Ng, F.S.L. orcid.org/0000-0001-6352-0351 (2023) Isotopic diffusion in ice enhanced by vein-water flow. The Cryosphere, 17 (7). pp. 3063-3082. ISSN 1994-0416
Abstract
Diffusive smoothing of signals on the water stable isotopes (18O and D) in ice sheets fundamentally limits the climatic information retrievable from these ice-core proxies. Past theories explained how, in polycrystalline ice below the firn, fast diffusion in the network of intergranular water veins “short-circuits” the slow diffusion within crystal grains to cause “excess diffusion”, enhancing the rate of signal smoothing above that implied by self-diffusion in ice monocrystals. But the controls of excess diffusion are far from fully understood. Here, modelling shows that water flow in the veins amplifies excess diffusion by altering the three-dimensional field of isotope concentration and isotope transfer between veins and crystals. The rate of signal smoothing depends not only on temperature, the vein and grain sizes, and signal wavelength, but also on vein-water flow velocity, which can increase the rate by 1 to 2 orders of magnitude. This modulation can significantly impact signal smoothing at ice-core sites in Greenland and Antarctica, as shown by simulations for the GRIP (Greenland Ice Core Project) and EPICA (European Project for Ice Coring in Antarctica) Dome C sites, which reveal sensitive modulation of their diffusion-length profiles when vein-flow velocities reach ∼ 101–102 m yr−1. Velocities of this magnitude also produce the levels of excess diffusion inferred by previous studies for Holocene ice at GRIP and ice of Marine Isotope Stage 19 at EPICA Dome C. Thus, vein-flow-mediated excess diffusion may help explain the mismatch between modelled and spectrally derived diffusion lengths in other ice cores. We also show that excess diffusion biases the spectral estimation of diffusion lengths from isotopic signals (by making them dependent on signal wavelength) and the reconstruction of surface temperature from diffusion-length profiles (by increasing the ice contribution to diffusion length below the firn). Our findings caution against using the monocrystal isotopic diffusivity to represent the bulk-ice diffusivity. The need to predict the pattern of excess diffusion in ice cores calls for systematic study of isotope records for its occurrence and improved understanding of vein-scale hydrology in ice sheets.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © Author(s) 2023. This work is distributed under the Creative Commons Attribution 4.0 License. (https://creativecommons.org/licenses/by/4.0/) |
Dates: |
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Institution: | The University of Sheffield |
Academic Units: | The University of Sheffield > Faculty of Social Sciences (Sheffield) > Department of Geography (Sheffield) |
Depositing User: | Symplectic Sheffield |
Date Deposited: | 30 Jun 2023 09:29 |
Last Modified: | 28 Jul 2023 09:28 |
Status: | Published |
Publisher: | Copernicus Publications |
Refereed: | Yes |
Identification Number: | 10.5194/tc-17-3063-2023 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:200706 |
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