"But then they noticed that after the big rise in spiny cysts and just when we see evidence for oxygen in the rock record, the acritarchs disappear. And that really doesn't make sense if you're evolving new groups because of the increase in oxygen."
"In 2009 a group of scientists led by Phoebe Cohen of Harvard University inspected acritarchs with transmission electron microscopes and concluded that they are not algae but rather animals, encased in protective cysts that animals form when conditions are not favorable to life," says Fike.
At the same time a group of scientists (including Fike) led by Chao Li of the University of California Riverside measured redox proxies in several different sections through the formation.
These measurements showed that the Nanhua Basin had had a layered chemical structure with deep iron-rich waters, near-shore wedges of sulfur-rich water and an oxygenated surface.
Both the sulfur- and the iron-rich waters would have been lethal to oxygen-loving species.
A Cautionary Tale
At the same time Fike acknowledges that spatial variability in redox proxies may make many geologists feel ill at ease because it might instead reflect an unusual depositional context or the reworking of the proxy after deposition instead of a signficant change in geochemistry.
By way of illustration, he describes a study of Amazonian mud belts, published this year by Robert Aller of Stony Brook University and colleagues in Geochimica et Cosmochimica Acta.
"The Amazon dumps mud rich in organic material into the Atlantic," Fike says. "The mud is deposited and the oxygen in it is consumed by biological activity, but then a storm churns it up, it gets reoxygenated, and redeposited. And this process happens over and over again."
By the time the muds become sediments, their chemistry is very different from what it was w
|Contact: Diana Lutz|
Washington University in St. Louis