[LINK] "Oxygenation: Gradual Process, Profound Results"

[rfmcdonald]

Centauri Dreams' Paul Gilster writes about a new study of the Earth's history. It took quite a long time for the Earth to build up current levels of oxygen, it turns out.

The researchers measured selenium isotopes in rock samples laid down under the ocean to track oxygen levels from 770 to 525 million years ago (across the full period believed to be covered by what is known as the Neoproterozoic Oxygenation Event). The marine shales are drawn from seven different geological sections, with samples from Canada (the Mackenzie Mountains), China (the so-called Yangtze Platform), Australia and the western US.

This is a fascinating period because three major glaciation events occurred during it: The Sturtian (‘snowball Earth’) of roughly 716 million years ago, the Marinoan (635 million years ago, or 635Ma), and the Gaskiers glaciation (about 580 Ma).

This would be a planet that looked nothing like what we’re familiar with, as the image above suggests. The land would have been covered in ice and the oceans frozen all the way to the equatorial regions. Temperature changes as these eras progressed would in each case melt the glaciers and produce a flow of nutrients into the oceans, one that would build the levels of organic carbon in seafloor sediments as oceanic plankton that flourished from that flow died. The growth in such carbon would lead to gradual increases in the level of oxygen.

A key finding of the paper is that rather than occurring after the Gaskiers glaciation, the oxygenation began much earlier, during or at the end of the Marinoan glaciation. From the paper:

“…the significance of the Se isotope record is not only that it adds to growing evidence that the late Proterozoic and Cambrian ocean and atmosphere reached a progressively more oxic state, coinciding with the diversification of animal life, but also that the process of oxidation was protracted, and not ultimately triggered by the Gaskiers deglaciation, as other data suggest.”

Moreover, it took approximately 100 million years for atmospheric oxygen to climb from less than 1 percent to over ten percent of today’s level. The occurrence of oxygenation in fits and starts over such a lengthy period of time is evidence, the team believes, that early animal evolution received its needed boost from these increased levels of oxygen. Says Pogge von Strandmann: “We were surprised to see how long it took Earth to produce oxygen and our findings dispel theories that it was a quick process caused by a change in animal behaviour.”