Extraterrestrial organic matter preserved in 3.33 Ga sediments from Barberton, South Africa
Résumé
Electron paramagnetic resonance (EPR) analysis of carbonaceous, volcanic, tidal sediments from the 3.33 Ga-old Josefsdal Chert (Kromberg Formation, Barberton Greenstone Belt), documents the presence of two types of insoluble organic matter (IOM): (1) IOM similar to that previously found in Archean cherts from numerous other sedimentary rocks in the world and of purported biogenic origin; (2) anomalous IOM localized in a 2 mm-thick sedimentary horizon. Detailed analysis by continuous-wave-EPR and pulse-EPR reveals that IOM in this layer is similar to the insoluble component of the hydrogenated organic matter in carbonaceous chondrites, suggesting that this narrow sedimentary horizon has preserved organic matter of extraterrestrial origin. This conclusion is supported by the presence in this thin layer of another anomalous EPR signal at g = 3 attributed to Ni-Cr-Al ferrite spinel nanoparticles, which are known to form during atmospheric entry of cosmic objects. From this EPR analysis, it was deduced that the anomalous sedimentary layer originates from deposition, in a nearshore environment, of a cloud of tiny dust particles originating from a flux of micrometeorites falling through the oxygen-poor Archean atmosphere.
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