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1. Climatological Significance of δD‐δ 18 O Line Slopes From Precipitation, Snow Pits, and Ice Cores at Summit, Greenland

   https://doi.org/10.1029/2022JD037037  

   Kopec, Ben G. ; Feng, Xiahong ; Osterberg, Erich C. ; Posmentier, Eric S. ( November 2022 , Journal of Geophysical Research: Atmospheres)

   Hydrogen (δD) and oxygen (δ¹⁸O) isotopic ratios are strongly correlated in precipitation over time and space, defining the meteoric water line, and the slope of this δD‐δ¹⁸O relationship reflects covariations of deuterium excess (d‐excess) with δD or δ¹⁸O. This δD‐δ¹⁸O line provides a tool for inferring hydrologic processes from the evaporation source to condensation site. Here, we present δD‐δ¹⁸O relationships on seasonal and annual timescales for daily precipitation, snow pits, and a 15‐m ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD‐δ¹⁸O slopes are less than 8 (summer = 7.70; winter = 7.77), while the annual slope is greater than 8 (8.27). We suggest that intra‐season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than 8. The summer line has a greater intercept (higher d‐excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d‐excess primarily to contributions of vapor sublimated from the Greenland Ice Sheet and other terrestrial sources. High sublimated moisture proportions result in a large separation between seasonal δD‐δ¹⁸O lines, and thus high annual slopes. Inter‐seasonal weighting of precipitation amount also influences annual slopes because slopes are weighed by the number of storms each season. Using snow pit measurements, we demonstrate that precipitation isotopic signals translate to the snowpack. We generate indices to determine Sublimation Proportion Index and Precipitation Weighting Index, and find that annual Owen core δD‐δ¹⁸O line slopes are significantly related to these indices, demonstrating that these factors are recorded in ice cores.

    

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2. Seasonal Deuterium Excess Variations of Precipitation at Summit, Greenland, and their Climatological Significance

   https://doi.org/10.1029/2018JD028750  

   Kopec, B. G. ; Feng, X. ; Posmentier, E. S. ; Sonder, L. J. ( January 2019 , Journal of Geophysical Research: Atmospheres)

   The hydrogen and oxygen isotopic composition of ice cores from Summit, Greenland, has provided invaluable information about variations in past climate. However, interpretations of these isotopic data have been made despite a paucity of direct isotopic studies of Summit precipitation. We provide insight to such interpretations by examining the annual cycle of deuterium excess (d‐excess) in precipitation samples from Summit and by considering the climatic controls on the annual cycle. Precipitation was collected daily from July 2011 to September 2014 at heights of 1, 2, and 4 m. The isotopic composition of precipitation sampled at 4 m above the snow surface is free of contamination from blowing snow. Precipitation d‐excess is high in the summer and low in the winter, a pattern opposite to that found at most high‐latitude locations, where summer d‐excess is low relative to winter. Low winter d‐excess values at Summit can be explained by varying degrees of Rayleigh distillation of moisture sourced from isotopically similar marine sources. However, the observed summer d‐excess maximum at Summit is anomalously high compared with other Arctic locations, and we propose that this is due to high d‐excess moisture contributed by sublimation of surface snow on the Greenland Ice Sheet. We demonstrate the plausibility of this hypothesis through simple isotopic mass balance calculations, analyses of cloud heights, and back trajectories to identify moisture sources. We show that Rayleigh distillation, sublimation, and the phase of the d‐excess annual cycle are all important factors that should be considered in ice core d‐excess interpretations.

    

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3. On the Interpretation of the ENSO Signal Embedded in the Stable Isotopic Composition of Quelccaya Ice Cap, Peru

   https://doi.org/10.1029/2018JD029064  

   Hurley, J. V. ; Vuille, Mathias ; Hardy, Douglas R. ( January 2019 , Journal of Geophysical Research: Atmospheres)

   Theδ¹⁸O signal in ice cores from the Quelccaya Ice Cap (QIC), Peru, corresponds with and has been used to reconstruct Niño region sea surface temperatures (SSTs), but the physical mechanisms that tie El Niño–Southern Oscillation (ENSO)‐related equatorial Pacific SSTs to snowδ¹⁸O at 5,680 m in the Andes have not been fully established. We use a proxy system model to simulate how QIC snowδ¹⁸O varies by ENSO phase. The model accurately simulates higher and lowerδ¹⁸O values during El Niño and La Niña, respectively. We then explore the relative roles of ENSO forcing on different components of the forward model: (i) the seasonality and amount of snow gain and loss at the QIC, (ii) the initial water vaporδ¹⁸O values, and (iii) regional temperature. Most (more than two thirds) of the ENSO‐related variability in the QICδ¹⁸O can be accounted for by ENSO's influence on South American summer monsoon (SASM) activity and the resulting change in the initial water vapor isotopic composition. The initial water vaporδ¹⁸O values are affected by the strength of upstream convection associated with the SASM. Since convection over the Amazon is enhanced during La Niña, the water vapor over the western Amazon Basin—which serves as moisture source for snowfall on QIC—is characterized by more negativeδ¹⁸O values. In the forward model, higher initial water vaporδ‐values during El Niño yield higher snowδ¹⁸O at the QIC. Our results clarify that the ENSO‐related isotope signal on Quelccaya should not be interpreted as a simple temperature response.

    

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4. Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA

   https://doi.org/doi.org/10.1016/j.gca.2021.01.003  

   Christensen, J. ( January 2021 , Geochimica)

   Teagle, Damon A (Ed.)

   The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ‘‘floes” forming at the atmosphere-water interface, ‘‘snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in d13C (
   9 to
   28‰ VPDB) and d18O (0 to
   20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the d13C-d18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe D44Ca data (
   0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8  10
   7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (a
   1)  1000) accompanying CO2 hydroxylation, estimated here to be
   17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and
   7.1 ± 1.1‰ (vs. ‘CO2(aq)+H2O’) for oxygen at 17.4 ± 1.0
   C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of
   17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and
   6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq)+H2O’) over the 17–28
   C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO- 3, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate d13C and d18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water d18O and atmospheric d13C values. There is evidence that older travertine at The Cedars recorded atmospheric d13C that predates large-scale combustion of fossil fuels. 

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5. Antarctic Water Tracks: Microbial Community Responses to Variation in Soil Moisture, pH, and Salinity

   https://doi.org/doi:10.3389/fmicb.2021.616730  

   George, Scott F ; Fierer, Noah ; Levy, Joseph S ; Adams, Byron ( January 2021 , Frontiers in microbiology)

   Zucconi, Laura (Ed.)

   Ice-free soils in the McMurdo Dry Valleys select for taxa able to cope with challenging environmental conditions, including extreme chemical water activity gradients, freeze-thaw cycling, desiccation, and solar radiation regimes. The low biotic complexity of Dry Valley soils makes them well suited to investigate environmental and spatial influences on bacterial community structure. Water tracks are annually wetted habitats in the cold-arid soils of Antarctica that form briefly each summer with moisture sourced from snow melt, ground ice thaw, and atmospheric deposition via deliquescence and vapor flow into brines. Compared to neighboring arid soils, water tracks are highly saline and relatively moist habitats. They represent a considerable area (∼5–10 km2) of the Dry Valley terrestrial ecosystem, an area that is expected to increase with ongoing climate change. The goal of this study was to determine how variation in the environmental conditions of water tracks influences the composition and diversity of microbial communities. We found significant differences in microbial community composition between on- and off-water track samples, and across two distinct locations. Of the tested environmental variables, soil salinity was the best predictor of community composition, with members of the Bacteroidetes phylum being relatively more abundant at higher salinities and the Actinobacteria phylum showing the opposite pattern. There was also a significant, inverse relationship between salinity and bacterial diversity. Our results suggest water track formation significantly alters dry soil microbial communities, likely influencing subsequent ecosystem functioning. We highlight how Dry Valley water tracks could be a useful model system for understanding the potential habitability of transiently wetted environments found on the surface of Mars. 

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