Institutions:

CASE WESTERN RESERVE UNIV,DEPT GEOL SCI/CLEVELAND//OH/44106
NEW MEXICO INST MIN & TECHNOL,DEPT EARTH & ENVIRONM SCI/SOCORRO//NM/87801
UNIV CALIF BERKELEY,SPACE SCI LAB/BERKELEY//CA/94720
USA,COLD REG RES & ENGN LAB/HANOVER//NH/03755
LAWRENCE LIVERMORE NATL LAB/LIVERMORE//CA/94550

Abstract:

During systematic sampling of volcanic ash (tephra) layers at a well-known Antarctic meteorite collection site (the Allan Hills main ice field), a band of
unusually dark and rounded (many spheroidal) particles was discovered. This debris layer (BIT-58) extends parallel to the stratigraphy of the ice
established from the tephra bands, apparently marking a single depositional event. The shapes, internal texture, major element composition, and levels
of cosmogenic nuclides of particles from within BIT-58 all strongly suggest that this material represents ablation debris from the passage of a large
H-group ordinary chondrite, Preliminary cosmogenic isotope dating suggests an age of 2.8 Ma, implying that the East Antarctic ice sheet has been
stable since that time. The relationship of the Bit-58 layer to known impact events is not clear.

Keywords:

+TERRESTRIAL