Debut of chromium signatures clocks great oxidation event

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

Prof Barley is one of a group of geobiologists that date the Great Oxidation Event, when earth’s atmospheric oxygen 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 core samples 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.

Provided by ScienceNetwork Western Australia