Modeling the densification of polar firn including heat diffusion: Application to close‐off characteristics and gas isotopic fractionation for Antarctica and Greenland sites

Abstract

.[1] Modeling the densification of polar firn is essential to estimate variations of close-off characteristics (density, close-off depth, delta age) in relation with past climate parameters (temperature and accumulation rates). Furthermore, the air composition in the firn is modified by gravitational and thermal fractionation, and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations. In this work, we propose a new model coupling firn densification and heat diffusion. In addition to the determination of the firn thickness and gas-ice age differences, the model allows a reconstruction of the time evolution of the temperature for different sites in Antarctica (Vostok) and Greenland (GISP2) and therefore the evolution of gravitational and thermal isotopic fractionations in firn. Under present-day conditions, the modeled profiles are in good agreement with the available temperature measurements in firn. For sites with low accumulation rates such as Vostok, the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur. The comparison of the modeled d15 N results to measurements allows to better constrain the d 18 O-temperature relationship, used to infer the surface temperature history, and for GISP2, the model validates the long-term borehole-based temperature.INDEX TERMS: 1827 Hydrology: Glaciology (1863); 1863 Hydrology: Snow and ice (1827); 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; KEYWORDS: firn densification, temperature reconstruction, gas fractionation Citation: Goujon, C., J.-M. Barnola, and C. Ritz, Modeling the densification of polar firn including heat diffusion: Application to close-off characteristics and gas isotopic fractionation for Antarctica and Greenland sites,

What was the surface temperature in central Antarctica during the last glacial maximum?

“…However, the model approach by Arnaud et al [27] leads to vages about 500 yr smaller than with our model [33]. Together with a 10% higher accumulation this model is in agreement with our data.…”

“…5 of Salamatin et al [1], T BH 2 is identical to T WI but the temperature increase from the last glacial to the Holocene is stretched to become 15 ‡C [1]. Present day temperature was taken as 357.3 ‡C [33] for all calculations. We also compare our empirical vage and COD to COD and vage deduced from N 15 N data.…”

“…Alternatively the COD can be forced to ¢t the N 15 N deduced COD by starting out from a higher surface density. However, the magnitude of the surface density would have to be 600 kg m 33 . Such a high density at the top of the ¢rn column di¡ers dramatically from values observed under dry sintering conditions and seems highly unlikely.…”

On the impact of impurities on the densification of polar firn

“…The models consider the overburden pressure as driving force (Kameda et al, 1994), the microscopic approach for the geometrical theory of pressure sintering of mono-sized spherical powders (Arnaud et al, 2000;Kameda et al, 1994) or couple the firn densification with heat diffusion (Goujon et al, 2003). Critical densities, at which the main densification process changes from grain boundary sliding to plastic deformation (Arnaud et al, 2000;Salamantin et al, 2006) are included in order to take care of the changing densification rate with depth.…”

“…Critical densities, at which the main densification process changes from grain boundary sliding to plastic deformation (Arnaud et al, 2000;Salamantin et al, 2006) are included in order to take care of the changing densification rate with depth. Main focus of the model studies is the prediction of air close-off within the firn column and thus detection of the age of the entrapped air (Arnaud et al, 2000;Goujon et al, 2003).…”