This 2.5-tonne lump of rock is a banded iron formation. It marks a turning point in the history of life on our beautiful planet. A crucial chemical transition. When oxygen started becoming abundant. And life took its next step towards complexity…
It’s after two o’clock. Let’s take a trip back in time…
We’re 2 billion years ago… or thereabouts… (You can’t be too specific with geological time.)
Free oxygen in the atmosphere reached an unprecedented high.
Free as a Rock
And you can “read” it, provided you can read a slab of rock the way an Earth scientist does.
About 2 metres along its base and 1.5 metre high, it represents a wonderful juxtaposition between the animate (complex life) and the inanimate (rock) and the very deep connection that exists between the two.
Those colourful wavy lines in the rock sample are bands of iron oxide (mostly haematite), interspersed with chert (silica).
Free Oxygen and Evolution
Before photosynthesis evolved, Earth’s atmosphere had no free oxygen (O2).
Free oxygen is what gives us the diversity of life we have on Earth today.
Photosynthetic prokaryotic organisms began producing O2 as a waste product from respiration long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago.
Around 2 billion years ago (2.4 to 2.0 Ga) during the Paleoproterozoic Era, the increased oxygen level in the atmosphere had a profound change that would ultimately make complex life – such as the giant cetaceans – possible.
The Great Oxygenation Event
This “mass rusting” led to the deposition of iron oxide on ocean floors, forming the banded iron formations we see today.
Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event – this mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen.
At first, the oxygen produced would have been rapidly removed from the atmosphere by weathering of minerals, notably iron.
In the absence of plant life, the rate of oxygen production by photosynthesis was slower in the Pre-Cambrian.
Back then, free oxygen O2 concentrations attained were less than 10% of today’s, and they probably fluctuated greatly.
Oxygen may even have disappeared from the atmosphere again around 1.9 Ga ago.
At current rates of primary production, today’s concentration of oxygen could be produced by photosynthetic organisms in 2,000 years.
These fluctuations in oxygen concentration had little direct effect on basic lifeforms, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period, .
Aerobic metabolism being more efficient than anaerobic pathways, the presence of oxygen created new possibilities for life to explore.
All of a sudden (in geological terms, at least), the presence of free oxygen O2 provided life with new opportunities.
The Alternative Theory
The Snowball Earth hypothesis provides an alternative explanation for these younger deposits.
Banded iron formations are indeed geologically significant.
They are the concrete record of a critical evolutionary shift in environmental conditions. They mark a real turning point.
An indelible mark on the history of our Rock of Ages…