THE POLYMICT PUZZLE

Unlithified coarse-sand size aggregations composed of sediment <250 microns, herein called polymicts, are observed in grain mounts of the 500-2000 micron fraction of till collected: 1) beneath the Siple Coast ice streams of West Antarctica, 2) from subglacial till delivered to the ice surface at Lonewolf Nunataks at Byrd Glacier, and 3) from LGM-age till from the Ross Sea continental shelf. Polymict grains are spheroidal- to rod- shaped and may contain a single 'core' grain. Till samples collected from Lonewolf Nunataks average 50% polymict fragments. At this site, both unlithified and lithified fragments were identified. In contrast, polymict grains from the Siple coast ice streams contain a single population of unlithified fragments. Ten depth intervals analyzed from cores collected beneath the active Whillans and Bindschadler ice streams average74% polymict grains, whereas the samples from beneath the inactive Kamb ice stream average <8%.

In three Ross Sea cores, the petrography data from downcore Ross Sea till samples show a broad mix of sand grain composition interrupted by intervals with >90% polymict grains. For example in core NBP94-01-02, collected in western Ross Sea, the 50-52 cm and 150-152 cm intervals have a variety of sand-sized grains that include quartz, feldspar, metamorphic, intermediate, and felsic igneous grains. It is the 111-116 cm interval that contains 91% polymict grains while the lower interval contains <9%, and the upper interval contains ~38%. For till samples with >10% clay-size particles, no correlation is evident between clay content and polymict content (r2=.097). Additionally, no correlation was observed between polymict abundance and water content of the sample or the preparation method. Thus we hypothesize that water and possibly freezing at the bed promotes cohesion of sediment and the grain shape may be subsequently modified during transport. Furthermore the association between active ice streams and polymict abundance in till suggests that polymict-rich intervals in Ross Sea till may be indicative of times when streaming ice flowed across the continental shelf.