Krypton-81 (81Kr) dating of dirty ice from the bottom 14 meters of the Greenland Ice Sheet Project 2 (GISP2) ice core, 2023

The object of this work is to date ice at the bottom of the Greenland Ice Sheet Project 2 (GISP2) ice core at Summit, Greenland, as a contribution to understanding the origin of the Greenland Ice Sheet. The GISP2 ice core penetrated 2800 m (meters) of clean ice, in proper stratigraphic order, extending back to 100 ka (thousands of years before the present). Below, the GISP2 ice core samples 240 m of clean but disturbed ice that was no longer stratigraphically continuous. This ice is dated to about 100-250 ka. This disturbed, clean ice is in turn underlain by a 14 m thick layer of visibly dirty ice, which overlies bedrock. The age of this dirty ice is presumably a minimum age of the GISP2 ice core. In principle, it would be straightforward to date this ice from its Ar (argon) isotope composition. With time, the 40Ar (Argon-40)/38Ar (Argon-38) ratio of air rises toward the future as radiogenic 40Ar is released from the crust and mantle into the atmosphere. However, trapped air at GISP2 is contaminated with radiogenic 40Ar from the underlying crust. An alternative method involves measuring the 81Kr (Krypton-81)/Kr ratio of the trapped air. 81Kr has a half-life of 229 kyr (thousand years). It is well mixed in the atmosphere and has an abundance that is nearly constant with time. When ice cores trap air, the air is isolated, and the ratio of 81Kr/Kr decreases exponentially with time. Age can be calculated from this ratio. 81Kr dating also has an important limitation: mixing of ice leads to 81Kr ages that are younger than the average age of the ice. This is a consequence of mixing an exponentially decaying property. The great experimental challenge of dating ice cores with 81Kr is that its abundance is extremely low, especially given limited availability of ice in the 14 m dirty ice layer at the bottom of GISP2. This challenge has led us to work with Zhengtian Lu and his colleagues at the University of Science and Technology of China. They have built an instrument that measures the abundance of 81Kr optically, achieving a much greater accuracy for measuring 81Kr of terrestrial materials at low levels. In this study, we extracted air from 3 dirty ice samples, and measured the 81Kr/Kr ratio. The deepest sample gave an age of 714 ka, which we take as a minimum age. Cosmogenic isotope studies suggest a maximum age of about 1.1 Ma (million years), indicating that the origin of the present Greenland ice sheet was associated with the Mid-Pleistocene Transition.