Isotopic diffusion in ice enhanced by vein-water flow
<p>Diffusive smoothing of signals on the water stable isotopes (<span class="inline-formula"><sup>18</sup></span>O and D) in ice sheets fundamentally limits the climatic information retrievable from these ice-core proxies. Past theories explained how, in polyc...
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-07-01
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| Series: | The Cryosphere |
| Online Access: | https://tc.copernicus.org/articles/17/3063/2023/tc-17-3063-2023.pdf |
| Summary: | <p>Diffusive smoothing of signals on the water stable
isotopes (<span class="inline-formula"><sup>18</sup></span>O 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 <span class="inline-formula">∼</span> 10<span class="inline-formula"><sup>1</sup></span>–10<span class="inline-formula"><sup>2</sup></span> m yr<span class="inline-formula"><sup>−1</sup></span>. 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.</p> |
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| ISSN: | 1994-0416 1994-0424 |