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In Brief September 2009

Joe McCall's eye is caught by some very old oxygen and the first meteorite to be traced from final plunge to find.

Geoscientist 19.9 September 2009

Marble Bar Chert Member. The bands showing blue are presumably the ‘black chert’ and the red and white cherts are clearly visible. The outcrop is near vertically disposed. Photo: Joe McCall.It has long been widely accepted that the Earth’s atmosphere and oceans contained little or no oxygen at any time prior to the so called ‘great oxygen event’ between 2300 and 2200Ma1,2. However, significant levels of free oxygen have recently been recognised in the Neoarchaean, so the question arises: “when did significant levels first appear?”3. Oxygen was probably produced by photosynthetic organisms - of which only bacteria were likely to have been extant in the Neoarchaean. The evidence for oxygen comes from organic carbon, microfossils(?) and stromatolites. The suggestion is that such bacteria existed right back to about 3500Ma. Once formed, the oxygen could have been removed by a variety of agencies and reactions, and the level might have fluctuated.

Hoashi et al 4 have now reported drilling to >200m depth, beyond the depth of present-day near-surface oxidation, in the Marble Bar Chert member of the Duffer Formation, Pilbara, Western Australia (as part of the Archaean Biosphere Drilling Project (ABDP)). The red beds (picture) in this on-end member contain very fine haematite, taken as evidence of oxygen’s being present at the time of deposition in the ocean.

The drilling shows that the haematite does continue below the deepest intersection. The haematite is in the red chert , which is concentrated in the upper part of the member: the black chert forms veins that intrude the white and red chert, but not the overlying Apex basalt, dated at ~3460 Ma. Therefore all three types of chert formed before the basalt eruption at ~3460 Ma, during deposition and diagenesis. The haematite forms very small crystals and neither siderite nor magnetite show evidence of oxidation. Hickman3, who has had a long career deciphering Pilbara rocks, believes that the depositional environment in the Marble Bar Member changed from anoxic/sideritic, when siderite was deposited, to oxic/haematitic (when haematite and magnetite formed up to 10%), and that the minerals were precipitated close to hyrdothermally active volcanic vents, the basin being successively flooded by water of a different composition. He hypothesises a caldera situation.

This new research seems to have established that the haematite is a primary precipitate, not an oxidation product of magnetite or siderite – which means that oxygenated sea-water was then in existence. Whether it established that cyanobacteria were extant seems beside the point: the famous putative cyanobacteria at North Pole in the Pilbara have been questioned5 and even said to be inorganic; though the presence of stromatolites there and elsewhere of this great age seems to be well established and the Neoarchean in the Pilbara is riddled with them.

Though Professor Malcolm Walter has very reasonable reservations about the Marble Bar evidence (for the existence of cyanobacteria) because these rocks are so ancient and altered5, this research does seem to establish that there was at least local oxidation of sea water, at least from time to time during the early Archaean. Whether this, even if possibly related to bacteria, was of global significance remains questionable.  Joe McCall


  1. Cloud, P E 1972 A working model for the primitive Earth American Journal of Science 272, 537-548
  2. Holland, H D 2002 Volcanic gases, black smokers and the great oxidation event Geochim Cosmochim Acta 66, 3811-2826
  3. Hickman, A H 2009 Oxygen on Earth 3460 million years ago, The Australian Geologist 151, 13-15
  4. Hoashi, M et al 2009 Primary haematite formation in an oxygenated sea 3 46 billion years ago Nature Geoscience, advance online publication, March 15 2009, 6pp
  5. Morton, M C 2009 Ancient rocks push back early oxygenation even further Earth 54(7), 24

Saw it coming

In October 2008, a small asteroid named 2008TC3, was discovered by telescope observation1. It had a flat reflectance spectrum in the 554-995 mm wavelength. It later hit the Earth, exploding at 37km altitude. In the absence of a firm link between individual meteorites (falls or finds) and their asteroidal parent bodies, asteroids are characterised and grouped into classes by virtue of their light reflectance. This not very satisfactory, but is the best that can be done. Though the asteroid Vesta has been confidently linked with the HED (Howardite, Eucite, Diogenite) achondrites, there is no certainty that this is correct, and few other reasonably valid linkages have been made. An impact in the Sudan, 20 hours after the asteroid discovery, resulted in a ground search, and 47 meteorite fragments were found, of total weight 3.95 kg, and named Almahata Sitta.

This was a remarkably lucky success for the Spacegurad Survey - asteroids of such size (4m diameter) hit the only Earth once per year2. A decade or so this fraction of the sky was covered only on a monthly basis, but is now covered several times per month, and improvements are in the pipeline for such NEA (Near Earth Asteroid) surveys. The prospect of linking asteroid observation and recovery from observed falls are thus very good and improving in the future. The mass was of a rare meteorite class - a polymict ureilite (achondrite), ultra fine-grained, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as of F class, which is now firmly linked to the dark carbon-rich ureilites. Such fine grained and porous ureilite material was not previously represented in museum collections.


  1. Jenniskens, P., Shaddad, M.H., Numan, D, et al. 2009. The impact and recovery of asteroid 2008 TC3. Nature 458, 485-488.
  2. Chapman, C.R., Harris, A.W. 2009. Near Earth Asteroid/Meteorite Impact Prospects for Linking telescopic Observations with Recovered Meteorites. Meteoritics and Planetary Science 44 (Supplement), A