BUBBLY-ICE DENSIFICATION IN ICE SHEETS - II - APPLICATIONS

A mathematical model for simulating the densification of bubbly glacie r ice is used to interpret the following experimental data from the Vo stok (central Antarctica) ice core: two ice-porosity profiles obtained by independent methods and a bubble-pressure profile obtained by dire ct measurements of air pressure within individual bubbles. The rheolog ical properties of pure polycrystalline ice are deduced from the solut ion of the inverse problem. The model and the inferred ice-now law are then validated, using porosity profiles from seven other ice cores dr illed in Antarctica and Greenland, in the temperature range from -55 d egrees to -20 degrees C. The following expression is adopted for the c onstitutive law: 2<(e)over dot> = (tau/mu(1) + tau(alpha)/mu(2))exp[Q( 1/T-s - 1/T)/R-s] where <(e)over dot> and tau are the effective strain rate and stress, respectively, ct is the creep exponent taken as 3.5, R-s is the gas constant and T(T-s) is the temperature (standard tempe rature). The numerical values obtained for the ''linear'' and ''non-li near'' viscosities are: mu(1) = 2.9 +/- 1.3 MPa year and mu(2) = 0.051 +/- 0.019 MPaalpha year, and the apparent activation energy Q is conf irmed to be 60 kJ mole(-1). The corresponding flow law is in good agre ement with results of both mechanical tests and independent estimation s based on the analysis of different natural phenomena associated with glacier-ice deformation. When the model is constrained by the porosit y and bubble-pressure profiles from Vostok, the mean air content in Ho locene ice is inferred to be about 0.088 cm(3) g(-1). The correspondin g mean air pressure in bubbles at the end of pore closure is about 0.0 83 MPa, whereas the atmospheric pressure at this depth level would be 0.063 MPa. The influence of the climatic change on the ice-porosity pr ofile is discussed. It resulted in an increased air content in ice at Vostok during the Last Glacial Maximum: 0.096 cm(3) g(-1).