O_enamelin fossil animals to examine paleo-aridity. We apply this approach to infer aridity at Quaternary fossil assemblages from present-day winter and year-round rainfall zones, including Elandsfontein (ca. 1–0.6 Ma), Hoedjiespunt (ca. 300–130 ka), and Nelson Bay Cave (23.5–3 ka). This analysis suggests that (1) at various times during the Pleistocene, Elandsfontein and Hoedjiespunt environments were wetter than last glacial maximum (LGM) to Holocene environments at Nelson Bay Cave (year-round rainfall zone); and (2) considered alongside other evidence from the year-round rainfall zone, wetter conditions across the Pleistocene–Holocene transition at Nelson Bay Cave suggests that climate changes at near-coastal sites may be out of phase with the adjacent interior. 

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3. Seasonal Hydroclimate Recorded in High Resolution δ ¹⁸ O Profiles Across Pinus palustris Growth Rings

   https://doi.org/10.1029/2021JG006505  

   Lukens, William E.; Narmour, Robert T.; Schubert, Brian A. (December 2021, Journal of Geophysical Research: Biogeosciences)

   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 (δ¹⁸O_cell) reflects local source water, climate, and tree physiology. The patterns of δ¹⁸O_cellwithin tree‐rings has the potential to extend pre‐instrument climate records with subannual resolution, but the influences on intra‐ring δ¹⁸O_cellprofiles are unexplored in many settings. In this study, high‐resolution δ¹⁸O_cellprofiles 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 δ¹⁸O_cellanalyses. The δ¹⁸O_cellvalues 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 δ¹⁸O_cellprofiles into two groupings: symmetrical δ¹⁸O_cellprofiles versus asymmetrical profiles that have depressed latewood δ¹⁸O_cellvalues. The slope of latewood δ¹⁸O_cellprofiles and mean δ¹⁸O_cellvalues 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 δ¹⁸O_cellof latewood. These results suggest that δ¹⁸O_cellproxies for tropical storm occurrence need to account for soil conditions at the site of tree growth. 

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4. Oxygen Isotopic Signatures of Major Climate Modes and Implications for Detectability in Speleothems

   https://doi.org/10.1029/2020GL089515  

   Midhun, M.; Stevenson, S.; Cole, J. E. (January 2021, Geophysical Research Letters)

   Abstract Natural and social systems worldwide are impacted by climate modes such as the El Niño/Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), making it imperative to understand their sensitivity to climate change. Paleoclimate studies extend the observational climate baseline, and speleothem records (δ¹⁸O_spel) are a common data source. However, relationships between δ¹⁸O_speland climate modes are uncertain; climate models provide a way to test the strength and stability of these relationships. Here, we use the isotope‐enabled Community Earth System Model's Last Millennium Ensemble combined with a forward proxy model to delineate the global expression of modal variability in “pseudo‐stalagmite” (δ¹⁸O_spel) records worldwide. The modeled δ¹⁸O_spelspatially correlates with modal signatures. However, substantial changes in modal variance only modestly affect individual δ¹⁸O_spelvariance. A network of δ¹⁸O_spelrecords, particularly one that straddles the Pacific, significantly improves the reconstructability of ENSO variance. 

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5. Paleoclimate Changes in the Pacific Northwest Over the Past 36,000 Years From Clumped Isotope Measurements and Model Analysis

   https://doi.org/10.1029/2021PA004266  

   Lopez‐Maldonado, Ricardo; Bateman, Jesse Bloom; Ellis, Andre; Bader, Nicholas E.; Ramirez, Pedro; Arnold, Alexandrea; Ajoku, Osinachi; Lee, Hung‐I; Jesmok, Gregory; Upadhyay, Deepshikha; et al (February 2023, Paleoceanography and Paleoclimatology)

   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. 

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