At a stormy meeting in August 2004at the 32nd International Geological Congress in Florence, a rearguard action was mounted by a group of stalwart geologists to thwart an attempt to expunge the last remnant of the stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century from the minds of all future geologists (see December 2004 issue of EPN). The Quaternary was under siege. Despite the fact that the International Commission on Stratigraphy (ICS) of the IUGS had already prepared the ground for a coup de gras by stating that, “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”, its defenders seem to have won (Mascarelli, A.L. 2009. Quaternary geologists win timescale vote. Nature, v. 459, p. 624). The ICS voted on 21 May 2009 to formally define the base of the Quaternary at 2.6 Ma when the Earth began to cool, glaciation began in the Northern Hemisphere and stone tools first appeared in Africa (it was formerly set at 1.8 Ma, for no obvious reason) and to pass that to IUGS for ratification. Another minority group is enraged, with rumours of chewed carpets, as the Quaternary has annexed 800 ka of what previously was designated as Pliocene: ‘It’s kind of a land grab’, commented Philip Gibbard, a Quaternary expert from Cambridge University, possibly with a hint of glee. To me, it is a milestone decision that gives a proper place to tool making, bipedal apes – ourselves – which makes a great deal more sense that the absurd notion of the Anthropocene (see Epoch, Age, Zone or Nonsense? in March 2008 issue of EPN), whose base some deluded colleagues are trying to set at the beginning of the Industrial Revolution!
Early signs of oxygen…but in the wrong place
The so-called ‘Great Oxidation Event’ is marked by the first occurrence of iron-oxide bearing subaerial sediments or palaeosols, widely regarded as occurring at around 2400 Ma. That is probably around the time that photosynthesis overtook the rate of oxidation reactions that previously consumed the oxygen that it produced, so that oxygen could build-up continually in the air. But that date is far earlier than the origin of subaerial photosynthesis and oxygenic photosynthesis must have arisen among oceanic bacteria before then, but only those inhabiting shallow water where the sunlight is. Banded iron formations that go back into the Archaean are often cited as evidence for when such photosynthesis got underway. Their dominant mineral hematite probably formed by oxidation of soluble iron-II and combination of iron-III with free biogenic oxygen, presumed by most workers to be in shallow water. Among the oldest hematite-rich formations is the Marble Bar Chert of Western Australia, dated to 3460 Ma (Hoashi, M. et al. 2009. Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, v. 2, p. 301-306). The hematite crystals in the chert seem to have formed at above 60ºC in ocean-floor hydrothermal springs that were discharging abundant dissolved iron-II. The authors estimate the basin in which the cherts formed to be between 200 to 1000 m deep. Since at such depths photosynthesis would not be possible, they claim that sufficient oxygen was produced by shallow-water photosynthesis to form oxygenated intermediate and deep ocean waters, reminiscent of far later times in Earth’s history. This is a minority view, and hinges on whether or not the hematite did form directly on the sea floor. One possibility is that it could have been precipitated colloidally from iron-II-rich ocean water in the photic zone where early photosynthesisers would be, to sink to the deeper sea floor. Eventually very fine iron oxide might recrystallise.
See also: Konhauser, K. 2009. Deepening the early oxygen debate. Nature Geoscience, v. 2, p. 241-242.