POLYMICT FREQUENCIES IN LGM TILL AS A PROXY FOR PALEO ICESTREAM FLOW IN THE ROSS EMBAYMENT, ANTARCTICA

Sand petrography and particle size data were collected on 24 samples from last-glacial maximum (LGM) till across the Ross Embayment, Antarctica. The goal was to assess polymict frequencies down core at approximately 10 cm intervals. Polymicts are rounded, sand-sized size aggregates of poorly sorted sand, silt and clay, which are common under active, modern ice streams. Point count data from LGM deposits show that there is little variability of polymicts down core, but there is spatial variability in polymict abundance and type across the Ross Embayment. No relationship is observed between clay abundance and polymict abundance. The consistent values downcore imply that the process, which formed polymicts, was constant. This is being interpreted to mean there was a uniform amount of water at the bed and consistent ice stream behavior over time.

Three different types of polymicts were found and classified by plasmic fabric and internal structure. The classification of polymicts is as follows: (a) type 1 have diffuse edges and no internal structure, (b) type 2 have moderately defined edges, and a weak inner core grain and plasmic fabric and (c) type 3 have well-defined edges, an inner core grain and plasmic fabric.

Polymicts in core ELT32-21 in the Central Ross Sea contain 56% of type 2 polymicts and 21% of type 3 polymicts. Core NBP94-07-39 located in the East-Central Ross Sea contains 62% type 2 polymicts and 31% of type 1 polymicts. In the Eastern Ross Sea, core NBP99-02-17 contains 97% type 1 polymicts. Preliminary results show that polymict type beneath the Bindschadler Ice Stream is dominated by type 1 polymicts, similar to core NBP99-02-17, and the tills from Lonewolf Nunataks in East Antarctica have polymict types most similar to ELT32-21.

Between core variability of polymicts is thought to be related to clay type. X-ray diffraction will be used to determine the clay mineralogy of these samples to identify possible controls on polymict abundance and type.