Evolution of air bubble and air hydrate ensembles: statistical analysis of the EPICA-DML ice core

Evolution of air inclusion size distributions in the Antarctic ice sheet was studied inEPICA-Dronning Maud Land ice core. The total concentration of air bubbles and air hydrates wasanalysed and validated by air mass balance. The inclusion size distributions were describedwith probability functions and with normalized distribution functions, allowing to distinguishbetween climatic signals and physical processes in a hydrate inclusion ensemble. Two typesof distribution functions were observed. A log-normal function characterizes a distribution ofair bubbles above the bubble-hydrate transition zone. The hydrate ensemble in the transitionzone assumes the same log-normal distribution, which is characterized by the presence ofhydrates or bubbles with sizes larger than 2.5 times the average. Such big hydrates disappearat the end of the transition zone and the distribution function assumes a Gaussian type. Toexplain this we suggest the hypothesis of bifurcation mechanical instability on sphericallyshaped hydrates. After bubble-hydrate transformation the hydrate assumes the spherical shapeof the pre-existing air bubble and becomes unstable due to the ice sheet pressure, as it is thecase for rigid inclusions in a plastic matrix. Spherically shaped hydrates become unstable andtransform to elliptical or multi-branch shapes as observed in the ice core. To validate thishypothesis a statistical analysis of number of hydrates having different shapes was conducted.The results confirm that evolution of the hydrate concentration with depth can be explainedby mechanical instability and splintering of the hydrates by ice flow.