Archaean asteroid impacts, banded iron formations and MIF-S anomalies: A discussion

Abstract

The origin of mass-independent fractionation (MIF-S) of sulphur isotopes (δ³³S) recorded in sediments older than 2.45 Ga is widely interpreted in terms of UV-triggered reactions under oxygen-poor ozone-depleted atmosphere conditions (Farquhar, J., Bao, H., Thiemens, M. [2000] Science, 289, 756; Farquhar, J., Peters, M., Johnston, D.T., Strauss, H., Masterson, A., Wiechert, U., Kaufman, A.J. [2007] Nature, 449, 706–709; Farquhar, J., Wing, B.A. [2003] Earth Planet. Sci. Lett., 213, 1–13; Kaufman, A.J., Johnston, D.T., Farquhar, J., Masterson, A.L., Lyons, T.W., Bates, S., Anbar, A.D., Arnold, G.L., Garvin, J., Buick, R. [2007a] Science, 317, 1900–1903; Kaufman, A.J., Farquhar, J., Johnston, D.T., Lyons, T.W., Arnold, G.L., Anbar, A. [2007b] Deep Time Drilling Project of the NASA Astrobiology Drilling Program). Observed mid-Archaean variability of MIF-S signatures raises questions regarding the extent of atmospheric anoxia (Ohmoto, H., Watanabe, Y., Ikemi, H., Poulson, H.R., Taylor, B. [2006] Nature, 406, 908–991; Farquhar et al., 2007). Late Archaean (∼2.7–2.5 Ga) and mid-Archaean (∼3.2 Ga) sequences in the Pilbara Craton (Western Australia) and Kaapvaal Craton (South Africa), in which MIF-S data were measured, contain asteroid impact ejecta units dated as 2.48, 2.56, 2.63, 3.24, 3.26 and 3.47 Ga old (Lowe, D.R., Byerly, G.R., Kyte, T., Shukolyukov, A., Asaro, F., Krull, A. [2003] Astrobiology, 3, 7–48; Simonson, B.M., Hassler, S.W. [1997] Aust. J. Earth Sci., 44, 37–48; Simonson, B.M., Glass, B.P. [2004] Ann. Rev. Earth Planet. Sci., 32, 329–361; Glikson, A.Y. [2004] Astrobiology, 4, 19–50; Glikson, A.Y. [2006] Earth Planet. Sci. Lett., 246, 149–160; Glikson, A.Y. [2008] Earth Planet. Sci. Lett., 267, 558–570). Mass balance calculations based on iridium and ⁵³Cr/⁵²Cr isotopic anomalies (Byerly, G.R., Lowe, D.R. [1994] Geochim. Cosmochim. Acta, 58, 3469–3486; Kyte, F.T., Shukloyukov, A., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2003] Geology, 31, 283–286) and on impact spherule size distribution (Melosh, H.J., Vickery, A.M. [1991] Nature, 350, 494–497) suggest projectiles several tens of kilometers in diameter (Byerly and Lowe, 1994; Shukloyukov, A., Kyte, F.T., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2000]. In: Koeberl, C., Gilmour, I. (Eds.), Impacts and the Early Earth, Springer-Verlag, Berlin, pp. 99–116; Kyte, F.T., Shukloyukov, A., Lugmair, G.W., Lowe, D.R., Byerly, G.R. [2003] Geology, 31, 283–286). Due to incomplete preservation these impacts represent a minimum rate of the Archaean impact flux. High UV radiation associated with low ozone levels in the Archaean atmosphere may have been further enhanced by large impacts, accentuating MIF-S anomalies. The appearance of iron-rich sediments above late and mid-Archaean impact ejecta units (Glikson, A.Y. [2006] Earth Planet. Sci. Lett., 246, 149–160; Glikson, A.Y., Vickers, J. [2007] Earth Planet. Sci. Lett., 254, 214–226) may be related either to microbial oxidation of ferrous iron or, alternatively, photochemical oxidation of ferrous to ferric iron. Given post-2.45 Ga diluting of possible MIF-S anomalies by the oxygenating ocean sulfate reservoir (Pavlov, A.A., Kasting, J.F. [2002] Astrobiology, 2, 27–41), similar MIF-S anomalies may have been associated with Proterozoic and Phanerozoic impacts, although to date little evidence exists in this regard (Marouka, T., Koeberl, C., Newton, J., Gilmour, I., Bohor, B.F. [2002] Geological Society of America Special Paper 356, pp. 337–344; Koeberl, C., Thiemens, M. [2008] Multi-sulfur isotopes in cretaceous–tertiary boundary samples from the Western interior-search for photochemical effects 2008. Joint Meeting of the Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM. <http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_148134.htm> (abstract)). Detailed sampling and isotopic analyses across the impact ejecta fallout units are required in order to test possible relationships between Archaean impacts and MIF-S anomalies.

Introduction

This discussion aims at examining the potential significance of relations in time and space between asteroid impact fallout units, banded iron formations and MIF-S anomalies in Archaean terrains. The identification of mass-independent fractionation of sulphur isotopes (MIF-S) in pre-2.45 Ga sediments is an issue of current debate between advocates of an oxygen-poor Archaean atmosphere (Farquhar et al., 2000, Farquhar et al., 2007, Farquhar and Wing, 2003, Kaufman et al., 2007a, Kaufman et al., 2007b) and those suggesting heterogeneous Archaean oxygen levels (Ohmoto et al., 2006), with implications to the nature of the early terrestrial atmosphere–biosphere system (Holland, 2002, Pavlov and Kasting, 2002, Catling and Claire, 2005, Kasting and Ono, 2006). The development of photosynthesis, and thereby limited release of oxygen as early as about 3.4 Ga, is suggested by identification of heliotropic stromatolite reefs in the Pilbara Craton (Allwood et al., 2006). The abrupt disappearance of positive MIF-S anomalies at ∼2.45 Ga poses a problem, as atmospheric enrichment in oxygen due to progressive photosynthesis could, perhaps, be expected to result in a gradual rather than an abrupt decline in MIF-S signatures (cf. Cloud, 1968, Margulis et al., 1976, Holland, 1984, Holland, 2002, Pavlov and Kasting, 2002, Catling and Claire, 2005, Kopp et al., 2005, Goldblatt et al., 2006, Kasting and Ono, 2006, Ono et al., 2006, Anbar et al., 2007, Farquhar et al., 2007, Kaufman et al., 2007a).

MIF-S (δ³³S) anomalies (Table 1 and Fig. 1, Fig. 2, Fig. 3) overlap mid-Achaean impact periods (∼3.26–3.24 Ga) and Late Archaean impact periods (∼2.63, ∼2.56, ∼2.48 Ga) (Lowe et al., 1989, Lowe et al., 2003, Simonson and Hassler, 1997, Simonson et al., 2000, Simonson and Glass, 2004, Glikson, 2001, Glikson, 2004, Glikson, 2005, Glikson, 2006, Glikson, 2007, Glikson, 2008, Glikson and Allen, 2004, Glikson et al., 2004, Glikson and Vickers, 2006), though no specific age correlations are observed. Estimates of projectile diameters derived from mass balance calculations of iridium levels, ⁵³Cr/⁵²Cr anomalies and size-frequency distribution (Melosh and Vickery, 1991) of fallout impact spherules (microkrystites), suggest Archaean projectiles of the 3.26, 3.24, 2.63, 2.56 and 2.48 Ga impact events reached several tens of kilometre in diameter (Byerly and Lowe, 1994, Shukloyukov et al., 2000, Kyte et al., 2003, Glikson and Allen, 2004, Glikson, 2005, Glikson, 2008), which would have led to major atmospheric effects, including large scale ejection of sulphur-bearing material and isotopic fractionation of sulphur.

Archaean impact ejecta units in the Pilbara and Kaapvaal Cratons are almost invariably overlain by ferruginous shale and banded iron formations (BIF) (Glikson, 2006, Glikson and Vickers, 2007). The origin of BIFs is interpreted, alternatively, in terms of oxidation of ferrous to ferric iron under oxygen-poor atmospheric and hydrospheric conditions (Cloud, 1968, Morris, 1993), direct chemolithotrophic or photoferrotrophic oxidation of ferrous iron (Konhauser et al., 2002, Konhauser et al., 2007), and UV-triggered photo-chemical reactions (Margulis et al., 1976, Cairns-Smith, 1978). Below I examine potential relations between Archaean impact clusters, formation of BIFs, impact-injected sulphates, consequent ozone depletion and enhanced UV radiation and MIF-S anomalies.