Tiny iron oxide stones reveal Earth’s ancient oceans were carbon-poor, challenging previous assumptions

Earth scientists often face huge challenges when researching Earth’s history: many significant events occurred such a long time ago that there is little direct evidence available. Consequently, researchers often have to rely on indirect clues or on computer models.

A team led by ETH Professor Jordon Hemingway, however, has now discovered a unique natural witness to this period: tiny egg-shaped iron oxide stones that can be used to directly measure the carbon reserves in the primordial ocean.

Viewed on the outside, they resemble grains of sand, but in terms of their formation, these so-called ooids are more like rolling snowballs: they grow by layers as they are pushed across the sea floor by the waves. In the process, organic carbon molecules adhere to them and become part of the crystal structure.

Examining these impurities, Hemingway’s team has succeeded in retracing the supply of organic carbon in the sea—by up to 1.65 billion years.

In the journal Nature, the researchers show that, between 1,000 and 541 million years ago, this store was considerably lower than previously assumed. These findings refute the common explanations of significant geochemical and biological events of that time and cast a new light on the history of Earth.

The ocean as a reservoir of life’s building blocks

How does carbon get into the oceans? On the one hand, carbon dioxide (CO₂) dissolves from the air into seawater and is transported to the depths by mixing processes and ocean currents, where it is retained for a long time.

On the other hand, organic carbon is produced by photosynthetic organisms such as phytoplankton or certain bacteria. Using the energy of sunlight and CO₂, these microscopic organisms produce organic carbon compounds themselves.

When the organisms die, they slowly sink towards the sea floor as marine snow. If it reaches the sea floor without being eaten by organisms along the way, the carbon is stored in the sea floor for millions of years.

But it is not only phytoplankton that provides a supply of carbon components. The building blocks of life are also reused: microorganisms decompose excrement and dead organisms, thereby releasing the building blocks again. These molecules form what is known as dissolved organic carbon, which drifts freely in the ocean: a huge reservoir of building blocks that contains 200 times more carbon than is actually ‘built into’ marine life.

The oxygen revolution changed everything

Based on anomalies in oceanic sedimentary rocks, researchers assumed that this building block reservoir must have been particularly voluminous between 1,000 and 541 million years ago.

For a long time, this assumption served as the foundation for explaining how ice ages and complex life emerged at the same time. The photosynthetic production of the building blocks of life is closely linked to the development of the atmosphere and more complex life forms. It was only through photosynthesis that oxygen began to accumulate in the atmosphere.

In two waves—referred to as the oxygen catastrophes—the oxygen content rose to its current level of 21%. Both events were accompanied by extreme ice ages that covered the entire planet in glaciers.

Nevertheless, life continued to tinker and potter with new inventions: during the first oxygen catastrophe 2.4 to 2.1 billion years ago, organisms developed a metabolism converting food into energy with the help of oxygen. This exceedingly efficient way of generating energy enabled the development of more complex lifeforms.

Carbon content much lower than assumed

Hemingway’s team is tracking such connections between geochemical and biological developments. The researchers have developed a new method that allows them to directly determine the size of the marine building block store at that particular time, based on the carbon particles in ooids.

“Our results contradict all previous assumptions,” Hemingway summarizes. According to the measurements taken by the ETH researchers, between 1,000 to 541 million years ago, the ocean did not contain more, but actually 90 to 99% less dissolved organic carbon than it does today. It was only after the second oxygen catastrophe that the values rose to the current level of 660 billion metric tons of carbon.

“We need new explanations for how ice ages, complex life and oxygen increase are related,” says lead author Nir Galili. He explains the massive shrinkage of the carbon store with the emergence of larger organisms at that time: single-celled and early multicellular organisms sank faster after their death, thereby increasing marine snowfall.

However, the carbon particles were not recycled in the deeper layers of the ocean because there was very little oxygen there. They settled on the sea floor, causing the reservoir of dissolved organic carbon to decline sharply. It was only when oxygen accumulated in the deep sea that the carbon reservoir grew back to its current volume.

From the primordial ocean to the present day

Although the periods studied are long past, the research findings are significant for the future. They change our view of how life on Earth and possibly also on exoplanets has developed. At the same time, they help us understand how Earth responds to disturbances, and humans are one such disturbance: the warming and pollution of the oceans caused by human activities are currently leading to a decline in marine oxygen levels.

Consequently, it cannot be ruled out that the events described could repeat themselves in the distant future.