THE INFLUENCE OF IMPACTS ON THE FIRST 87% OF EARTH HISTORY

Most of Earth’s supracrustal rocks are Phanerozoic in age, but the first 88% of Earth history (~ 4 billion years) resides in the Precambrian. The Moon is covered with Precambrian craters, but only a few dozen impacts that age have been documented on Earth. Relating evolution to impacts is also a daunting task because “hard parts” didn’t evolve until the end of the Precambrian, but interesting connections are beginning to emerge. The first and arguably most important stems from the fact that evidence of life and stable surface environments both appeared virtually simultaneously. Apparently neither one could get established until impact activity decreased significantly ca. 3.9 Ga (billion years ago), marking the end of the Hadean. Another important point is that the Archean and Proterozoic have different types of impact records. The Archean persisted from 3.8 to 2.5 Ga, and unlike younger times, its impact archive consists of distinctive spherule-rich layers rather than conventional impact craters. Spherule layers from 6 or more large Archean impacts have been found, and there are already some tantalizing coincidences. Stratigraphically, spherule layers occur very close to the shift from volcanic to sediment-dominated successions in the Barberton Greenstone Belt, the base of Earth’s first large banded iron formation (in the Hamersley basin), and the base of Earth’s first large microbial edifice (the Transvaal carbonates of South Africa). These could be signs that large impacts played significant roles in key tectonic and biologic turning points early in Earth history. In contrast, the Proterozoic contains the two largest impact structures known (Vredefort and Sudbury) but very few spherule layers. Given a duration of almost 2 billion years, evidence of impacts is surprisingly scarce. Of 23 Proterozoic impact structures reported, 13 are Neoproterozoic (from 0.9 to 0.54 Ga). There is almost no evidence of impacts in the Mesoproterozoic (from 1.6 to 0.9 Ga), which was probably the longest sustained period of stability in Earth surface history. Perhaps a paucity of large impacts was a key factor in maintaining this stability. The impact record picked up significantly in the Neoproterozoic as Earth’s surface underwent its greatest climate fluctuations ever and life reached new levels of complexity.