CONSISTENT FLOW PATTERNS OF THE ARGOSY GLACIER DETERMINED FROM THE PROVENANCE OF GLACIAL TILL IN ONG VALLEY, ANTARCTICA

The U-Pb zircon age populations from three tills in Ong Valley, Antarctica, are statistically the same, which indicates the source of these tills is the same provenance. We interpret this to indicate that the Argosy Glacier that deposited these drifts flowed in a consistent manner for millions of years.

Ong Valley contains three till units that have previously been dated with cosmogenic nuclides to yield minimum ages of 11 kyrs, 1.1 Myrs, and 1.57 Myrs. The middle and youngest drifts overlie relict glacier ice, which is preserved under thin (< 1m) layers of till. We collected samples from the three till units, the two buried ice bodies, and from the modern Argosy Glacier front. We also collected samples of the local bedrock, the Hope Granite and Argosy Gneiss. From these samples, we isolated over 800 total zircon minerals and U-Pb dated them with LA-ICP-MS. All detrital samples show major clusters of zircon ages at 535 Ma, 1,690 Ma, and 3,050 Ma, and minor peaks at 1,030 Ma and 2,500 Ma. We used a Kolmogorov-Smirnoff test to see if two samples are drawn from the same population, and all three till units are statistically indistinguishable. Zircon sands collected from relict glacier ice buried under the middle and youngest tills and from the modern Argosy glacier do not match any of the tills individually. When all the ice samples are combined into one unit, they match all of the tills. We suggest this result arises from a bias induced by our sampling strategy. Ice samples were collected from individual bands of debris in the ice, which may be more homogeneous than the tills. The till samples were each about 5 cm thick, and we combined multiple pits and multiple depths from each drift. The till samples likely represent multiple debris bands from the ice, not just a single one.

If the zircon populations had changed between the tills, then something must have changed in the flow of the Argosy Glacier to bring in different zircon minerals. Either the Argosy Glacier could have flowed over new areas or it could have eroded into deeper rock layers. We see no evidence of either of these options in the data, and our results are consistent with the notion that this portion of the East Antarctic Ice Sheet and the Central Transantarctic Mountains has remained cold and only slowly changing for at least the past 1.57 Myrs.