Significance

Giant meteorite impacts during Earth’s early history likely had significant effects on early life. We studied the effects on the surface environment and life of a Paleoarchean impactor ~50 to 200× larger than the famous K-Pg impactor. The impact caused a tsunami, partial ocean evaporation, and darkness that likely harmed shallow-water photosynthetic microbes in the short-term, while life in the deeper oceans and hyperthermophiles were less impacted. The impact also released phosphorus into the environment, and the tsunami brought iron-rich deep-water to the surface. As a consequence, there was a temporary bloom of iron-cycling microbes. Giant impacts were not just agents of destruction but also conferred transient benefits on early life.

Abstract

Large meteorite impacts must have strongly affected the habitability of the early Earth. Rocks of the Archean Eon record at least 16 major impact events, involving bolides larger than 10 km in diameter. These impacts probably had severe, albeit temporary, consequences for surface environments. However, their effect on early life is not well understood. Here, we analyze the sedimentology, petrography, and carbon isotope geochemistry of sedimentary rocks across the S2 impact event (37 to 58 km carbonaceous chondrite) forming part of the 3.26 Ga Fig Tree Group, South Africa, to evaluate its environmental effects and biological consequences. The impact initiated 1) a giant tsunami that mixed Fe2±rich deep waters into the Fe2±poor shallow waters and washed debris into coastal areas, 2) heating that caused partial evaporation of surface ocean waters and likely a short-term increase in weathering and erosion on land, and 3) injection of P from vaporization of the S2 bolide. Strata immediately above the S2 impact event contain abundant siderites, which are associated with organic matter and exhibit light and variable δ13Ccarbvalues. This is consistent with microbial iron cycling in the wake of the impact event. Thus, the S2 impact likely had regional, if not global, positive and negative effects on life. The tsunami, atmospheric heating, and darkness would likely have decimated phototrophic microbes in the shallow water column. However, the biosphere likely recovered rapidly, and, in the medium term, the increase in nutrients and iron likely facilitated microbial blooms, especially of iron-cycling microbes.