IMPACT DELIVERY OF REDUCED PHOSPHORUS IN THE ARCHEAN

Phosphorus (P) is a critical element in modern biochemical systems and presumably was so during the origin of life. However, formation of prebiotic molecules on the early Earth using common terrestrial phosphorus minerals such as apatite is retarded by a lack of reactivity and poor solubility. Pasek et al. (2013) suggested that the aqueous oxidation of schreibersite (F, Ni)₃ P was a possible source of reduced P species. Schreibersite is a phosphide mineral present in meteorites and interplanetary dust that was likely transported to the early earth during the heavy bombardment period. These reduced forms of P likely led to the synthesis of organophosphorus compounds required for biochemical reactions essential to living organisms.

As an assessment of impact delivery of phosphorus, we investigated phosphorus speciation across the Bee Gorge impact spherule horizon from the Wittenoom Formation in the Hamersley Basin, Pilbara Craton, Western Australia. Analytical analysis was conducted using a coupled High Performance Liquid Chromatograph (HPLC) – Inductively Coupled Plasma Mass Spectrometer (ICPMS) to determine concentrations of orthophosphate (+5), phosphite (+3) and hypophosphite (+1) in extracts of this rock.

The Bee Gorge Member is comprised of carbonate and calcareous siltstone deposited at ~2.54 Ga. This unit contains a ~10cm thick layer of sand-size spherules of silicate melt interpreted as microkrystites within anorbital wave ripples resulting from a major bolide impact-generated tsunami (Hassler et al. 2000). Hence, the Bee Gorge impact layer was likely deposited in a deep shelf marine environment unaffected by atmospheric interference, thereby providing an undisturbed redox record of its formation.

Reduced P compounds such as phosphite are present in the Wittenoom Formation primarily in the carbonate layer above the spherules. In contrast, little phosphite is found above and below these spherule and carbonate layers. The results from our study support the hypothesis that meteorites are a plausible source for the reduced forms of P necessary to support the origin of life on the early Earth.