GRAVITATIONAL ENRICHMENT OF KR-84 AR-36 RATIOS IN POLAR ICE CAPS - A MEASURE OF FIRN THICKNESS AND ACCUMULATION TEMPERATURE/

Measurements of Kr-84/Ar-36 ratios in Greenland ice show that gravitat ional separation in the firn layer is responsible for the enrichments relative to atmospheric ratios. The Kr-84/Ar-36 ratio is enriched by 1 2.8 per mil and is 24 times the O-18/O-16 enrichment in trapped O-2, a s predicted for gravitational fractionation. Because gravitational enr ichment depends on firn thickness, which in turn depends on annual mea n temperature, noble gas ratios provide a method for determining paleo temperatures and ancient firn thicknesses in polar ice caps, The gravi tational effects are modulated by about 10 to 15 percent by atmospheri c concentration changes caused by temperature effects on oceanic gas s olubilities. The availability of five noble gases should make it possi ble to deconvolute the solubility and gravitational enrichments for ca libration of O-18 paleotemperatures throughout the polar ice sheets.