Mass dependent isotope fractionation during impacts induced the Archaean mass-independent fractionation of sulphur: Evidence against Great Oxidation Event(All rights reserved)

 A prevailing hypothesis, low-oxygen level of the Archaean atmosphere,

relies strongly on the presence of strong mass-independent fractionation

(MIF) of the sulfur isotopes in sulfide- and sulfate-bearing minerals older

than 2.4 billion years1,2,3,4. Actually, there is “a broad overlap between

MIF signals observed within Archaean sedimentary sequences and

periods of enhanced asteroid impacts represented by impact ejecta/fallout

units” (Fig. 1)5. Impact processes (vaporization and condensation) are

sufficient to explain the MIF signals following the principle: the earlier

the condensed material, the more enriched in lighter isotopes6. The nature

of the MIF of the sulfur isotopes is that the fractionation of isotope is still

mass dependent during impacts, which means the measured nonzero

⊿33S values of Archean sulfide- and sulfate-bearing minerals indicate

that their different condensation sequences. Thus, it is clear that the

signals are the markers of impact rather than O2 poor atmosphere.

Furthermore, the lack of MIF-S in several Archaean units before 2.4

billion years also supports this idea7,8.

 
References:

1 Farquhar, J., Bao, H. M. & Thiemens, M. Atmospheric influence of Earth's earliest sulfur cycle. Science 289, 756-758 (2000).

2 Farquhar, J. et al. Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry. Nature 449, 706-U705, doi:10.1038/nature06202 (2007).

3 Farquhar, J. & Wing, B. A. Multiple sulfur isotopes and the evolution of the atmosphere. Earth Planet. Sci. Lett. 213, 1-13, doi:10.1016/s0012-821x(03)00296-6 (2003).

4 Kaufman, A. J. et al. Late Archean biospheric oxygenation and atmospheric evolution. Science 317, 1900-1903, doi:10.1126/science.1138700 (2007).

5 Glikson, A. Archaean asteroid impacts, banded iron formations and MIF-S anomalies: A discussion. Icarus 207, 39-44, doi:10.1016/j.icarus.2009.11.024.

6 Huang, H. Isotope fractionation during impacts. Available from Nature Precedings

(2010).

7 Bao, H. M., Rumble, D. & Lowe, D. R. The five stable isotope compositions of Fig Tree barites: Implications on sulfur cycle in ca. 3.2 Ga oceans. Geochimica et Cosmochimica Acta 71, 4868-4879, doi:10.1016/j.gca.2007.05.032 (2007).

8 Ohmoto, H., Watanabe, Y., Ikemi, H., Poulson, S. R. & Taylor, B. E. Sulphur isotope evidence for an oxic Archaean atmosphere. Nature 442, 908-911, doi:10.1038/nature05044 (2006).