Fossil Shells Reveal How Ancient Oceans Helped Regulate Earth’s CO2

Over the last 50 million years, Earth slowly changed from a warm planet with no permanent ice into the cooler world we live on today. Ice sheets eventually formed at both poles, and global temperatures dropped by as much as 15 to 20 degrees Celsius.

This shift did not happen suddenly. It unfolded over tens of millions of years and was closely tied to a large decline in atmospheric carbon dioxide. Estimates suggest CO₂ levels fell by roughly 1,500 parts per million during this time.

Scientists have long tried to understand what drove this change. Mountain building, rock weathering, volcanic activity, and ocean biology have all been proposed. Still, no single explanation has fully accounted for the scale and timing of the shift.

Now, a new study points to something often treated as a background detail. The chemistry of the oceans themselves.

Why Ocean Chemistry Deserves Attention

Seawater contains a mixture of dissolved elements, including calcium, magnesium, sodium, and sulfate. These ions enter the ocean through rivers, seafloor reactions, and volcanic processes, and they leave through sediment burial and mineral formation.

Calcium is especially important. It combines with carbonate to form calcium carbonate, the material used by corals, plankton, and many shell-forming organisms. When this material sinks and becomes buried in sediments, carbon is effectively removed from the atmosphere for long periods.

Because of this, calcium sits at a key intersection between ocean chemistry, biology, and the long-term carbon cycle. Changes in calcium concentration can influence how carbon moves between the ocean and the atmosphere.

The Problem With Looking Back in Time

Reconstructing ancient seawater chemistry is difficult. There is no direct way to sample oceans from millions of years ago.

Most previous estimates came from fluid inclusions trapped inside evaporite minerals, such as ancient salt deposits. While valuable, these deposits are rare and unevenly spread through time. They provide only a few snapshots across long geological intervals.

As a result, scientists have lacked continuous, high-resolution records of how major seawater ions changed during key climate transitions.

Fossils as Chemical Recorders

To fill this gap, the researchers turned to fossils. They focused on large benthic foraminifera, single-celled marine organisms that lived on shallow seafloors in warm regions during the early Cenozoic.

These organisms build calcium carbonate shells that can preserve chemical information from the seawater they lived in. If the shells remain well preserved, their composition can act as a reliable archive of past ocean chemistry.

The team analyzed fossil shells dating from about 56 million to 31 million years ago, covering much of the Eocene Epoch and extending into the early Oligocene.

Why Sodium Matters So Much

The key measurement was the ratio of sodium to calcium in the fossil shells. Sodium plays a special role because it stays in seawater for a very long time. Its residence time is greater than 40 million years.

This means sodium concentration changes very slowly. Over the time period studied, it can be treated as nearly constant. Any long-term change in the sodium-to-calcium ratio therefore reflects changes in calcium, not sodium.

Using this approach, the researchers reconstructed absolute calcium concentrations in seawater at roughly one-million-year resolution.

Making Sure the Signal Was Real

Before trusting the data, the team carefully checked whether the shells had been altered after burial. They used scanning electron microscopy to examine shell structure and measured trace elements linked to contamination.

The shells retained their original microstructure and mineralogy. Element ratios showed no signs of clay contamination or secondary mineral growth.

These checks were critical. Even small chemical changes after burial could distort long-term trends.

A Clear Decline in Seawater Calcium

The reconstructed record shows that calcium concentrations in seawater were much higher in the early Eocene than they are today. Over time, calcium levels declined steadily.

This long-term decrease closely mirrors Earth’s gradual cooling trend. Where data overlap, the results agree with earlier estimates based on evaporite fluid inclusions, lending confidence to the reconstruction.

The new record, however, fills in large gaps and provides a far more detailed picture of how seawater chemistry evolved.

Adding Magnesium to the Story

The researchers also reconstructed magnesium concentrations by combining their calcium record with existing magnesium-to-calcium data from similar fossils.

Magnesium followed a different path. Levels decreased during the Paleogene before rising through the Neogene to reach modern values.

Together, the calcium and magnesium records represent the most complete reconstruction yet of major seawater ions across the Cenozoic.

Revisiting Ancient Carbon Dioxide Levels

Seawater chemistry affects how scientists interpret geochemical proxies used to estimate past CO₂ levels. One important example is the boron isotope method, which depends on seawater composition.

Using their new ion reconstructions, the researchers recalculated surface ocean chemistry and revised atmospheric CO₂ estimates for the Eocene.

The updated values are higher than many previous estimates. During the Early Eocene Climatic Optimum, atmospheric CO₂ likely reached between 1,200 and 3,000 parts per million.

A Strong Long-Term Pattern

When calcium concentrations and atmospheric CO₂ estimates were compared on million-year timescales, a strong relationship emerged.

Both declined together across much of the Cenozoic. Statistical analysis showed a tight correlation, even after smoothing out short-lived climate events.

This suggests that whatever was driving long-term changes in CO₂ was closely linked to changes in seawater calcium.

Correlation Is Not the Same as Cause

The study does not claim that calcium alone controlled Earth’s climate. Both calcium and CO₂ could be responding to the same geological forces.

Processes such as silicate rock weathering, volcanic degassing, and changes in seafloor spreading can influence both atmospheric CO₂ and ocean chemistry at the same time.

To explore whether calcium could actively influence CO₂ levels, the researchers used a carbon cycle model.

Testing the Idea With a Simple Model

The team used a box model called DeepCarb to simulate exchanges of carbon between the atmosphere, oceans, and sediments over long timescales.

They ran experiments in which seawater calcium was forced to decline, as well as scenarios where silicate weathering increased.

The goal was not to predict exact outcomes, but to test whether changes in calcium could plausibly drive changes in atmospheric CO₂.

How Calcium Could Affect CO₂

Calcium controls the saturation state of seawater with respect to calcite. This affects how easily marine organisms produce calcium carbonate shells and how readily those shells dissolve in the deep ocean.

Lower calcium reduces calcite saturation. This can decrease shell production and alter how carbon is stored in marine sediments.

Because calcification releases protons into seawater, changes in shell production can shift ocean chemistry in ways that influence atmospheric CO₂ over long periods.

What the Model Shows

In some model scenarios, changes in seawater calcium produced CO₂ declines similar to those seen in the geological record. In others, silicate weathering dominated the outcome.

The results depend strongly on how sensitive weathering and biological processes are assumed to be. Those sensitivities remain poorly constrained.

The model cannot prove causation. It does show that a calcium-driven mechanism is physically possible.

A Carbon Cycle With Many Moving Parts

The findings reinforce the idea that Earth’s carbon cycle is not controlled by a single process. Instead, it reflects the balance between tectonics, ocean chemistry, biology, and climate feedbacks.

Seawater chemistry may be one of several quiet but persistent influences operating over millions of years.

Rather than replacing existing explanations, the study adds another layer to how scientists understand long-term climate regulation.

Why This Matters Now

These processes act far too slowly to offset modern human emissions. They will not solve today’s climate crisis.

However, understanding them improves reconstructions of Earth’s past climate and sharpens the tools used to interpret geological records.

It also helps clarify the limits of natural climate regulation, showing that Earth’s stabilizing feedbacks operate within wide bounds, not tight controls.

What Still Needs Answering

Key uncertainties remain. Scientists still need better constraints on how weathering responds to climate and how ancient marine ecosystems reacted to changing ocean chemistry.

More fossil data and improved models will be needed to resolve these questions.

What is clear is that Earth’s oceans have been active players in the planet’s long-term carbon story, not just passive reservoirs.

The research was published in PNAS on January 09, 2026.