Small pre-Cambrian oxygen jump in atmosphere helped enable animals take first breaths
Not big ones? Can someone just make a list of pop science media assumptions? Would save trouble. Maybe we could mechanize it.
Here is one: We must always be looking for a “small” event that kickstarted life or some advance in life.
If oxygen was a driver of the early evolution of animals, only a slight bump in oxygen levels facilitated it, according to a multi-institutional research team that includes a Virginia Tech geoscientist.
The discovery, published in the journal Nature, calls into question the long held theory that a dramatic change in oxygen levels might have been responsible for the appearance of complicated life forms like whales, sharks, and squids evolving from less complicated life forms, such as microorganisms, algae, and sponges.
The researchers discovered oxygen levels rose in the water and atmosphere, but at lower levels than was thought necessary to trigger life changes.
“We suggest that about 635 million to 542 million years ago, Earth passed some low, but critical, threshold in oxygenation for animals,” said Benjamin Gill, an assistant professor of geoscience in the College of Science. “That threshold was in the range of a 10 to 40 percent increase, and was the second time in Earth’s history that oxygen levels significantly rose.”
The Big Easy.
Here’s the abstract:
Sedimentary rocks deposited across the Proterozoic–Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals1, 2. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans3, 4. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances1, 5, 6, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities7, 8, 9. (paywall) – Erik A. Sperling, Charles J. Wolock, Alex S. Morgan, Benjamin C. Gill, Marcus Kunzmann, Galen P. Halverson, Francis A. Macdonald, Andrew H. Knoll, David T. Johnston. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature, 2015; 523 (7561): 451 DOI: 10.1038/nature14589
See also: Evolution: The Fossils Speak, but Hardly with One Voice
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