Cover image credits: Graeme Churchard (CC-BY via Wikimedia Commons).
For millennia, oxygen has been leaving telltale signatures in rocks, at the bottom of the oceans, as fossils, and even as biosignatures 200 kilometres/120 miles high up in the atmosphere.
In school, we learned how photosynthesis results in the formation of oxygen, yet was there oxygen before life on Earth evolved? Where did that oxygen come from? Did the Earth's atmosphere always have 21% oxygen, if not, what changed?
This blog looks at formations across the Indian subcontinent (and elsewhere in the world) that provide evidence of oxygen's strange equilibrium and what they can tell us about how life evolved on Earth.
Early Earth - life without oxygen?
It seems counterintuitive to call oxygen 'the elixir of life'. After all, anaerobic life — organisms that do not need oxygen for growth, evolved before oxygen-loving aerobic life. There was very little 'free' oxygen in the early atmosphere. The original source of oxygen was as a result of chemical reactions of crushed silicate rock during tectonic activity, and when solar energy split water molecules (into hydrogen and oxygen). This early oxygen reacted with iron in the rocks and oceans, and was 'locked' into the crust — it wasn't available for organisms.
Oxygen forms slowly. New rock formation via volcanism, or the exposure of more rock surfaces via weathering are faster processes that lead to the oxidization of rocks. If rock formation/exposure outpaces the rate at which oxygen forms, there is little chance for it to build up or accumulate in the atmosphere.
The surfaces of Venus and Mars oxidized rapidly, and their atmospheres were never able to accumulate free oxygen. On Earth, a different story played out.
3.8 billion years ago, the earliest life forms, anaerobic prokaryotes established themselves in the vast oceans. Shortly after (400 million years after, or 3.4 billion years ago), remarkable organisms known as cyanobacteria, evolved the ability to photosynthesize - they could generate energy from sunlight, by using water as fuel. The by-product of photosynthesis was oxygen.
Yet how do we know of these early organisms and their metabolism?
The crocodile-skin textured stromatolites at Jhamarkotra, near Udaipur in Rajasthan as featured in a recent article in the Hindu.
Image credits: Devayani Khare (CC-BY SA)
Enter Stromatolites! In their quest for sunlight, cyanobacteria live in shallow waters; where they trap sediments and deposit them in lens-like layers. Over time, these sediments build up to form a layered sedimentary rock called a stromatolite. Stromatolites preserve records of cyanobacteria colonies that expanded and flourished — almost like microbial reefs — and are thereby, considered fossils. (I've written about dinosaur fossils and other fossils from across India before). While stromatolites are mostly fossils, researchers have found some living colonies in Western Australia and in a Bahamas lagoon.
In India, stromatolites can be found in Shimoga and the Chitradurga region of Karnataka, at Chakrata in Uttarakhand, at Sonbhadra near Benaras in Uttar Pradesh, in Himachal Pradesh's Pin Valley, at various sites in Rajasthan, among other locations — occurring in a bewildering number of shapes and shades.
Page 43 from Indica: A Deep Natural History of the Indian Subcontinent, written by Pranay Lal, which details the incredible diversity of stromatolite formations across the country.
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The Great Oxidation Event
As cyanobacteria flourished, more oxygen was released into the environment. At first, the newly created oxygen was oxidized by the highly reactive iron present in rocks, leaving behind iron-rich, rust-red soil like that in Goa and Karnataka, and some parts of Odisha.
The gradual increase in oxygen caused ferric and ferrous iron to settle in alternating bands at the bottom of the oceans and seas. Over time, these bands hardened into layers known as banded iron formations (BIFs). Note: This is just one of the possible explanations for banded iron deposits the world over.
India's peninsula is rich in BIFs, especially where the Singhbhum, Bastar, Dharwar & Bundelkhand cratons lie. In Karnataka, some spectacular outcrops can be seen north-east of Bengaluru towards Chitradurga district, near Sandur in Bellary district, in Shimoga, along the highway on the route to Hubli, each displaying a range of hues from dark red to brownish yellow. Similarly, there are deposits in Madhya Pradesh, Bihar and Odisha — all of which represent India's high-grade iron ores earning it a place among the world's top 10 iron ore producers.
Jasper, an opaque, reddish mineral is often found in banded iron formations - this is a specimen from Michigan, USA, showing deformations. Similar rock outcrops can be seen in India.
Image credits: Jasper Knob (CC-BY SA via Wikimedia Commons) With the oxidation of rocks, it took nearly 500 million years before oxygen could accumulate in the atmosphere — that is, for the rate of oxygen generation to outpace the rate at which rocks oxidized. Somewhere between 2.4 - 2.1 billion years ago, oxygen released by photosynthesizing cyanobacteria began to accumulate over the vast expanses of ocean, and started escaping into the atmosphere, in an event geologists call 'The Great Oxidation Event (GOE)'. The GOE was crucial for the Earth's evolutionary history — it led to four Ice Ages, widespread wildfires, a (much-contested) mass extinction event, and the adaptation and evolution of multicellular life.
Ice, fire & multicellular life
How Cyanobacteria Took Over the World
Video credits: Journey to the Microcosmos
Some say the world will end in fire,
Some say in ice.
~ Robert Frost
As oxygen built up and reached the upper stretches of the atmosphere, UV radiation from the sun caused the oxygen (O2) molecules to split and react to create ozone (O3). Around 600 million years ago, an ozone layer accumulated around the Earth buffering UV rays from reaching the surface. The ozone layer created a cooler, safer space for life to survive and thrive, both in the oceans and on land. This cooling effect of the ozone layer would ultimately, lead to four Ice Ages.
700 million years ago, complex oxygen-producing organisms like microbes, multicellular organisms and early plants evolved. Some say, these oxygen-generating organisms caused a mass extinction of anaerobic life, yet it is difficult to estimate species loss for that time period.
With the addition of oxygen to the atmosphere and the creation of new minerals by processes under the Earth's crust, organisms evolved new metabolic pathways — multicellular life evolved and flourished. Interestingly, the first Ice Age, a period also known as 'Snowball Earth', was crucial — it marks when complex, multicellular life was first found in fossil records.
Based on evidence from coal swamps, mountain building activity, rates of erosion, and the chemical changes wrought by these processes, we can surmise that oxygen levels fluctuated between 13-30 per cent of the atmosphere. As oxygen levels rose and fell, wildfires swept across the landmasses — we have evidence for wildfires as early as when plants evolved on land. As wildfires burned, they left their searing mark in Earth's history from regulating oxygen, causing carbon dioxide fluctuations and carbon sequestration, to causing mass extinctions, and even creating conditions for specific evolutionary traits.
In so many ways, oxygen has been responsible for much of the drama played out in geological time. As oxygen catalyzed life on Earth, life over the past two billion years, has tinkered with oxygen levels in the atmosphere — and the dance continues.