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Abstract On polar ice sheets, water vapor interacts with surface snow, and through the exchange of water molecules, imprints an isotopic climate signal into the ice sheet. This exchange is not well understood due to sparse observations in the atmosphere. There are currently no published vertical profiles of water isotopes above ice sheets that span the planetary boundary layer and portions of the free troposphere. Here, we present a novel data set of water‐vapor isotopes (O, D, ) and meteorological variables taken by fixed‐wing uncrewed aircraft on the northeast Greenland Ice Sheet (GIS). During June–July (2022), we collected 104 profiles of water‐vapor isotopes and meteorological variables up to 1,500 m above ground level. Concurrently, surface snow samples were collected at 12‐hr intervals, allowing connection to surface‐snow processes. We pair observations with modeling output from a regional climate model as well as an atmospheric transport and water‐isotope distillation model. Climate model output of mean temperature and specific humidity agrees well with observations, with a mean difference of +0.095°C and −0.043 g/kg (−2.91%), respectively. We find evidence that along an air parcel pathway, the distillation model is not removing enough water prior to onsite arrival. Below the mean temperature inversion (200 m), water‐isotope observations indicate a kinetic fractionating process, likely the result of mixing sublimated vapor from the ice sheet surface along with an unknown fraction of katabatic wind vapor. Modeled does not agree well with observations, a result that requires substantial future analysis of kinetic fractionation processes along the entire moisture pathway.
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The role of sublimation as a driver of climate signals in the water isotope content of surface snow: laboratory and field experimental results
Abstract. Ice core water isotope records from Greenland and Antarctica are a valuableproxy for paleoclimate reconstruction, yet the processes influencing theclimate signal stored in the isotopic composition of the snow are beingchallenged and revisited. Apart from precipitation input, post-depositionalprocesses such as wind-driven redistribution and vapor–snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal subsequently stored in the ice. Recent field studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor–snow exchange is an important driving mechanism. Here we investigate how vapor–snow exchange processes influence the isotopic composition of the snowpack. Controlled laboratory experiments under forced sublimation show an increase in snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer due to sublimation, with an attenuated signal down to 3 cm snow depth over the course of 4–6 d. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Our observations confirm that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 d field experiments at the East Greenland Ice Core Project site (northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation contribute to the climate signal recorded in the water isotopes in surface snow, in both laboratory and field settings. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such, particularly in regions of low accumulation.
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- PAR ID:
- 10648171
- Publisher / Repository:
- Published by Copernicus Publications on behalf of the European Geosciences Union.
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 15
- Issue:
- 10
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 4949 to 4974
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. We document the isotopic evolution of near-surface snow at the East Greenland Ice Core Project (EastGRIP) ice core site in northeast Greenland using a time-resolved array of 1 m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age–depth model was developed and applied to each profile, mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occur during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3 ‰–5 ‰ increase after aging 1 year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work indicate that the summertime post-depositional changes in d (Δd) can be explained by a combination of surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven diffusion throughout the column. The interannual Δd is also partly explained with isotope-gradient-driven diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source-region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages through metamorphic processes. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity, which can be explained solely by isotope-gradient-driven diffusion. Our results are dependent on the site characteristics (e.g., wind, temperature, accumulation rate, snow properties) but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/tc-15-4949-2021
