The Extraterrestrial Mass Flux on the Early Earth

We have measured iridium and platinum in ancient metasediments to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr (the Hadean era). The craters of the Moon record an intense "late heavy bombardment" (LHB) beginning as early as 4.15 Gyr, and terminating ca.3.85 +/- 0.05 Gyr. Recent geochemical studies of metasediments and other supracrustal rocks from Akilia Island, in southern West Greenland, appear to extend both the record of marine sedimentation and the record of metabolically-sophisticated life to > 3.8 Gyr. Additional information is contained in the original extended abstract.