Gas enclosure in ice : age difference and fractionation

Ice from polar ice sheets and from glaciers bear a unique archive of the past atmospheric composition. However, the air extracted from the ice gives not only information about the atmospheric composition. Through the complex manner of gas occlusion the extracted gases can also provide information about physical properties at the drill site in the past. Since air in the porous firn layer on the surface of the ice sheet exchanges with the overlying atmosphere it has a mean age which is younger than the age of the surrounding ice when it gets enclosed in bubbles at 50-150 m below the surface. The difference between the age of the ice and the mean age of the gas at close-off depth has been calculated using a dynamic model for firn densification and diffusional mixing of the air in the firn including the heat transport in the firn and temperature dependence of the close-off density. The gravitational fractionation of 015N which is constant in the atmosphere gives an independent measure of the close-off depth. These results are generally in agreement with the modelled close-off depth. The occlusion process adds new information to the gas record. Since the occlusion process is temperature sensitive the depth difference between corresponding events in the ice and in the gas record allows in principle to reconstruct the temperature history. Temperature reconstructions for Greenland based on borehole temperature measurements suggest that the temperature calculated based on the spatial 0180ltemperature relation does at least not hold for the glacial/interglacial temperature increase. Under the assumption that the 0180 and the CH4 signal in the GRIP and GISP2 ice core are synchronous, justified by the close resemblance of the two records, they corroborate that the changed 0180/temperature relation is also valid for short term variations like DansgaardOeschger events. Recently a new aspect of the 015N record has been brought up. During fast temperature changes the 015N concentration in the firn changes due to thermal diffusion. This effect can be used to determine independently temperature changes during fast climatic events. Physics a/Ice Core Recams Edited by T. Hondoh Hokkaido University Press, 2000, Sapporo 308 Gas enclosure in ice: age difference andfi'actionation