  Non-mass dependent chromium isotopic signatures have been successfully used
to determine the presence and identification of extra-terrestrial materials in
terrestrial impact rocks. Paleoproterozoic spherule layers from Greenland
(Graenseso) and Russia (Zaonega), as well as some distal ejecta deposits (Lake
Superior region) from the Sudbury impact (1,849 +/- 0.3 Ma) event, have been
analyzed for their Cr isotope compositions. Our results suggest that 1) these
distal ejecta deposits are all of impact origin, 2) the Graenseso and Zaonega
spherule layers contain a distinct carbonaceous chondrite component, and are
possibly related to the same impact event, which could be Vredefort (2,023 +/-
4 Ma) or another not yet identified large impact event from that of similar
age, and 3) the Sudbury ejecta record a complex meteoritic signature, which is
different from the Graenseso and Zaonega spherule layers, and could indicate
the impact of a heterogeneous chondritic body.
