Pongola glaciation

Earth’s first major glacial epochs

The global glaciations of the Neoproterozoic that reached low latitudes – the so-called ‘Snowball Earth ’ events have dominated accounts of ancient glaciations since the start of the 21st century. Yet they are not the oldest examples of large-scale effects of continental ice sheets. Distinctive tillites or diamictites that contain large clasts of diverse, exotic rocks occur in sedimentary sequences of Archaean and Palaeoproterozoic age. The oldest are dated at around 2.9 Ma in the Pongola Supergroup of Swaziland, South Africa and formed at an estimated palaeolatitude of 48°; within the range of the equatorward extent of Pleistocene ice sheets. No evidence has turned up for glaciation of that age in other regions, and therefore for a ‘Snowball Earth’ at that time. The surprise is not the antiquity of the Pongola glaciation but the fact that tillites formed by glaciers are not more common in the early part of geological history. The sun has increased in its warming effect since the Earth formed so that the very absence of glaciations over huge spans of early Precambrian time points strongly towards an early atmosphere far richer in greenhouse gases than it is now.

Evidence for Palaeoproterozoic glaciation is more widespread, important tillites occurring in the Great Lakes region of North America and in the Transvaal and Griqualand regions of South Africa. Those of South Africa formed at a latitude of around 10°, suggesting ‘Snowball’ conditions, and in each region there are multiple tillites in the stratigraphic column. Accurate dating of volcanic ash horizons in the sequences of both areas (Rasmussen, B. et al. 2013. Correlation of Paleoproterozoic glaciations based on U-Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups. Earth and Planetary Science Letters, v. 382, p. 173-180) has made it possible to correlate three glacial deposits precisely between the two now widely separated areas. The dating also reveals that four glacial events occurred over a period of 200 Ma between 2.45 and 2.22 billion years ago: longer than the duration of the Mesozoic Era of the Phanerozoic and about the same as the time span during which 3 or 4  ‘Snowball’ events plastered the planet with ice in the Cryogenian and Ediacaran Periods of the Neoproterozoic. This episode of the first large-scale glaciations neatly brackets the first appearance of significant amounts of oxygen in the Earth’s atmosphere during the Great Oxidation Event from 2.45 to 2.2 Ga. It is hard to avoid the conclusion that the two were connected as an increase in oxygen in the air must have influenced the concentration of greenhouse gases, especially that of methane, the most powerful of several that delay loss of heat to space by radiation from the surface. Once oxygen production by photosynthetic organisms exceeded a threshold atmospheric methane would very rapidly have been oxidized away to CO2 plus water vapour, leaving excess oxygen in the air to prevent the build-up of methane thereafter as is the case nowadays. But what pushed atmospheric composition beyond that threshold? A key piece of evidence lies in the record of different carbon isotopes in seawater of those times, which emerges from their study in Precambrian limestones.

After the end of the Archaean Eon at 2.5 Ga the proportion of marine 13C to 12C increased dramatically. Its accepted measure (δ13C) changed rapidly from the near-zero values that had previously characterised the Archaean to more than 10; an inflated value that lingered for much of the half-billion years that spanned the Great Oxidation Event and the Palaeoproterozoic glaciations (Martin, A.P et al. 2013. A review of temporal constraints for the Palaeoproterozoic large, positive carbonate carbon isotope excursion (the Lomagundi–Jatuli Event). Earth-Science Reviews, v. 127, p. 242-261). Later times saw δ13C return to hovering between slightly negative and slightly positive values either side of zero until the Neoproterozoic when once more ‘spikes’ affected the C-isotope record during the period of the better known ‘Snowball’ events. What lay behind this very broad carbon-isotope anomaly?

To increase 13C at the expense of 12C requires to removal from seawater of very large amounts of the lighter isotope. The only likely mechanism is the prolonged and permanent burial of masses of organic material, the only substances that selectively take up 12C. In turn, that implies a huge increase in biological productivity and its efficient burial without being oxidised to CO2 plus water. There are three possibilities: oxygen was absent from the ocean floor; sedimentation was too fast for oxidising bacteria to keep pace or such bacteria did not evolve until the end of the Lomagundi–Jatuli Event. It seems likely that such a dramatic change in the biosphere may have marked some fundamental shift in biological evolution not long after the close of the Archaean. Whichever, the biosphere somehow increased its capacity to generate oxygen. Since oxygen is anathema to many kinds of anaerobic bacteria and archaea, probably the only kinds of organism at the outset of these events, it is possible to imagine continual extinctions yet to maintain high biological productivity new organisms may have emerged to replace those that vanished. By 2.0 Ma, the first putative eukaryote cells (those with nuclei and a variety of organelles) had appeared.