Talk Description
Early Earth habitability must have been strongly affected by large meteorite impacts. While the impactor flux gradually decreased from the Hadean to the Archean, at least 16 major impact events (bolide diameter >10 km) are recorded in the Archean rock record. Although these impacts doubtfully caused complete ecosystem annihilation, they still had severe consequences for surface conditions. Here we study the sedimentology, petrography, and geochemistry of sedimentary rocks across the 3.26 Ga S2 impact event (bolide diameter 37-58 km) in a shelfal and a shallow water section to evaluate the environmental effects of the impact and its possible consequences for early life. The two sections show similar transitions in sedimentation: (1) Below the S2 spherule layer, black-and-white banded cherts reflect background sedimentation. (2) A conglomerate containing impact-derived spherules reflects the initial passage of a tsunami. (3) This is overlain by a ~1-meter-thick, normally graded black chert bed representing the settling of fine, suspended particles after the tsunami passed. This black chert layer contains euhedral pseudomorphs formed as interstitial evaporates, reflecting partial ocean evaporation. In both sections, the grain size and/or the Al2O3 (wt%) abruptly increase across the impact event, suggesting an increase in detrital input that continued for some time after the passage of the initial tsunami wave. (4) Lastly, the sections show an abrupt (shallow-water section) to more gradual (shelfal) transition to iron-bearing cherts (FeO* up to 7.4 wt%). The increase in FeO* does not correlate with relative deepening or changes in geochemical proxies for provenance, volcanic activity, or hydrothermal input. It is hence most parsimoniously explained by the mixing of a stratified water column by the tsunami, making Fe2+-rich deep waters available to otherwise Fe2+-poor shallow water environments. Meteorite impacts are typically seen as agents of mass destruction and extinction. In the short term, the S2 impact certainly would have had disastrous consequences for the early biosphere. However, in the medium term, mixing of the ocean would have made Fe2+ as a potential electron donor available to the upper water column (photic zone) and the tsunami-induced erosion and aggressive weathering in a post-impact hothouse may have provided an in injection of nutrients, such as P, to the otherwise nutrient starved Archean oceans. Meteorite impacts may thus have had at least transient benefits for the early biosphere.