Studying Earth’s earliest history is notoriously difficult. Events from billions of years ago left behind little direct evidence, forcing scientists to rely on indirect signals or computer models. Now, a research team led by ETH Professor Jordon Hemingway has uncovered a rare natural archive: microscopic, egg-shaped iron oxide stones known as ooids. These structures capture and preserve organic carbon from ancient oceans, providing a direct way to measure marine carbon reserves dating back up to 1.65 billion years.

On the surface, ooids look like grains of sand, but their growth resembles that of rolling snowballs. As they are pushed along the seabed by waves, they accumulate mineral layers. During this process, organic carbon molecules attach to the layers and become part of the stone’s crystal structure, effectively locking in a record of the ocean’s past.

Reconstructing Ancient Carbon Stores

By analyzing these carbon inclusions, Hemingway’s team reconstructed marine organic carbon supplies over long stretches of geological time. Writing in Nature, they report that between 1,000 and 541 million years ago, the ocean stored far less organic carbon than previously thought. These results challenge long-standing theories about the links between ancient geochemical cycles, climate events, and the rise of complex life.

The Ocean as a Reservoir of Life’s Building Blocks

Carbon reaches the oceans in two primary ways:

• Dissolved CO₂ from the atmosphere mixes into seawater and can remain in the deep ocean for extended periods.
  
• Organic carbon production by photosynthetic organisms such as phytoplankton and bacteria. When these organisms die, they sink as “marine snow.” If not consumed, the carbon reaches the seabed, where it can be stored for millions of years.
  

Beyond this, the ocean also contains dissolved organic carbon (DOC), released as microorganisms recycle organic matter. Today, this reservoir is massive — holding around 200 times more carbon than is present in living marine organisms.

Rethinking the Oxygen Revolution

Until now, many scientists assumed that Earth’s primordial DOC reservoir was especially large between 1,000 and 541 million years ago, helping explain why ice ages and more complex life forms appeared during this period. Photosynthesis not only generated organic carbon but also drove the rise of atmospheric oxygen, culminating in two dramatic “oxygen catastrophes” that transformed Earth’s biosphere.

• First oxygen event (2.4–2.1 billion years ago): Enabled the evolution of oxygen-based metabolism, making energy use more efficient and supporting early complex life.
  
• Second oxygen event: Raised oxygen levels toward today’s 21%, but was accompanied by severe ice ages that enveloped Earth in glaciers.
  

Carbon Levels Lower Than Expected

The ETH team’s method using ooids revealed a surprising result: ancient oceans contained 90–99% less dissolved organic carbon than today’s levels during the key interval of 1,000–541 million years ago. Only after the second oxygen event did DOC values rebound to modern levels of about 660 billion tonnes.

“Our results contradict all previous assumptions,” says Jordon Hemingway.

Lead author Nir Galili adds: “We need new explanations for how ice ages, complex life and oxygen increase are related.” He suggests that the emergence of larger organisms accelerated the sinking of marine snow, depleting the DOC reservoir. Because oxygen was scarce in deep waters at the time, carbon was buried on the sea floor rather than recycled, shrinking the ocean’s carbon store. Only once oxygen penetrated the deep ocean did the reservoir refill.

Lessons for Today and the Future

Though these findings concern events more than half a billion years ago, they hold modern relevance. They reshape how scientists understand the interplay between oxygen, carbon, climate, and life’s evolution — insights that may also apply to studying exoplanets.

Crucially, they also serve as a warning. Human-driven warming and pollution are already reducing oxygen levels in today’s oceans. Similar processes to those seen in Earth’s past could potentially re-emerge in the distant future.

Key Takeaways

• ETH Zurich team discovered ooids as direct archives of ancient marine carbon.
• Between 1,000–541 million years ago, dissolved organic carbon was up to 99% lower than assumed.
• This challenges existing theories linking carbon reservoirs, oxygenation, and ice ages.
• Results suggest larger organisms accelerated carbon burial in oxygen-poor seas.
• Findings offer new perspectives on Earth’s evolution, exoplanet habitability, and modern ocean risks.

Original Publication
Authors: Nir Galili, Stefano M. Bernasconi, Alon Nissan, Uria Alcolombri, Giorgia Aquila, Marcella Di Bella, Thomas M. Blattmann, Negar Haghipour, Francesco Italiano, Madalina Jaggi, Ifat Kaplan-Ashiri, Kang Soo Lee, Maxwell A. Lechte, Cara Magnabosco, Susannah M. Porter, Maxim Rudmin, Robert G. M. Spencer, Roman Stocker, Zhe Wang, Stephan Wohlwend and Jordon D. Hemingway.
Journal: Nature
DOI: 10.1038/s41586-025-09383-3
Article Title: The geologic history of marine dissolved organic carbon from iron oxides
Article Publication Date: 13-Aug-2025

Original Source: https://ethz.ch/en/news-and-events/eth-news/news/2025/09/minute-witnesses-from-the-primordial-sea.html

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