Hints of Free Oxygen on the KaapVaal Craton
(Published in The EarthSphere Blog. Cover Image: Greenstone by WM House; ArcheanArt)
Prologue
My last article in the Forgotten Origins series discussed the rise of blue-green algae and their importance in developing our biosphere. Dates covering over a billion years have been proposed for when free oxygen first appeared in Earth’s primordial oceans and atmosphere. This article picks up the story and examines evidence of when Earth’s oxygenation first started.
(Thanks to my readers for pointing out an age dating problem in my last article, A Blue-Green Revolution, where the word billions mysteriously turned into millions)
Free Oxygen: The Gateway to More Efficient Metabolism
Free oxygen reacts with a variety of molecules to form new compounds. Billions of years ago, as enough free oxygen became available in the oceans, these newly-formed compounds accumulated in the seas and precipitated out of the briny waters as solids. Over time, deposits of these new materials collected on the ocean floor and were buried, eventually becoming part of the rock record.
Today geologists analyze these ancient rocks to determine when oxygen first appeared. The appearance of free oxygen matters because it was the gateway to more efficient metabolism for Earth’s population of single celled inhabitants. We know that 2.3 to 2.5 billion years ago the oxygen content in rocks worldwide started rising. The geological signature is so distinct the period is known as the “Great Oxidation Event.” Some researchers have correlated the rise of cyanobacteria (blue-green algae) with the timing of this oxidation event.
But about a decade ago, research into South African rocks pushed early oxygenation back to about 3 billion years. One study looked at levels of molybdenum and iron in 2.95-billion-year-old rocks. Certain molybdenum isotopes serve as markers for organic manganese oxidation, which occurs during photosynthesis. Their work indicated these ancient sediments were deposited in an environment with significant free oxygen.
A second South African study investigated chromium isotopes in 3-billion-year-old rocks. The research results suggested these isotopes formed in an environment where atmospheric oxygen was about 100,000 time higher than could be explained if only non-biological chemical reactions were originally present. These rocks whispered to us from over the eons, speaking of oxygen from photosynthesis in a primitive world whose origins are all but forgotten. The rocks provided us with information about our distant past and the first production of free oxygen.
The Pangola Supergroup
The rocks investigated in both South African studies were from the Pangola Supergroup. In geological terms, a group is a collection of geological formations that share certain lithological characteristics. A supergroup is just a collection of related geological groups. I realize this explanation is a bit of a letdown for something with the word “Super” in it. Nevertheless, the Pangola Supergroup is one of the rare examples of early Archean age rocks in a relatively pristine state.
Plate tectonics puts a lot of wear and tear on rocks. Plate movement constantly pushes and pulls the continents, ensuring that most rocks are either raised up so weathering can grind them back into sand, or buried deep underground where high pressures and temperatures squeeze and chemically alter them through metamorphism. The older the rocks, the more likely they have eroded away or changed into something new. But occasionally, a group of rocks escapes this fate and remains relatively close to its original condition.
The Pangola Supergroup is one of the lucky few whose sedimentary formations are still usable for studying a world long passed. The Pangola Supergroup is located on the southeastern edge of an even larger geological feature called the Kaapvaal Craton.
Kaapvaal Craton
The Kaapvaal Craton holds the secrets of early life on Earth. Entombed in its rocks are geological indications of single-cell prokaryotes and other simple organisms that gave life a firmer foothold on the planet. The Craton underlies the northeastern third of South Africa and was formed during the Archean Eon between 3.6 and 2.7 billion years ago. But the oldest portions of the Kaapvaal Craton are exposed on its eastern edge in the Barberton Greenstone Belt.
Greenstone belts contain a variety of rock types. However, they are primarily composed of metamorphosed mafic and ultramafic volcanic rocks mixed with sedimentary formations. Both mafic and ultramafic are terms indicating that these igneous rocks have low silica content, typical of oceanic crust. Greenstone belts are widely interpreted as representing the remains of ancient oceans. Therefore, sedimentary rock formations in greenstone belts are important in the search for early life.
Even without direct fossil evidence, chemical analysis of the Kaapvaal rocks points to a partially oxygenated world about 3 billion years ago. Given the vagaries of rock preservation, it isn’t unreasonable to think that oxygen-producing blue-green algae formed the Australian stromatolites, which clock in at 3.4 billion years. If this is true, then a billion years passed between the first free oxygen production and Earth’s Great Oxygenation Event.
What was going on over this vast stretch of time, and why did it take so long for life to fully oxygenate our biosphere?
(Excerpts from Vanishing Origins, read the book on Wattpad as it unfolds)
Or, see my current set of Medium articles as I chronologically trace the development of life on our planet: EarthSphere Page — Forgotten Origins
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Sources:
When Did Earth’s First Whiffs of Oxygen Emerge? (by Becky Oskin; Live Science)
Timeline of Photosynthesis on Earth (Source: Scientific American) — https://www.scientificamerican.com/article/timeline-of-photosynthesis-on-earth/
The origins of life on Earth (Source: Australian Academy of Science)