My Rise of Animals post has led to a dozen readers e-mailing me to ask for more details about Neoproterozoic oxygen levels, since I particularly stressed the importance of oxygen for the radiation of animals. This post is a summary of Neoproterozoic palaeoclimate.
For easy reference, the Neoproterozoic ranges from 1000 Ma to 542.5 Ma (the Proterozoic-Cambrian boundary). During the first 300 Ma, oxygen levels increased steadily, as did overall biological activity. From sulphur isotope data, we know that there was a revolution in the way microorganisms oxidised sulphides, particularly related to the radiation of the sulphide-oxidising bacteria at the time (Canfield & Teske, 1996). Critically, this new process, which involves step-wise ocidation of sulphides through various intermediates instead of a direct sulphide → sulphate conversion, can only take place in water with a concentration of at least 13-46 µM oxygen (Canfield & Teske, 1996), and this is how we know that oxygen levels also rose at this time, during a second oxygenation event which led to the overthrowing of the Canfield Ocean equilibrium.
In the last 150 million years of the Neoproterozoic, the Earth was in the grips of at least two very severe ice ages (Hoffman et al., 1998), together lumped together as the Snowball Earth periods, as known from typical glacier deposits (tillites, dropstones) from all over the world, both as it’s configured today and palaeogeographically (Evans, 2000). Many reasons have been proposed for this global cooling, including palaeogeography disrupting previous climate cycles (Hoffman & Schrag, 2002) and increased erosion of silicates into the oceans leading to more CO2 trapping in the oceans (Hoffman & Schrag, 2002); once the cooling had started, increased albedo from the ice sheets made it worse (Baum & Crowley, 2001).
But obviously, somehow the Earth’s climate restabilised. The key to this is simple: with too much ice, the water cycle simply stops working (Kirschvink, 1992). Because volcanoes are atill active though, CO2 levels continued to rise – but without rain to dissolve it and flush it into the oceans, it all accumulated in the atmosphere, leading to a greenhouse and melting of the ice (Caldeira & Kasting, 1992). Much like a pendulum, the climate now swings to an extremely warm one, before eventually stabilising as the water cycle is restored and CO2 levels are restored to reasonable levels.
Important for the evolution of animal life is the fact that during the Snowball Earth, oxygen levels in the oceans plummeted and anoxia was widespread – with ice covering the surface, oxygen couldn’t get into the oceans; the cold temperatures don’t help either; and when organisms died, their decay quickly depleted whatever oxygen was left in the water. And this isn’t just theory: we know that throughout the Snowball Earth, the oceans were anoxic because of the reappearance of banded-iron formations.
And this explains why it wasn’t until after the last of the glaciations that true multicellular animal life appeared (sponges were there before, but they’re barely multicellular anyway); see the Rise of Animals post to see why oxygen is important. The recovery from the last glaciation period, the Gaskiers glaciation, is exemplary of the extreme rebound I mentioned earlier, as you can see in the diagram above (Fike et al., 2006). But, as I said in the Rise of Animals post, the exact interplay between the geochemistry and biological evolution isn’t completely clear, but this can be resolved by doing high-resolution studies comparing acritarch (biomineralised microfossils, very common in rocks of the time) abundances and diversity with the geochemical excursions. (And hey, if anyone of you dear readers is the head of a research lab, I’m always on the lookout for interesting research projects to stick my nose into. No, this post was not just shameless advertising, I just thought I’d throw that little bit of information out there.)
Baum SK & Crowley TJ. 2001. GCM response to late Precambrian (~590 Ma) ice-covered continents. Geophysical Research Letters 28, 583–586.
Caldeira K & Kasting JF. 1992. Susceptibility of the early Earth to irreversible glaciation caused by carbon dioxide clouds. Nature 359, 226–228.
Canﬁeld DE & Teske A. 1996. Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature 382, 127–132.
Evans DAD. 2000. Stratigraphic, geochronological and paleomagnetic constraints upon the Neoproterozoic climatic paradox. American Journal of Science 300, 347–433.
Fike DA, Grotzinger JP, Pratt LM & Summons RE. 2006. Oxidation of the Edicaran Ocean. Nature 444, 744-747.
Hoffman PF & Schrag DP. 2002. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129–155.
Hoffman PF, Kaufman AJ, Halverson GP & Schrag DP. 1998. A Neoproterozoic snowball Earth. Science 281, 1342–1346.
Kirschvink JL. 1992. Late Proterozoic low-latitude global glaciation: the snowball earth. In: Schopf JW & Klein C (eds.). The Proterozoic Biosphere.
Research Blogging necessities :)
Evans, D. (2000). Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox American Journal of Science, 300 (5), 347-433 DOI: 10.2475/ajs.300.5.347
Hoffman, P., & Schrag, D. (2002). The snowball Earth hypothesis: testing the limits of global change Terra Nova, 14 (3), 129-155 DOI: 10.1046/j.1365-3121.2002.00408.x