POLYMICT FREQUENCY IN ROSS SEA TILL, ANTARCTICA

Sand petrography data were collected from 9 thin sections from the last-glacial maximum (LGM) till in the Ross Sea. The goal is to assess sand petrography data from four cores across the Ross Sea by looking for the frequency of polymicts down core at 10-centimeter intervals down to 1 meter. Preliminary data from two cores in the Ross Sea have been recorded. Polymicts are rounded, sand-sized size aggregates of poorly sorted sand, silt and clay that appear to be associated with streaming ice.

Three types of polymicts were identified that were determined from plasmic fabric and internal structure. The classification includes polymicts with: (a) diffuse edges and no internal structure, (b) moderately defined edges, and a weak inner core grain and plasmic fabric and (c) well-defined edges, an inner core grain and plasmic fabric.

Preliminary data on polymict abundance in east-central Ross Sea core NBP94-07-39, shows variation from 70% to 100% of the coarse sand fraction. The lowest abundance is in the top of the core and is >90% below a depth of 32-34 centimeters. The data show that there is variability in polymict type downcore. For instance, the diffuse edge-type polymicts vary from 32-61% downcore. The non-polymict fraction is composed of quartz and felsic igneous grains in most samples.

In core ELT32-21 from the western Ross Sea, there is a decrease in polymicts downcore. At 30 cm, the sample contains 38% polymicts and at 55 cm, polymict abundance drops to 20%. The most common type of polymict observed is intermediate type polymicts that include an inner structure and moderately defined edges. The non-polymict fraction is dominantly composed of quartz, schist and extrusive igneous grains.

Particle size data and clay mineralogy will be assessed to identify their possible controls on polymict type and abundance.