On the nature of the dirty ice at the bottom of the GISP2 ice core

We present data on the triple Ar isotope composition in trapped gas from clean, stratigraphically disturbed ice between 2800 and 3040m depth in the GISP2 ice core, and from basal dirty ice from 3040 to 3053m depth. We also present data for the abundance and isotopic composition of O₂ and N₂, and abundance of Ar, in the basal dirty ice. The Ar/N₂ ratio of dirty basal ice, the heavy isotope enrichment (reflecting gravitational fractionation), and the total gas content all indicate that the gases in basal dirty ice originate from the assimilation of clean ice of the overlying glacier, which comprises most of the ice in the dirty bottom layer. O₂ is partly to completely depleted in basal ice, reflecting active metabolism. The gravitationally corrected ratio of ⁴⁰Ar/³⁸Ar, which decreases with age in the global atmosphere, is compatible with an age of 100-250ka for clean disturbed ice. In basal ice, ⁴⁰Ar is present in excess due to injection of radiogenic ⁴⁰Ar produced in the underlying continental crust. The weak depth gradient of ⁴⁰Ar in the dirty basal ice, and the distribution of dirt, indicate mixing within the basal ice, while various published lines of evidence indicate mixing within the overlying clean, disturbed ice. Excess CH₄, which reaches thousands of ppm in basal dirty ice at GRIP, is virtually absent in overlying clean disturbed ice, demonstrating that mixing of dirty basal ice into the overlying clean ice, if it occurs at all, is very slow. Order-of-magnitude estimates indicate that the mixing rate of clean ice into dirty ice is sufficient to maintain a steady thickness of dirty ice against thinning from the mean ice flow. The dirty ice appears to consist of two or more basal components in addition to clean glacial ice. A small amount of soil or permafrost, plus preglacial snow, lake or ground ice could explain the observations.