Effect of atmospheric water vapor on modification of stable isotopes in near-surface snow on ice sheets

[1] A Rayleigh fractionation model is developed to investigate postdepositional modification of stable isotopes of water in the near-surface snow of East Antarctica. The processes of forced ventilation, pore-space diffusion, and intra-ice-grain diffusion are parameterized through characteristic time constants. Routine meteorological observations, their derived products, and general glaciological conditions from the South Pole are used to simulate the d18O of near-surface snow as it evolves in time. The sensitivity of d18O of near-surface snow to wind speed, surface temperature, and accumulation rate is tested through model experiments. A steady wind of 5–10 m s1 results in annual mean d18O enrichment of 3–7%. Enrichment of the heavy isotope occurs predominantly in buried winter snow layers during summer. However, in many simulations, there is a slight depletion of d18O summer layers that occurs as atmospheric water vapor is deposited within the snow during winter. Postdepositional modification of water stable isotopes in near-surface snow also depends on the snow accumulation rate; high accumulation rates quickly advect snow layers away from the influence of the atmosphere, preventing significant modification. Low accumulation rates (e.g., at Vostok and Dome C) allow significant postdepositional modification because the near-surface snow is exposed to forced ventilation for several annual cycles. Postdepositional modification during the Last Glacial Maximum (LGM) at Summit, Greenland is estimated to be greater than the present-day postdepositional modification. Therefore the temperature difference for LGM may be larger than that previously inferred from the d18O record. However, because of the compensating effects of lower temperatures and smaller accumulation during the LGM in Antarctica, the postdepositional enrichment in East Antarctica may be approximately the same for LGM as for the modern climate