Setting a chronology for the basal ice at Dye-3 and GRIP: Implications for the long-term stability of the Greenland Ice Sheet

- Trapped gas studies give new information about basal ice of GRIP and Dye-3 ice cores.
- Basal ice at GRIP and Dye-3 was dated by measuring Ar isotopes in trapped air.
- GRIP dating documents the presence of ice over central Greenland for the past 1 Myr.
- Dye-3 ages show that the South Greenland ice lobe was present throughout the Eemian.

The long-term stability of the Greenland Ice Sheet (GIS) is an important issue in our understanding of the climate system. Limited data suggest that the northern and southern sections extend well back into the Pleistocene, but most age constraints do not definitively date the ice. Here, we re-examine the GRIP and Dye-3 ice cores to provide direct ice core observations as to whether the GIS survived previous interglacials known to be warmer (∼130 ka) or longer (∼430 ka) than the present interglacial. We present geochemical analyses of the basal ice from Dye-3 (1991-2035 m) and GRIP (3020-3026 m) that characterize and date the ice. We analyzed the elemental and isotopic composition of O2, N2, and Ar, of trapped air in these two cores to assess the origin of trapped gases in silty ice. Dating of the trapped air was then achieved by measuring the paleoatmospheric δ40Ar/38Ar and the 17O anomaly (Δ17) of O2. The resulting age is a lower limit because the trapped air may be contaminated with crustal radiogenic 40Ar. The oldest average age of replicates measured at various depths is 970±140 ka for the GRIP ice core and 400 ka±170 ka for Dye-3. Δ17 data from Dye-3 also argue strongly that basal ice in this core predates the Eemian. This confirms that the Greenland Ice Sheet did not completely melt at Southern Greenland during the last interglacial, nor did it completely melt at Summit Greenland during the unusually long interglacial ∼430 kyr before present.