Trinity geologists re-write Earth’s evolutionary history books

The study site landscape is shown with boulders of the paleosol in the foreground. -  Quentin Crowley
The study site landscape is shown with boulders of the paleosol in the foreground. – Quentin Crowley

Geologists from Trinity College Dublin have rewritten the evolutionary history books by finding that oxygen-producing life forms were present on Earth some 3 billion years ago – a full 60 million years earlier than previously thought. These life forms were responsible for adding oxygen (O2) to our atmosphere, which laid the foundations for more complex life to evolve and proliferate.

Working with Professors Joydip Mukhopadhyay and Gautam Ghosh and other colleagues from the Presidency University in Kolkata, India, the geologists found evidence for chemical weathering of rocks leading to soil formation that occurred in the presence of O2. Using the naturally occurring uranium-lead isotope decay system, which is used for age determinations on geological time-scales, the authors deduced that these events took place at least 3.02 billion years ago. The ancient soil (or paleosol) came from the Singhbhum Craton of Odisha, and was named the ‘Keonjhar Paleosol’ after the nearest local town.

The pattern of chemical weathering preserved in the paleosol is compatible with elevated atmospheric O2 levels at that time. Such substantial levels of oxygen could only have been produced by organisms converting light energy and carbon dioxide to O2 and water. This process, known as photosynthesis, is used by millions of different plant and bacteria species today. It was the proliferation of such oxygen-producing species throughout Earth’s evolutionary trajectory that changed the composition of our atmosphere – adding much more O2 – which was as important for the development of ancient multi-cellular life as it is for us today.

Quentin Crowley, Ussher Assistant Professor in Isotope Analysis and the Environment in the School of Natural Sciences at Trinity, is senior author of the journal article that describes this research which has just been published online in the world’s top-ranked Geology journal, Geology. He said: “This is a very exciting finding, which helps to fill a gap in our knowledge about the evolution of the early Earth. This paleosol from India is telling us that there was a short-lived pulse of atmospheric oxygenation and this occurred considerably earlier than previously envisaged.”

The early Earth was very different to what we see today. Our planet’s early atmosphere was rich in methane and carbon dioxide and had only very low levels of O2. The widely accepted model for evolution of the atmosphere states that O2 levels did not appreciably rise until about 2.4 billion years ago. This ‘Great Oxidation Event’ event enriched the atmosphere and oceans with O2, and heralded one of the biggest shifts in evolutionary history.

Micro-organisms were certainly present before 3.0 billion years ago but they were not likely capable of producing O2 by photosynthesis. Up until very recently however, it has been unclear if any oxygenation events occurred prior to the Great Oxidation Event and the argument for an evolutionary capability of photosynthesis has largely been based on the first signs of an oxygen build-up in the atmosphere and oceans.

“It is the rare examples from the rock record that provide glimpses of how rocks weathered,” added Professor Crowley. “The chemical changes which occur during this weathering tell us something about the composition of the atmosphere at that time. Very few of these ‘paleosols’ have been documented from a period of Earth’s history prior to 2.5 billion years ago. The one we worked on is at least 3.02 billion years old, and it shows chemical evidence that weathering took place in an atmosphere with elevated O2 levels.”

There was virtually no atmospheric O2 present 3.4 billion years ago, but recent work from South African paleosols suggested that by about 2.96 billion years ago O2 levels may have begun to increase. Professor Crowley’s finding therefore moves the goalposts back at least 60 million years, which, given humans have only been on the planet for around a tenth of that time, is not an insignificant drop in the evolutionary ocean.

Professor Crowley concluded: “Our research gives further credence to the notion of early and short-lived atmospheric oxygenation.

This particular example is the oldest known example of oxidative weathering from a terrestrial environment, occurring about 600 million years before the Great Oxidation Event that laid the foundations for the evolution of complex life.”

The ups and downs of early atmospheric oxygen

This photo shows one-billion-year-old weathering profiles from Michigan. Ancient soils like these provide evidence for low atmospheric oxygen levels through much of Earth's history. -  Noah Planavsky.
This photo shows one-billion-year-old weathering profiles from Michigan. Ancient soils like these provide evidence for low atmospheric oxygen levels through much of Earth’s history. – Noah Planavsky.

A team of biogeochemists at the University of California, Riverside, give us a nontraditional way of thinking about the earliest accumulation of oxygen in the atmosphere, arguably the most important biological event in Earth history.

A general consensus asserts that appreciable oxygen first accumulated in Earth’s atmosphere around 2.3 billion years ago during the so-called Great Oxidation Event (GOE). However, a new picture is emerging: Oxygen production by photosynthetic cyanobacteria may have initiated as early as 3 billion years ago, with oxygen concentrations in the atmosphere potentially rising and falling episodically over many hundreds of millions of years, reflecting the balance between its varying photosynthetic production and its consumption through reaction with reduced compounds such as hydrogen gas.

“There is a growing body of data that points to oxygen production and accumulation in the ocean and atmosphere long before the GOE,” said Timothy W. Lyons, a professor of biogeochemistry in the Department of Earth Sciences and the lead author of the comprehensive synthesis of more than a decade’s worth of study within and outside his research group.

Lyons and his coauthors, Christopher T. Reinhard and Noah J. Planavsky, both former UCR graduate students, note that once oxygen finally established a strong foothold in the atmosphere starting about 2.3 billion years ago it likely rose to high concentrations, potentially even levels like those seen today. Then, for reasons not well understood, the bottom fell out, oxygen plummeted to a tiny fraction of today’s level, and the ocean remained mostly oxygen free for more than a billion years.

The paper appears in Nature on Feb. 19.

“This period of extended low oxygen spanning from roughly 2 to less than 1 billion years ago was a time of remarkable chemical stability in the ocean and atmosphere,” Lyons said.

His research team envisions a series of interacting processes, or feedbacks, that maintained oxygen at very low levels principally by modulating the availability of life-sustaining nutrients in the ocean and thus oxygen-producing photosynthetic activity.

“We suggest that oxygen was much lower than previously thought during this important middle chapter in Earth history, which likely explains the low abundances and diversity of eukaryotic organisms and the absence of animals,” Lyons said.

The late Proterozoic-the time period beginning less than a billion years ago following this remarkable chapter of sustained low levels of oxygen-was strikingly different, marked by extreme climatic events manifest in global-scale glaciation, indications of at least intervals of modern-like oxygen abundances, and the emergence and diversification of the earliest animals. Lyons notes that the factors controlling the rise of animals are under close scrutiny, including challenges to the long-held view that a major rise in atmospheric oxygen concentrations triggered the event.

“Despite the new ideas about animal origins, we suspect that oxygen played a major if not dominant role in the timing of that rise and, in particular, in the subsequent emergence of complex ecologies for animal life on and within the sediment, predator-prey relationships, and large bodies” said Lyons. “But, again, feedbacks always rule the day. Environmental change drives evolution, and steps in the progression of life change the environment.”

No single factor is likely to be the whole story, and there is much more to be written in the tale. Lyons and coauthors, along with research groups from around world over, are focusing current efforts on the timing and drivers of oxygenation in the late Proterozoic, favoring a combination of global-scale mountain building, evolutionary controls on the way carbon is cycled in the biosphere, and concomitant climate events.

“We are faced with a lot of chicken-and-egg questions when it comes to unraveling the timing and sequence of oxygenation of the ocean and atmosphere,” Lyons said. “But now, armed with new and better data, more sophisticated numerical simulations, and highly integrated investigations in the lab and the field, Earth’s oxygenation history seems much longer and more dynamic than envisioned before, and we are getting closer to understanding the mechanisms behind such change.”

A new wrinkle in ancient ocean chemistry

UC Riverside's Chris Reinhard analyzes metal content in 2.5 billion-year-old black shale using a mass spectrometer seen to his left. -  UCR Strategic Communications.
UC Riverside’s Chris Reinhard analyzes metal content in 2.5 billion-year-old black shale using a mass spectrometer seen to his left. – UCR Strategic Communications.

Scientists widely accept that around 2.4 billion years ago, the Earth’s atmosphere underwent a dramatic change when oxygen levels rose sharply. Called the “Great Oxidation Event” (GOE), the oxygen spike marks an important milestone in Earth’s history, the transformation from an oxygen-poor atmosphere to an oxygen-rich one paving the way for complex life to develop on the planet.

Two questions that remain unresolved in studies of the early Earth are when oxygen production via photosynthesis got started and when it began to alter the chemistry of Earth’s ocean and atmosphere.

Now a research team led by geoscientists at the University of California, Riverside corroborates recent evidence that oxygen production began in Earth’s oceans at least 100 million years before the GOE, and goes a step further in demonstrating that even very low concentrations of oxygen can have profound effects on ocean chemistry.

To arrive at their results, the researchers analyzed 2.5 billion-year-old black shales from Western Australia. Essentially representing fossilized pieces of the ancient seafloor, the fine layers within the rocks allowed the researchers to page through ocean chemistry’s evolving history.

Specifically, the shales revealed that episodes of hydrogen sulfide accumulation in the oxygen-free deep ocean occurred nearly 100 million years before the GOE and up to 700 million years earlier than such conditions were predicted by past models for the early ocean. Scientists have long believed that the early ocean, for more than half of Earth’s 4.6 billion-year history, was characterized instead by high amounts of dissolved iron under conditions of essentially no oxygen.

“The conventional wisdom has been that appreciable atmospheric oxygen is needed for sulfidic conditions to develop in the ocean,” said Chris Reinhard, a Ph.D. graduate student in the Department of Earth Sciences and one of the research team members. “We found, however, that sulfidic conditions in the ocean are possible even when there is very little oxygen around, below about 1/100,000th of the oxygen in the modern atmosphere.”

Reinhard explained that at even very low oxygen levels in the atmosphere, the mineral pyrite can weather on the continents, resulting in the delivery of sulfate to the ocean by rivers. Sulfate is the key ingredient in hydrogen sulfide formation in the ocean.

Timothy Lyons, a professor of biogeochemistry, whose laboratory led the research, explained that the hydrogen sulfide in the ocean is a fingerprint of photosynthetic production of oxygen 2.5 billion years ago.

“A pre-GOE emergence for oxygenic photosynthesis is a matter of intense debate, and its resolution lies at the heart of understanding the evolution of diverse forms of life,” he said. “We have found an important piece of that puzzle.”

Study results appear in the Oct. 30 issue of Science.

“Our data point to oxygen-producing photosynthesis long before concentrations of oxygen in the atmosphere were even a tiny fraction of what they are today, suggesting that oxygen-consuming chemical reactions were offsetting much of the production,” said Reinhard, the lead author of the research paper.

The researchers argue that the presence of small amounts of oxygen may have stimulated the early evolution of eukaryotes – organisms whose cells bear nuclei – millions of years prior to the GOE.

“This initial oxygen production set the stage for the development of animals almost two billion years later,” Lyons said. “The evolution of eukaryotes had to take place first.”

The findings also have implications for the search for life on extrasolar planets.

“Our findings add to growing evidence suggesting that biological production of oxygen is a necessary but not sufficient condition for the evolution of complex life,” Reinhard said. “A planetary atmosphere with abundant oxygen would provide a very promising biosignature. But one of the lessons here is that just because spectroscopic measurements don’t detect oxygen in the atmosphere of another planet doesn’t necessarily mean that no biological oxygen production is taking place.”

To analyze the shales, Reinhard first pulverized them into a fine powder in Lyons’s laboratory. Next, the powder was treated with a series of chemicals to extract different minerals. The extracts were then run on a mass-spectrometer at UC Riverside.

“One exciting thing about our discovery of sulfidic conditions occurring before the GOE is that it might shed light on ocean chemistry during other periods in the geologic record, such as a poorly understood 400 million-year interval between the GOE and around 1.8 billion years ago, a point in time when the deep oceans stopped showing signs of high iron concentrations,” Reinhard said. “Now perhaps we have an explanation. If sulfidic conditions could occur with very small amounts of oxygen around, then they might have been even more common and widespread after the GOE.”

Said Lyons, “This is important because oxygen-poor and sulfidic conditions almost certainly impacted the availability of nutrients essential to life, such as nitrogen and trace metals. The evolution of the ocean and atmosphere were in a cause-and-effect balance with the evolution of life.”