Heavy Bombardment of the Earth at - 3 . 85 Ga : The Search for Petrographic and Geochemical Evidence

The Moon experienced an interval of intense bombardment peaking at -3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It can be assumed that the early Earth must have been subjected to an even more intense impact flux resulting from its larger size and because of its proximity to the Moon. Siderophile-element analyses (e.g., Ir abun­ dance) of the oldest sediments on Earth could be used to indicate past escalated influxes of extraterrestrial material. In addition, shocked minerals may also be present in the oldest extant rocks of sedimentary origin as detrital minerals. and remnants of impact ejecta might exist in early Archean formations. Searches for impact signatures have been initiated in the oldest sedi­ ments on the Earth, from the early Archean (>3.7 Ga) terrane of West Greenland; some of these rocks have been interpreted to be at least 3.8 Ga in age. So far, unequivocal evidence of a late heavy bombardment on the early Earth remains elusive. We conclude that either the sedimen­ tation rate of the studied sediments was too fast and therefore too diluting to record an obvi­ ous signal, or the ancient bolide flux has been overestimated, or the bombardment declined so rapidly that the Greenland sediments, some even at -3.85 Ga in age, do not overlap in time with it.