Studying Earth’s history is like trying to piece together a story where most of the pages have been lost. The planet is over 4.5 billion years old, and while mountains rise and erode, seas advance and retreat, and rocks are recycled by plate tectonics, much of the evidence of past worlds disappears forever. For scientists who want to understand how life and climate evolved, this creates a frustrating challenge: the most important events are often buried in deep time with only scattered clues left behind.
Yet every so often, nature offers a surprise—a hidden archive that manages to preserve whispers from an ancient age. A team led by ETH Zürich geoscientist Professor Jordon Hemingway has discovered one such archive in the form of tiny, egg-shaped iron oxide stones called ooids. Though they look as ordinary as grains of sand, these microscopic spheres have revealed a revolutionary new story about the state of Earth’s oceans up to 1.65 billion years ago.
Their discovery does more than rewrite textbooks. It reshapes our understanding of how carbon cycles through the oceans, how oxygen transformed the planet, and how complex life eventually emerged.
The Ocean as a Reservoir of Life
To understand the importance of the discovery, one must first appreciate the role oceans play in Earth’s carbon cycle. Oceans are not just vast bodies of water—they are living, breathing reservoirs of carbon, which is the building block of all life.
Carbon enters the ocean in two major ways. Some of it comes directly from the atmosphere: carbon dioxide dissolves into seawater, is carried downward by currents, and can remain trapped in the deep ocean for millennia. But a much larger portion comes from living organisms. Microscopic phytoplankton and bacteria use sunlight and dissolved CO₂ to create organic molecules. When these organisms die, their remains drift downward as “marine snow,” a slow shower of organic matter that feeds life in the depths and, if it escapes consumption, locks carbon into the sea floor for millions of years.
Even so, not all carbon follows this path of burial. Some remains dissolved in seawater as dissolved organic carbon (DOC)—a massive pool of molecules that floats freely through the ocean. Today, this reservoir is staggering in scale: it contains nearly 200 times more carbon than all marine organisms combined. For decades, scientists assumed that Earth’s primordial oceans contained even more DOC, providing the raw material that shaped both climate and evolution.
But ooids tell a different story.
Ooids: Nature’s Rolling Time Capsules
At first glance, ooids seem trivial, no different from ordinary sand. But their formation makes them unique. As they roll across the sea floor, pushed by currents and waves, they grow layer by layer like tiny snowballs. In each layer, fragments of the environment—minerals, chemical traces, and even organic molecules—become trapped inside.
This means that ooids are more than mere grains of sediment. They are time capsules, preserving the chemical fingerprints of ancient oceans with remarkable fidelity.
By analyzing the carbon bound inside these iron-rich layers, Hemingway’s team was able to directly measure the amount of dissolved organic carbon in oceans stretching back more than a billion years. Unlike previous methods that relied on indirect assumptions, this approach provided a rare, direct glimpse into Earth’s long-vanished waters.
A Radical Revelation About Ancient Oceans
The results shocked the scientific community. For years, researchers believed that between 1,000 and 541 million years ago—the period leading up to the explosion of complex life—the oceans must have been overflowing with organic carbon. This belief helped explain why ice ages occurred and how oxygen accumulated in the atmosphere.
But ooids told the opposite story. Hemingway’s team found that the oceans actually contained 90 to 99% less dissolved organic carbon than they do today. Instead of being a vast carbon-rich soup, the seas of this era were startlingly lean.
This finding overturns decades of assumptions. If the oceans had so little organic carbon, then the prevailing explanations for how Earth underwent dramatic climate shifts, how oxygen rose to modern levels, and how multicellular organisms appeared may no longer hold true.
The Oxygen Revolution and Its Consequences
To grasp why this matters, it helps to step back to one of Earth’s greatest transformations: the rise of oxygen. Early Earth’s atmosphere contained little free oxygen. Life was dominated by microbes that thrived without it. But with the evolution of photosynthesis, organisms began releasing oxygen as a byproduct. Over billions of years, this slowly changed everything.
The planet experienced two great surges of oxygen, often called the “oxygen catastrophes” because they were devastating to anaerobic organisms. The first, around 2.4 billion years ago, transformed the atmosphere and set the stage for more complex metabolisms. The second, between 600 and 541 million years ago, coincided with the rise of animals and the emergence of ecosystems we would recognize as truly modern.
Scientists long assumed that during the interval between these oxygen surges, the oceans stored huge amounts of dissolved organic carbon. This massive reservoir would have regulated oxygen, climate, and nutrient cycles. But Hemingway’s results reveal that the ocean carbon store was dramatically smaller than believed, forcing scientists to rethink the link between oxygenation, ice ages, and the origins of complex life.
Why Did the Carbon Store Shrink?
If the oceans were not carbon-rich, then what explains the difference? Hemingway’s team suggests that biology itself played the decisive role.
As single-celled organisms gave way to larger and more complex forms, the dynamics of “marine snow” changed. Bigger organisms sank faster, carrying their carbon-rich bodies more quickly to the sea floor. In the oxygen-poor deep ocean, decomposition slowed, preventing much of this carbon from recycling back into the dissolved pool. Instead, it became buried in sediments.
This rapid sinking effectively drained the ocean’s reservoir of dissolved organic carbon. Only later, when oxygen levels rose in the deep sea, could recycling restart, allowing DOC to grow again to its modern volume.
Lessons from the Past for the Future
Why should we care about oceans that existed more than half a billion years ago? Because Earth’s history is not only a story of the past—it is a guide to our future.
Today, human activity is disrupting the same delicate balance of carbon and oxygen that shaped ancient oceans. Burning fossil fuels is warming the planet, while pollution and rising temperatures are reducing oxygen levels in modern seas. Some regions are already experiencing “dead zones” where oxygen has dropped too low to support life.
If oxygen depletion spreads, it could alter the way carbon is stored in the oceans—potentially echoing the processes Hemingway’s team identified in deep time. By understanding how the ocean once responded to global change, scientists can better predict how it may react again in the centuries to come.
A New Chapter in Earth’s Story
The discovery hidden inside tiny ooids is more than a technical breakthrough. It is a reminder that the Earth itself carries memories—layered in rocks, buried in sediments, locked in microscopic grains of stone. By learning to read those memories, scientists can challenge long-held assumptions and rewrite the story of our planet.
For decades, we believed the oceans of a billion years ago were vast storehouses of life’s building blocks. Now we know they were far more fragile, their carbon reserves startlingly small. This revelation forces us to reimagine how life survived, adapted, and ultimately thrived through one of the most transformative chapters in Earth’s history.
And as we look forward, it leaves us with a sobering truth: Earth’s oceans are not eternal constants. They are dynamic systems, vulnerable to disruption, yet also resilient in ways we are only beginning to understand. The past teaches us humility—and urgency—as we navigate the uncertain waters of our own future.
More information: Nir Galili et al, The geologic history of marine dissolved organic carbon from iron oxides, Nature (2025). DOI: 10.1038/s41586-025-09383-3
