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1. 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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2. Determining the source of speleothem δ18O variability from in situ measurements of seasonal and inter-annual isotope trends in precipitation, cave drip water and modern calcite from sites in the Central Peruvian Andes

   Olson, E.J. (December 2022, American Geophysical Union Conference. Chicago, IL)

   In the past decade, Huagapo and Pacupahuain Caves in the Central Peruvian Andes have become sources of speleothem oxygen isotope (δ18O) paleoclimate records. These studies identify the South American Summer Monsoon (SASM) as the main climate system controlling δ18O variability. While this interpretation is verified through inter-proxy record comparisons on millennial scales, interpretation of the high-resolution variability within these records is limited by a lack of modern proxy calibration studies at these sites. Here we present results from a modern cave monitoring study undertaken to address the controls on the δ18O values of precipitation at these sites and how surface and in-cave processes affect the δ18O value of speleothem calcite. Speleothem calcite δ18O values reflect an integrated signal of atmospheric processes (e.g., rainout, Raleigh distillation, upstream moisture recycling, changes in moisture source), evaporation and mixing during infiltration in the soil and epikarst, and in-cave processes such as degassing and evaporation. In consideration of these factors, we compare isotope trends in precipitation, cave drip water and modern farmed calcite from the two cave sites. We find that precipitationδ18O values during peak monsoon activity (January -February) shows considerable inter-annual variation with averages of -16.7‰ for 2020, -18.5‰ for 2021 and -13.8‰ in 2022. We investigate the source of this variability in regional atmospheric circulation patterns using weather station data and back trajectories. The mean annual precipitation (MAP) from outside Huagapo Cave is δ18O = -15.5+/- 6‰, while seasonal samples of drip water δ18O = -14.5+/- 1‰, are offset from MAP possibly due to evaporation during infiltration. Cave drip waterδ18O has low variability over inter-annual and seasonal timescales indicating homogenization in the epikarst. Using geochemical and sensor data (e.g. cave relative humidity, temperature, and drip rate) as inputs for a karst based forward model, we simulate modern speleothem δ18O to quantitatively assess the combined effects of hydroclimate processes integration to the isotope record. 

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3. How teeth record and attenuate seasonal signals

   https://doi.org/10.1016/j.jas.2024.106148  

   Green, Daniel R; Smith, Tanya M; Olack, Gerry; Williams, Ian S; Tafforeau, Paul; Colman, Albert S; Uno, Kevin T (December 2025, Journal of Archaeological Science)

   Variability of oxygen isotopes in environmental water is recorded in tooth enamel, providing a record of seasonal change, dietary variability, and mobility. Physiology dampens this variability, however, as oxygen passes from environmental sources into blood and forming teeth. We showcase two methods of high resolution, 2-dimensional enamel sampling, and conduct modeling, to report why and how environmental oxygen isotope variability is reduced in animal bodies and teeth. First, using two modern experimental sheep, we introduce a sampling method, die-saw dicing, that provides high-resolution physical samples (n = 109 and 111 sample locations per tooth) for use in conventional stable isotope and molecular measurement protocols. Second, we use an ion microprobe to sample innermost enamel in an experimental sheep (n = 156 measurements), and in a Pleistocene orangutan (n = 176 measurements). Synchrotron and conventional μCT scans reveal innermost enamel thicknesses averaging 18 and 21 μm in width. Experimental data in sheep show that compared to drinking water, oxygen isotope variability in blood is reduced to 70–90 %; inner and innermost enamel retain between 36 and 48 % of likely drinking water stable isotope range, but this recovery declines to 28–34 % in outer enamel. 2D isotope sampling suggests that declines in isotopic variability, and shifted isotopic oscillations throughout enamel, result from the angle of secretory hydroxyapatite deposition and its overprinting by maturation. This overprinting occurs at all locations including innermost enamel, and is greatest in outer enamel. These findings confirm that all regions of enamel undergo maturation to varying degrees and confirm that inner and innermost enamel preserve more environmental variability than other regions. We further show how the resolution of isotope sampling — not only the spatial resolution within teeth, but also the temporal resolution of water in the environment — impacts our estimate of how much variation teeth recover from the environment. We suggest inverse methods, or multiplication by standard factors determined by ecology, taxon, and sampling strategy, to reconstruct the full scale of seasonal environmental variability. We advocate for combined inverse modeling and high-resolution sampling informed by the spatiotemporal pattern of enamel formation, and at the inner or innermost enamel when possible, to recover seasonal records from teeth. 

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4. Stable isotope composition of surface waters across the Pamir, Central Asia: Implications of precipitation seasonality

   https://doi.org/10.1016/j.jhydrol.2025.132815  

   Chen, Shenqiang; Rugenstein, Jeremy KC; Mulch, Andreas (June 2025, Journal of Hydrology)

   The Pamir range, located in Central Asia, mainly receives moisture from the mid-latitude westerlies, but its western side (i.e., Tajikistan Pamir) receives much of its precipitation in the winter and spring and its eastern side (i.e., Chinese Pamir) in the summer. Thus, the Pamir provides a natural laboratory to study the distribution of surface water stable isotopes across a large mountain range that ultimately receives moisture from one single source but has different precipitation seasonality regimes between its two sides. In this study, we present stable oxygen (δ18O) and hydrogen (δ2H) isotope data for 113 surface water samples from the Chinese Pamir. Our new data, along with previously published stable isotope data, show that the slope of the Chinese Pamir local meteoric water line is higher than that of the Global Meteoric Water Line (GMWL), and almost all of the data plot above the GMWL, implying that the Chinese Pamir surface waters have not experienced significant isotopic modification by evaporation. The Chinese Pamir surface waters have substantially higher δ18O and d-excess values and a steeper apparent δ18O lapse rate than surface water samples collected from the Tajikistan Pamir. We suggest that this contrast results from the shift in precipitation seasonality across the Pamir, with dominantly winter and springtime precipitation on the Tajikistan side and summertime precipitation on the Chinese side of the Pamir. This predominant summertime precipitation regime results in surface waters with high δ18O values in the Chinese Pamir. Further, this summertime moisture is dominantly convectively recycled moisture, resulting in high d-excess values in surface waters. The percentage of summertime moisture, which has high δ18O values, decreases west and with elevation in the Chinese Pamir, resulting in a steep apparent δ18O lapse rate of − 3.2 ‰/km. The importance of precipitation seasonality in modulating δ18O values across the Pamir suggests that proxy-derived records of past environments in the region must consider the mechanisms that today cause the seasonality contrast. 

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5. Triple oxygen isotope evidence for a hot Archean ocean

   https://doi.org/10.1130/G50230.1  

   McGunnigle, J.P.; Cano, E.J.; Sharp, Z.D.; Muehlenbachs, K.; Cole, D.; Hardman, M.F.; Stachel, T.; Pearson, D.G. (May 2022, Geology)

   Triple oxygen isotope (δ17O and δ18O) values of high- and low-temperature altered oceanic crust and products of basalt alteration experiments were measured to better constrain ocean isotope compositions in deep time. The data define an array of δ18O and Δ′17O (Δ′17O=δ′17O – λRL × δ′18O + γ) values from mantle values toward 1‰ and –0.01‰, respectively, with a λ of ~0.523. The altered oceanic crust data were used to construct a model for estimating δ18O-Δ′17O values of the ancient oceans if the continental weathering flux (FCW) and/or hydrothermal oceanic crust alteration flux (FHT) changed through time. A maximum lowering of 7‰ and 4‰, respectively, is achieved in the most extreme cases. The δ18O value of the ocean cannot be raised by more than 1.1‰. Eclogites from the Roberts Victor kimberlite (South Africa), with a protolith age of 3.1 Ga, have δ18O-Δ′17O values that precisely overlap with those of the modern altered oceanic crust, suggesting that the Archean oceans had similar isotope values as today. Published triple isotope data for Archean cherts show that all samples have been altered to some degree and suggest an Archean ocean surface temperature of ~70–100 °C. An ocean as light as –2‰ is still consistent with our eclogite data and reduce our temperature estimates by 10 °C. 

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