Accepted biogeochemical wisdom suggests that about 2000 Ma ago, the terrestrial environment changed from one in which oxygen was a rare free element to an increasingly oxygenated world. One line of support for this involves the first appearances around that time of redbeds and lateritic palaeosols, that signify a surge in the O2 content of the atmosphere. The other pointer is the disappearance of banded iron formations (BIFs), suggesting that soluble iron-2 was no longer available in the oceans due to its oxidation near its main source at mid-ocean ridges. The first unambiguous microfossils of eukaryotes, which need oxygen for their metabolism, also appeared some two billion years ago.
There is, however, a different view; that there was a transition between the anoxic world of the Archaean and Early Palaeoproterozoic and that marked by pervasion of atmosphere and hydrosphere by oxygen. It stems from studies of sulphur isotopes in Proterozoic marine sediments by Donald Canfield of Odense University Denmark (Canfield, D.E., 1998. A new model for Proterozoic ocean chemistry. Nature, v. 396, p. 450-453). Canfield found evidence for steadily increasing sulphate ions in seawater from 2300 Ma, which he suggested would have led to increasing production of hydrogen sulphide in the deep oceans by sulphate-reducing bacteria. He proposed that it was combination with deep-ocean sulphide ions that shut off the supply of soluble iron-2, essential for the production of shallow-water BIFs. Today, sulphide precipitation is restricted to hydrothermal vents and most iron is removed by combination with oxygen in sediments on the main ocean floors. In short, Canfield proposed a transitional ocean akin to the Black Sea, with an oxic near-surface zone but anoxic at depth. Not only iron would have been removed in sedimentary sulphides, but many other metals, leading to their depletion in seawater. Ariel Anbar of the University of Rochester and Andrew Knoll of Harvard examine the biological repercussions of this transitional ocean (Anbar, A.D. & Knoll, A.H. 2002. Proterozoic ocean chemistry and evolution: a bioinorganic bridge? Science, v. 297, p. 1137-1142).
Iron and molybdenum are crucial elements for eukaryotes, albeit only in small quantities, because they are central to the enzymes that fix nitrogen. Insufficient quantities would put early eukaryotes at an evolutionary disadvantage to prokaryote life. Moreover it would reduce ocean productivity. This, they propose, can help explain the lack of evolution among eukaryotes until the late Proterozoic. The carbon isotope record of seawater (derived from limestones) shows a strange pattern that supports a period of biological stasis from 2000 to about 1200 Ma. From the end of the Archaean until 2 billion years ago, there are huge fluctuations (to highly positive and negative values) in the proportion of heavy 13C, and so too in the Neoproterozoic. The period in between shows no significant carbon-isotope fluctuation, d13C remaining at around zero, which Anbar and Knoll attribute to very low biological productivity. In their model, it was the release of massive amounts of metals by continental erosion during the “Snowball Earth” glacial periods of the Neoproterozoic that was able to kick start life, especially that of the eukaryotes. Emergence of the efficient, multicelled algal photosynthesizers drove up oxygen levels, eventually to oxygenate the deep oceans.
A cautionary note needs to be thrown in, however, especially when using analogies with the modern Black Sea (see Analogue of Archaean carbon cycle in Black Sea reefs). Biogenic carbonates on the Black Sea bed show huge negative excursions in their d13C, because organisms that formed them metabolized methane, thereby incorporating methane’s strong depletion in heavy carbon. As well as there being little direct evidence for Anwar and Knoll’s idea, the methane part of the carbon cycle needs to be factored into interpretations of the carbon-isotope record.
See: Kerr, R.A. 2002. Could poor nutrition have held life back? Science, v. 297, p. 1104-1105.
Isotopic evidence for early life may be from metamorphic processes
Controversy has surrounded reports of carbon-isotope evidence from the oldest recognisable sedimentary rocks that can be interpreted as signs of life 3800 Ma ago. The problem is that the data came from carbon trapped in resistant minerals, such as apatite, in the metamorphosed Isua supracrustal rocks of west Greenland. A detailed study of carbon in various forms in the Isua metasediments (van Zullen, M.A. et al. 2002. Reassessing the evidence for the earliest traces of life. Nature, v. 418, p. 627-630) strongly suggests that the isotopic evidence for life is flawed. It seems likely that both graphite and carbonates in the Isua rocks originated by chemical reactions that took place during metamorphism; they are probably metasomatic in origin. The wide range of d13C values found in both graphites and carbonates could have formed by isotopic exchange between graphite and carbonate during metamorphism. Graphite inclusions in apatite, the source of carbon isotopes claimed to reflect the earliest biological activity, are petrographically no different from inclusions in other minerals. Indeed, the sample originally used to suggest the isotopic influence of early life is of metasomatic origin.
All is not lost, however, for graphite that is highly depleted in heavy carbon-13 (a sign, albeit ambiguous, for organic processes) also occurs in turbidites that show graded bedding. These rocks show no petrographic signs of metasomatism, and may contain signs of life. Ominously, the US, Norwegian and Estonian co-workers, having looked in detail at carbon found in low concentration within BIFs and cherts from Isua, conclude that at least some is recent organic matter that groundwater flow has carried into the rocks.