Debut of chromium signatures clocks great oxidation event

Nov 25, 2011
Dr Barley says he contributed core samples of banded ironstone that he obtained from the Pilbara. Flickr: Bron

Banded ironstone core samples from the Pilbara have aided in dating the first appearance of atmospheric oxygen at 2.48 billion years ago.

UWA Associate Professor Mark Barley says the theory -- published in the journal Nature by Prof Barley and his colleagues -- rested on the of the they used as evidence.

Prof Barley is one of a group of geobiologists that date the , when ’s formed, at between 2.48 and 2.32 billion years ago.

He says the groups’ argument depended on pinpointing the time that chromium—previously bonded in igneous rocks—began to appear in the ocean’s waters.

“This was evidence for the most primitive form of aerobic respiring life, aerobic respiring bacteria which oxidise pyrite that released acid that dissolved rocks and soils on land, including chromium, that was then carried to the oceans by the flow of water,” Prof Barley says.

“The aerobic respiring chemolitho-autorobic bacteria require coexistence with cyanobacteria producing oxygen to do this.”

The advent of breathable oxygen had been previously dated at 2.7 billion years BP—at date Prof Barley said comes from unreliable data.

He says rocks of that age are often “overprinted” by data from later metamorphic processes.

“The banded iron formations have good representation of the geochemistry in the earth’s early ocean, but also a lot of [the formations] were later altered,” he says.

“The later changing of banded iron formations into iron ores happened after the main event of oxidation.

Prof Barley says geobiologists are working towards a better database, for more evidence of when the types of bacteria linked to the rise of oxygen were really functioning.

“We got a good group of samples from banded iron formations and analysed the chromium isotopes and other elements because that gives the strongest evidence of oxidation,” he says.

“If you have a good deep drill hole that’s not close to a big iron ore deposit, you have got the appropriate chemistry record.”

Dr Barley says he contributed of banded ironstone that he obtained from the Pilbara.

“I added some key samples to gaps in the global database,” he says.

Dr Barley says this analysis provided no evidence of dissolved chromium in the oceans older than 2.48 billion years, and therefore no evidence of atmospheric oxygen.

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omatumr
1 / 5 (4) Nov 25, 2011
the Great Oxidation Event, when earths atmospheric oxygen formed, between 2.48 and 2.32 billion years ago


Does Professor Mark Barley think plant life first started generating O2 from CO2 at that time?

With kind regards,
Oliver K. Manuel
Former NASA principal
Investigator for Apollo
http://myprofile....anuelo09

Shootist
2.3 / 5 (3) Nov 27, 2011
With kind regards,
Oliver K. Manuel


Free oxygen is rare in nature (@ standard temp and pressure) (yes?) and when it does occur, it binds quickly and, unless replaced, will some diminish. Yes?

Certainly something has kept the partial pressure of O2 up around 18-20% for an eon, or three.

Oliver, are you suggesting the O2 arrived, by means miraculous, prior to the evolution of CO2 ingesting organisms? Or, that CO2 breathers existed earlier than the Boffin Barley's reported observations indicate?
Loodt
1 / 5 (1) Nov 28, 2011
In Ian Plimer's book he mentions this great Oxidation Event.

Prior to the earliest plants developing the ability to consume CO2 and releasing O2, the oceans were acidic with a lot of dissolved Fe. The Banded Iron formations in Australia and South Africa were formed when plants started releasing oxygen. Read about it, geology is a fascinating subject!
JustAnyone
not rated yet Nov 28, 2011
Please pardon my lack of geophysical background... It seems the alternative dating method Dr. Barley mentions relies on deposition of Chromium (oxides?) from the atmosphere starting at a specific time.

* Is the Chromium in the form of a chromium oxide, or another chemical?
* Is the dating being done via isotope measurement of the oxygen, the chromium, or another chemical that's also present (uranium)?
* Is the isotope dating method possible because of different relative (atmospheric vs. sample) concentration due to the atmosphere constantly being exposed to cosmic rays vs. not so in the dirt?

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