Cosmogenic nuclide dating of two stacked ice masses: Ong Valley, Antarctica

Abstract

We collected a debris-rich ice core from a buried ice mass in Ong Valley, located in the Transantarctic Mountains in Antarctica. We measured cosmogenic nuclide concentrations in quartz obtained from the ice core to determine the age of the buried ice mass and infer the processes responsible for the emplacement of the debris currently overlaying the ice. Such ice masses are valuable archives of paleoclimate proxies; however, the preservation of ice beyond 800 kyr is rare, and therefore much effort has been recently focused on finding ice that is older than 1 Myr. In Ong Valley, the large, buried ice mass has been previously dated at > 1.1 Ma. Here we provide a forward model that predicts the accumulation of the cosmic-ray-produced nuclides 10Be, 21Ne, and 26Al in quartz in the englacial and supraglacial debris and compare the model predictions to measured nuclide concentrations in order to further constrain the age. Large downcore variation in measured cosmogenic nuclide concentrations suggests that the englacial debris is sourced both from subglacially derived material and recycled paleo-surface debris that has experienced surface exposure prior to entrainment. We find that the upper section of the ice core is 2.95 + 0.18 / −0.22 Myr old. The average ice sublimation rate during this time period is 22.86 + 0.10 / −0.09 m Myr−1, and the surface erosion rate of the debris is 0.206 + 0.013 / −0.017 m Myr−1. Burial dating of the recycled paleo-surface debris suggests that the lower section of the ice core belongs to a separate, older ice mass which we estimate to be 4.3–5.1 Myr old. The ages of these two stacked, separate ice masses can be directly related to glacial advances of the Antarctic ice sheet and potentially coincide with two major global glaciations during the early and late Pliocene epoch when global temperatures and CO2 were higher than present. These ancient ice masses represent new opportunities for gathering ancient climate information.