Abstract

We collected a debris rich ice core from a buried ice mass in Ong Valley, located in 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 kyrs is rare and therefore much effort has been recently focused on finding ice that is older than 1 Ma. 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 Myrs. 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 Myrs 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.

Highlights

  • Ice cores from glaciers and ice sheets are used as an archive for paleoclimate proxies, including: atmospheric gases, chemical 30 compounds, and airborne particles (Dansgaard et al, 1969; Fredskild and Wagner, 1974; Castellano et al, 2004; Willerslev et al, 2007); the potential age of ice core records is limited by the fact that ice sheets are subject to deformation, ice flow, and basal melting

  • Visual inspection of the ice core indicate that it is primarily composed of debris-rich ice containing poorly sorted sediment ranging from clay to clasts exceeding the diameter of the borehole (Fig. 4)

  • The results show that cosmogenic nuclide concentrations in the englacial debris show large downcore variations (Table 1)

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Summary

Introduction

Ice cores from glaciers and ice sheets are used as an archive for paleoclimate proxies, including: atmospheric gases, chemical 30 compounds, and airborne particles (Dansgaard et al, 1969; Fredskild and Wagner, 1974; Castellano et al, 2004; Willerslev et al, 2007); the potential age of ice core records is limited by the fact that ice sheets are subject to deformation, ice flow, and basal melting. The lack of ice older than ~1 Ma severely limits our direct paleoclimate record and creates uncertainties when modeling future climate predictions which include modeled configuration of the past Antarctica Ice Sheet (Bulthuis et al, 2019; Noble et al, 2020). This is important during the Pliocene epoch (Dolan et al, 2018; Haywood et al, 2009), in which global surface temperatures and CO2 levels were higher than present 50 (Pagani et al, 2010; Seki et al, 2010) and which is considered analog for current anthropogenic warming

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