Evidence of a Large Asteroid/Comet Impact During the Siderian PaleoProterozoic

First detection of extraterrestrial material in ca. 2.49 Ga impact spherule layer in Kuruman Iron Formation, South Africa

Authors:

Simonson et al

Abstract:

Thin layers rich in formerly molten spherules interpreted as distal ejecta from large impacts by extraterrestrial bodies have been found in 8 stratigraphic units deposited between ca. 2.63 Ga and 2.49 Ga and attributed to a minimum of 4 separate impacts. Here we report geochemical evidence of extraterrestrial material in the only one of these spherule layers where it has not been previously reported, the Kuruman spherule layer (KSL) in the Kuruman Iron Formation, a banded iron formation (BIF) in the Griqualand West Basin (South Africa). We identified the KSL in 3 drill cores separated by as much as ~350 km and analyzed 2 core samples that have a mean Ir concentration of ~12.9 ppb and nearly chondritic interelement ratios of platinum group elements Ir, Ru, Pt, and Rh. This suggests that the samples contain ~1%–3% by mass extraterrestrial material even though the spherules are highly diluted by ambient sediment. Our geochemical data strongly support the correlation of the KSL with the Dales Gorge spherule layer (DGSL) in a penecontemporaneous BIF in the Hamersley Basin (Western Australia). The KSL and DGSL are close matches in terms of major and various trace element contents and the DGSL has a comparable Ir content of ~11.5 ppb. Therefore it is very likely the KSL and DGSL are distal ejecta from a ca. 2.49 Ga impact by a single extraterrestrial object greater than 10 km across. The lack of any significant changes in the stratigraphic succession in either basin also implies that large impacts alone are not sufficient to cause long-term changes in Earth's surface environments.