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1. Using a Paired Chironomid δ ¹⁸ O and Aquatic Leaf Wax δ ² H Approach to Reconstruct Seasonality on Western Greenland During the Holocene

   https://doi.org/10.1029/2020PA004169  

   Corcoran, Megan_C; Thomas, Elizabeth_K; Morrill, Carrie (April 2021, Paleoceanography and Paleoclimatology)

   Abstract The Arctic hydrological cycle is predicted to intensify as the Arctic warms, due to increased poleward moisture transport during summer and increased evaporation from seas once ice‐covered during winter. Records of past Arctic precipitation seasonality are important because they provide a context for these ongoing changes. In some Arctic lakes, stable isotopes of oxygen and hydrogen (δ¹⁸O and δ²H, respectively) vary seasonally, due to seasonal changes in precipitation δ¹⁸O and δ²H. We reconstruct precipitation seasonality from Lake N3, a well‐dated lake sediment archive in Disko Bugt, western Greenland, by generating Holocene records of two proxies that are produced at different times of the year, and therefore record different lake water seasonal isotopic compositions. Aquatic plants synthesize waxes throughout the summer, and their δ²H reflects winter‐biased precipitation δ²H at Lake N3, whereas chironomids synthesize their head capsules between late summer and winter, and their δ¹⁸O reflects summer‐biased precipitation δ¹⁸O at Lake N3. During the middle Holocene at Lake N3, aquatic plant leaf wax was strongly²H‐depleted, while chironomid chitin was¹⁸O‐enriched. We guide interpretations of these records using sensitivity tests of a lake water and energy balance model, where we change precipitation amount and isotope seasonality inputs. The sensitivity tests suggest that the contrasting trends between proxies were likely caused by an increase in precipitation amount during all seasons and an increase in precipitation isotope seasonality, in addition to proxy‐specific mechanisms, highlighting the importance of understanding lake‐ and proxy‐specific systematics when interpreting records from sediment archives. 

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2. Carbonate facies‐specific stable isotope data record climate, hydrology, and microbial communities in Great Salt Lake, UT

   https://doi.org/10.1111/gbi.12386  

   Ingalls, Miquela; Frantz, Carie M.; Snell, Kathryn E.; Trower, Elizabeth J. (March 2020, Geobiology)

   Abstract Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_water, δ¹³C_DIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸O_carb, δ¹³C_carb, ∆₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_org) and dissolved inorganic carbon (DIC; δ¹³C_DIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆₄₇isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆₄₇)), δ¹⁸O_carb, and calculated δ¹⁸O_waterin isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation. 

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3. Variability and Controls on δ ¹⁸ O, d‐excess, and ∆′ ¹⁷ O in Southern Peruvian Precipitation

   https://doi.org/10.1029/2020JD034009  

   Aron, Phoebe G.; Poulsen, Christopher J.; Fiorella, Richard P.; Levin, Naomi E.; Acosta, Rene P.; Yanites, Brian J.; Cassel, Elizabeth J. (December 2021, Journal of Geophysical Research: Atmospheres)

   Abstract 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 δ¹⁸O_pand δ²H_pvalues from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′¹⁷O_p, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ¹⁸O_pand that seasonal δ¹⁸O_pvariations are related to upstream rainout and local convection. Spatial δ¹⁸O_pvariations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excess_pcycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excess_pand ∆′¹⁷O_pclearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′¹⁷O_pvalues 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 δ¹⁸O_p, d‐excess_p, and ∆′¹⁷O_pdata and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes. 

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4. An Isotope Mass Balance Analysis of Evaporative Loss From Lake Turkana, Kenya Using δ ¹⁸ O and δD of Natural Waters

   https://doi.org/10.1029/2023WR036076  

   Saslaw, M.; Yang, D.; Lee, D.; Poulsen, C_J; Henkes, G_A (May 2024, Water Resources Research)

   Abstract Measurements of oxygen and hydrogen stable isotope ratios (δ¹⁸O and δD) in meteoric waters provide insight to overlapping effects of evaporation, precipitation, and mixing on basin scale hydrology. This study of waters collected between 2016 and 2021 in the Turkana Basin, northern Kenya, uses δ¹⁸O and δD to understand water balance in Lake Turkana, a large, low‐latitude, alkaline desert lake. The Omo River, a major river system in the Ethiopian Highlands, is historically understood to provide approximately 90% of the water input to Lake Turkana. Discharge of the Omo is prohibitively difficult to measure, but stable isotope ratios in the lake may provide a meaningful method for monitoring the lake's response to changes in input. Precipitation in the Turkana Basin is low (<200 mm/year) with negligible rainfall on the lake's surface, and all water loss from the lake is evaporative. We compare new measurements with previous data from the region and records of lake height and precipitation from the same time period. We show that a Bayesian approach to modeling evaporation using atmospheric conditions and river δ¹⁸O and δD yields results consistent with published water balance models. Continued sampling of lake and meteoric waters in the Turkana Basin will be a useful way to monitor the lake's response to regional and global climate change. 

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5. The Connected Isotopic Water Cycle in the Community Earth System Model Version 1

   https://doi.org/10.1029/2019MS001663  

   Brady, E.; Stevenson, S.; Bailey, D.; Liu, Z.; Noone, D.; Nusbaumer, J.; Otto‐Bliesner, B. L.; Tabor, C.; Tomas, R.; Wong, T.; et al (August 2019, Journal of Advances in Modeling Earth Systems)

   Abstract Because of the pervasive role of water in the Earth system, the relative abundances of stable isotopologues of water are valuable for understanding atmospheric, oceanic, and biospheric processes, and for interpreting paleoclimate proxy reconstructions. Isotopologues are transported by both large‐scale and turbulent flows, and the ratio of heavy to light isotopologues changes due to fractionation that can accompany condensation and evaporation processes. Correctly predicting the isotopic distributions requires resolving the relationships between large‐scale ocean and atmospheric circulation and smaller‐scale hydrological processes, which can be accomplished within a coupled climate modeling framework. Here we present the water isotope‐enabled version of the Community Earth System Model version 1 (iCESM1), which simulates global variations in water isotopic ratios in the atmosphere, land, ocean, and sea ice. In a transient Last Millennium simulation covering the 850–2005 period, iCESM1 correctly captures the late‐twentieth‐century structure of δ¹⁸O and δD over the global oceans, with more limited accuracy over land. The relationship between salinity and seawater δ¹⁸O is also well represented over the observational period, including interbasin variations. We illustrate the utility of coupled, isotope‐enabled simulations using both Last Millennium simulations and freshwater hosing experiments with iCESM1. Closing the isotopic mass balance between all components of the coupled model provides new confidence in the underlying depiction of the water cycle in CESM, while also highlighting areas where the underlying hydrologic balance can be improved. The iCESM1 is poised to be a vital community resource for ongoing model development with both modern and paleoclimate applications. 

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