More than a decade ago the oldest sedimentary rocks in the world at Isua in West Greenland hit the headlines, and not for the first time. Inclusions of graphite in crystals of the mineral apatite from the Isua supracrustals had yielded carbon isotopes unusually deficient in 13C relative to 12C, which is often regarded as a sign that life was involved in the carbon cycle at the time. The Isua rocks have been reliably dated at around 3.8 billion years (Ga) so that added over 400 Ma to the time at which life was present on Earth. Sedimentary rocks formed at 3.4 Ga contain the first tangible signs in the form of stromatolites thought to have been secreted by biofilms of blue-green bacteria which are oxygen-generating photosynthesisers. Sadly, limestones at Isua, indeed all the putative sedimentary rocks there were metamorphosed and deformed plastically so that such features, if they were ever present, had been obliterated. Apatite was thought to be so strong and resistant to heating that carbon within its crystals would have preserved original isotopic ‘signatures’. Detailed studies to test this hypothesis refuted the early age for life, which reverted back to around 3.4 Ga. But Isua presents too good an opportunity for its geochemical secrets to be left uninvestigated.
The latest targets are its iron isotopes. Isua includes metamorphosed banded ironstones composed largely of magnetite and quartz. Magnetite is iron oxide (Fe3O4) and begs the question of how such an oxygen-rich mineral formed in such volumes in sediment if photosynthesizing life had not made elemental oxygen available. That would oxidize soluble ferrous ions (Fe2+) to the insoluble ferric form (Fe3+) in order for iron oxide to precipitate from sea water in large amounts. There is no other means known for oxygen to be produced in a planet’s surface environment. A team at the University of Wisconsin’s NASA Astrobiology Institute, led by Andrew Czaja and joined by Stephen Moorbath of the University of Oxford, who set the entire West Greenland story rolling by leading its geochronological investigation since the early 1970s, have made a breakthrough (Czaja, A.D. et al. 2013. Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770 Ma Isua Supracrustal Belt (West Greenland). Earth and Planetary Science Letters, v. 363, p. 192-203).
Any element that has more than one naturally occurring isotope offers the possibility of studying various kinds of chemical process by looking for changes to the relative proportions of the different isotopes. Having different relative atomic masses isotopes of an element have slightly different chemical properties so that one is likely to be more favoured in a reaction than another. In the case of iron, the most important reactions in surface processes are those that depend on reducing and oxidising conditions, i.e. producing soluble Fe2+ and insoluble Fe3+ respectively. Oxidation and precipitation of iron oxides and hydroxides tend to favour the heavier isotope 56Fe over the more common 54Fe resulting in an increase in the 56Fe/54Fe ratio (δ56Fe). This is found throughout the Isua ironstones, but may again reflect metamorphism. However, such was the detail of this study that δ56Fe values were measured for many individual bands. Instead of showing roughly the same values throughout the rock, each band had a different value. That strongly suggests that values produced during sedimentation had been preserved. It seems that a bacterial mechanism of oxidation was involved. Moreover, by comparing the 3.8 Ga Isua ironstones with examples dated at 2.5 Ga from Australia the team found different isotopic values that implicates different kinds of bacteria involved in producing apparently similar rock types. The twist is that the most likely bacterial type involved at Isua may have been a photosynthesiser, but not of the kind that releases elemental oxygen instead transferring it from water to combine directly with the ions of iron that its photosynthesis had oxidised. The younger ironstones seem more likely to have involved cyanobacteria that do excrete oxygen; shortly after their formation the Earth’s surface increasingly became oxygen-bearing.
Throughout the Precambrian, BIFs appear and then vanish from the record only to reappear when geologist least expect them, for instance around the time of the Snowball Earth events in the Neoproterozoic Era. Iron isotopes could well become handy tools to probe the processes that formed them.
- Rusty rocks reveal ancient origin of photosynthesis (newscientist.com)
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