🌡️ Why did Earth cool down after the disappearance of the dinosaurs?

What mechanisms allowed Earth to cool significantly after the age of the dinosaurs, transforming it from a tropical climate to a partially glaciated environment?

New research provides a surprising answer by exploring ocean chemistry. For millions of years, a slow and profound transformation in seawater composition is thought to have gradually altered the planet's climate balance.


Published in the Proceedings of the National Academy of Sciences (PNAS), the work of an international team reveals a marked reduction in the concentration of dissolved calcium in the oceans. Estimated at more than 50% over the last 66 million years, this drop directly affected the amount of carbon dioxide present in the atmosphere. A lower presence of this greenhouse gas means less heat is retained, thus contributing to the global cooling observed over this very long period.

To trace this chemical evolution, scientists relied on valuable clues preserved in marine sediments. Their analyses focused on the fossilized shells of tiny organisms called foraminifera, which preserve the signature of the seawater in which they lived. These natural archives have made it possible to establish a detailed timeline, highlighting a close link between calcium content and atmospheric carbon dioxide levels.

Furthermore, the computer models developed by the team indicate that with high calcium concentrations, the oceans stored less carbon. This carbon then remained more available as CO₂ in the air, maintaining a high global temperature. Gradually, the decrease in calcium altered biological processes, encouraging the trapping of carbon in ocean sediments and its slow removal from the atmosphere.


This chemical transformation appears to be associated with deep geological phenomena. The researchers found that the decline in calcium coincides with a slowdown in seafloor spreading, a volcanic process that continuously shapes the ocean floor. This slowdown altered the chemical exchanges between water and rock, leading to the gradual decrease in calcium concentrations.

Thus, this research renews our understanding of climate mechanisms over very long timescales. It demonstrates that ocean chemistry is not merely a consequence of climate change but can be an active driver of it.

The role of foraminifera in reconstructing past climate

Foraminifera are microscopic marine organisms whose calcareous shells preserve valuable information. During their life in the ocean, they incorporate chemical elements from the surrounding water into their shells. The composition of these shells therefore directly reflects that of the seawater at the time of their formation.

After the death of these organisms, their shells slowly accumulate on the seabed, forming layers of sediment. Scientists can extract sediment cores to go back in time. By chemically analyzing the fossil shells found at different depths, they reconstruct the oceanic and atmospheric conditions of very ancient periods.

This method is important for examining past climates because it provides direct data. Unlike other indicators, foraminifera offer a continuous record over millions of years. Their study has already allowed for a better understanding of ice ages or intense warming episodes in Earth's history.

For this specific research, the analysis of the ratio between different isotopes in the shells made it possible to deduce past calcium levels.