Triple oxygen isotope ratios (Delta'O-17) offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of Delta'O-17 in paleoclimate applications is hampered by a limited understanding of how precipitation Delta'O-17 values vary across time and space. To improve applications of Delta'O-17, we present delta O-18, d-excess, and Delta'O-17 data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation Delta'O-17 tracks evaporation but appears insensitive to many controls that govern variation in delta O-18, including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on Delta'O-17 variation in the data set and we observe higher seasonally amount-weighted average precipitation Delta'O-17 values in the winter (40 +/- 15 per meg [+/- standard deviation]) than in the summer (18 +/- 18 per meg). This seasonal precipitation Delta'O-17 variability likely arises from a combination of sub-cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal Delta'O-17 pattern, which is absent in d-excess and opposite in sign from delta O-18, appears in other data sets globally; it showcases the influence of seasonality on Delta'O-17 values of precipitation and highlights the need for further systematic studies to understand variation in Delta'O-17 values of precipitation.

Seasonal changes in light and physicochemical conditions have strong impacts on cyanobacteria, but how they affect community structure, metabolism, and biogeochemistry of cyanobacterial mats remains unclear. Light may be particularly influential for cyanobacterial mats exposed to sulphide by altering the balance of oxygenic photosynthesis and sulphide-driven anoxygenic photosynthesis. We studied temporal shifts in irradiance, water chemistry, and community structure and function of microbial mats in the Middle Island Sinkhole (MIS), where anoxic and sulphate-rich groundwater provides habitat for cyanobacteria that conduct both oxygenic and anoxygenic photosynthesis. Seasonal changes in light and groundwater chemistry were accompanied by shifts in bacterial community composition, with a succession of dominant cyanobacteria from Phormidium to Planktothrix, and an increase in diatoms, sulphur-oxidizing bacteria, and sulphate-reducing bacteria from summer to autumn. Differential abundance of cyanobacterial light-harvesting proteins likely reflects a physiological response of cyanobacteria to light level. Beggiatoa sulphur oxidation proteins were more abundant in autumn. Correlated abundances of taxa through time suggest interactions between sulphur oxidizers and sulphate reducers, sulphate reducers and heterotrophs, and cyanobacteria and heterotrophs. These results support the conclusion that seasonal change, including light availability, has a strong influence on community composition and biogeochemical cycling of sulphur and O2 in cyanobacterial mats.

Oxygen isotope distributions from lacustrine carbonates provide insights into climate and hydrological change, but it is difficult to isolate the influences of catchment precipitation δ18O, water temperature, and evaporation on lacustrine carbonate δ18O values. Recent work shows the potential for using a combination of clumped (Δ47) and triple oxygen isotope (Δ′17O) measurements to identify the roles of temperature and evaporation on carbonate δ18O values in lakes, allowing precipitation δ18O values to be inferred and facilitating paleoclimate reconstructions. However, modern calibration of this approach has been mostly limited to arid regions with a high ratio of evaporative losses over inputs (XE) and low relative humidity (h<0.7). Developing this tool for paleoclimate and paleoelevation reconstructions requires expanding the modern calibrations to a greater range of climatic and hydrologic conditions. We sampled four lakes in different hydrologic states under a single, high humidity climate regime (h=0.7–0.9) in the Lake Junín region of central Peru. Clumped isotope temperatures from lake carbonates reflect water temperatures during carbonate formation. Lake hydrology is the main control on the Δ′17O values of carbonates and waters: Δ′17O values are lowest in the larger lakes with higher XE when compared to smaller, headwater lakes where evaporation is minimal and Δ′17O is indistinguishable from that of precipitation. Reconstructed unevaporated catchment precipitation δ′18O (δ′18Orucp) values from lake waters rely on accurate characterization of λlake, the triple oxygen isotope evaporation slope. We explore the influence of humidity on λlake using both new observations and modeled data. Accounting for local humidity improves λlake estimates, which allows for more accurate reconstructions of δ′18Orucp. We generate a δ′18Orucp value of −15.2±2.1‰ from modern carbonates and waters (n=15) in the Lake Junín region, which is similar to amount weighted mean annual precipitation −14.1 (±2.2‰). This study illustrates that (1) Δ′17O can be used to differentiate between lakes with differing XE in humid climates, (2) lake carbonate Δ′17O and δ18O values can be used to evaluate the influence of evaporation on lake water δ18O values in a range of climates, and (3) modeling λlake under appropriate humidity conditions improves δ′18Orucp estimates from lake carbonate Δ′17O.

This is the supplementary data for the publication in Earth and Planetary Science Letters. It includes all supplementary figures and tables cited in this publication to make them more readily available. Beverly, E.J., Levin, N.E., Passey, B.H., Aron, P.G., Yarian, D.A., Page, M., Pelletier, E.M., 2021. Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti , Tanzania. Earth Planet. Sci. Lett. 567, 1–13. https://doi.org/10.1016/j.epsl.2021.116952

The isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ 18 O p and δ 2 H p values from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′ 17 O p , from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ 18 O p and that seasonal δ 18 O p variations are related to upstream rainout and local convection. Spatial δ 18 O p variations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excess p cycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excess p and ∆′ 17 O p clearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′ 17 O p values in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ 18 O p , d‐excess p , and ∆′ 17 O p data and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.

Transpiration (T) is perhaps the largest fluxes of water from the land surface to the atmosphere and is susceptible to changes in climate, land use and vegetation structure. However, predictions of future transpiration fluxes vary widely and are poorly constrained. Stable water isotopes can help expand our understanding of land–atmosphere water fluxes but are limited by a lack of observations and a poor understanding of how the isotopic composition of transpired vapour (δ T ) varies. Here, we present isotopic data of water vapour, terrestrial water and plant water from a deciduous forest to understand how vegetation affects water budgets and land–atmosphere water fluxes. We measured subdiurnal variations of δ 18 O T from three tree species and used water isotopes to partition T from evapotranspiration (ET) to quantify the role of vegetation in the local water cycle. We find that δ 18 O T deviated from isotopic steady‐state during the day but find no species‐specific patterns. The ratio of T to ET varied from 53% to 61% and was generally invariant during the day, indicating that diurnal evaporation and transpiration fluxes respond to similar atmospheric and micrometeorological conditions at this site. Finally, we compared the isotope‐inferred ratio of T to ET with results from another ET partitioning approach that uses eddy covariance and sap flux data. We find broad midday agreement between these two partitioning techniques, in particular, the absence of a diurnal cycle, which should encourage future ecohydrological isotope studies. Isotope‐inferred estimates of transpiration can inform land surface models and improve our understanding of land–atmosphere water fluxes.

Paleosol carbonates are used extensively in the rock record to reconstruct vegetation using carbon isotopes, but poor constraints on temperature and soil evaporation has limited the use of oxygen isotopes in these reconstructions. Advances in carbonate clumped isotope thermometry allow for independent controls on temperature (T ), but proxies for evaporation remain elusive. The sensitivity of OOO distributions to kinetic fractionation makes it possible to track evaporation using triple oxygen isotope distributions (∆ʹ O). Recent work on lacustrine carbonates shows that ∆ʹ O is sensitive to evaporation, but little is known about variation of ∆ʹ O in soil carbonates or their potential to track evaporation. Here we report isotopic data from modern soil carbonates collected from 11 sites along a transect in the Serengeti Ecosystem where Mean Annual Precipitation (MAP) and Aridity Index (AI = MAP/PET) range from 499 to 846 mm yr and 0.33 to 0.55, respectively. δ C values range from ‐2.7 to 1.8‰ and reflect C grasslands. δ O values vary by ~8‰ along the transect and increase with increased aridity (lower MAP, higher AI). Mean T from these soil carbonates is 23 ± 4 ̊C and is within error of mean annual air temperature (MAAT) or 1-2 ̊C warmer at all sites. T does not vary significantly across sites or with depth, and soil temperature measurements and modeling suggest that this invariability is likely due to small changes in temperature at the equator. Using T temperatures, reconstructed soil water δ O values are up to 6‰ greater than δ O values of local precipitation and springs, indicating considerable evaporation in the soils. The ∆ʹ O values of these soil carbonates range from -161 to -111 per meg and decrease with increasing aridity and increasing δ O. This clear trend in ∆ʹ O values across an aridity gradient supports Monday, 26 October 2020 3:30 PM 3:45 PM GSA e-Attend Platform Meeting Rooms ∆47 18 17 16 17 17 17

Water resources management in Maui, Hawaii, remains a challenge due to uneven distribution of rainfall, water scarcity in populated areas, and an ill-defined hydrological system. Based on data collected in June 2014, Niu et al. (2017) suggested that noble gases in this rapid groundwater infiltration system record seasonality and may shed light on water sources, recharge locations, and groundwater flow paths. This study goes one step further by analyzing rainwater, basal and perched (springs) aquifer data collected in June 2014 and February 2016. It shows for the first time that both rain and spring water display sharp temporal variations which are related to either seasonal (summer/winter) variations or to the strong 2015–2016 El Niño year. These spring water temporal variations also suggest that perched aquifer groundwater is extremely young and likely unreliable as a long-term water supplier. In contrast, noble gas temporal variations are absent in the basal aquifer. This, in turn, is consistent with tritium derived groundwater ages on the order of a few decades for the basal aquifer. This study further demonstrates that Ne is an excellent candidate to estimate the source elevation of rain and thus, to distinguish between orographic and synoptic-scale rain.

The past decade has seen a remarkable expansion of studies that use mass-dependent variations of triple oxygen isotopes (16O, 17O, 18O) in isotope hydrology and isotope geochemistry. Recent technological and analytical advances demonstrate that small deviations of δ′18O and δ′17O from a mass-dependent reference relationship are systematic and are explained by well-known equilibrium and kinetic fractionations. Measurements of δ′18O and δ′17O complement traditional metrics like deuterium-excess, constrain isotope effects of kinetic fractionation that are impossible to discern with δ18O alone, and help reconstruct past environmental conditions from geologic records. In this review, we synthesize published meteoric (derived from precipitation) water triple oxygen isotope data with a new, near-global surface water dataset of δ′18O, δ′17O, δ2H, deuterium-excess, and ∆′17O, where ∆′17O is defined as δ′17O – λref δ′18O, δ′ notation is a logarithmic definition of the common δ value (δ′=ln(δ + 1), and λref is equal to 0.528. The expanded dataset shows that meteoric water δ′18O and δ′17O fit multiple regression lines and indicates that one global meteoric water line does not adequately describe all triple oxygen isotope data. Instead, this isotope system may be sensitive to processes such as moisture transport, rainout, and evaporation that do not affect the water cycle equally across the globe. This review provides a practical guide to understand ∆′17O variation in waters, explains the utility of this isotope system in hydrologic and paleoclimate studies, and outlines directions of future work that will expand the use of ∆′17O.

Forests play an integral role in the terrestrial water cycle and link exchanges of water between the land surface and the atmosphere. To examine the effects of an intermediate disturbance on forest water cycling, we compared vertical profiles of stable water vapor isotopes in two closely located forest sites in northern lower Michigan. At one site, all canopy-dominant early successional species were stem girdled to induce mortality and accelerate senescence. At both sites, we measured the isotopic composition of atmospheric water vapor at six heights during three seasons (spring, summer, and fall) and paired vertical isotope profiles with local meteorology and sap flux. Disturbance had a substantial impact on local water cycling. The undisturbed canopy was moister, retained more transpired vapor, and at times was poorly mixed with the free atmosphere above the canopy. Differences between the disturbed and undisturbed sites were most pronounced in the summer when transpiration was high. Differences in forest structure at the two sites also led to more isotopically stratified vapor within the undisturbed canopy. Our findings suggest that intermediate disturbance may increase mixing between the surface layer and above-canopy atmosphere and alter ecosystem-atmosphere gas exchange.