Oxygen-isotope palaeothermometry of the Eocene Harudi Formation, Kutch Basin, western India: evidence for an early inception of the Middle Eocene Climatic Optimum

Deep time is often imagined as a slow, steady drift, but the Earth's climate history is actually a story of sudden jumps and dramatic reversals. During the Eocene epoch, roughly 56 to 34 million years ago, the planet was a greenhouse world, significantly warmer than it is today. Yet even within this hot era, the climate did not remain static; it experienced brief, intense spikes of heat known as hyperthermal events, followed by longer periods of warmth that stood out against a general cooling trend. One of the most significant of these warm phases is the Middle Eocene Climatic Optimum, a period where global temperatures surged for hundreds of thousands of years. Understanding exactly when this warming began and how it felt in specific regions is crucial for scientists trying to piece together how the Earth's climate system responds to change. To do this, researchers often look at the chemical fingerprints left behind in ancient shells, which act as natural thermometers recording the temperature of the seawater in which they grew.

For decades, a specific stretch of rock in western India, known as the Harudi Formation, remained a mystery in this global puzzle. This layer of sediment, rich in fossils of ancient whales, crocodiles, and giant sea snails, sits in the Kutch Basin and represents a critical window into the middle Eocene. However, despite its importance, scientists had very little data on the actual water temperatures of this time and place. The difficulty lay in the rocks themselves; the area is famous for its fossils, but the shells found there are often chemically altered by millions of years of pressure and groundwater, making their temperature records unreliable. Without pristine, unaltered shells, the chemical signals needed to calculate past temperatures are lost or scrambled. This left a significant gap in the map of ancient climates for western India, forcing researchers to rely on data from other rock layers nearby that might not tell the same story.

A team of researchers has now filled this gap by carefully selecting six fossil samples from the Harudi Formation that had survived in perfect condition. They chose five oyster shells and one shell from a large, single-celled sea organism called a Nummulites, all collected from a specific road cut where the rock layers are clearly visible. Before attempting any temperature calculations, the team had to be absolutely certain that these shells had not been chemically tampered with over millions of years. They used a powerful X-ray technique to scan the internal structure of each sample, confirming that every single one was made of pure, stable calcite, the same mineral the animals originally built their shells from. This verification was essential, as it proved the shells had not been recrystallized or altered by later geological processes, ensuring that the chemical data they held was a true record of the ancient ocean.

With the samples verified as pristine, the researchers analyzed the oxygen isotopes trapped within the crystal structure of the shells. The ratio of different types of oxygen atoms in a shell changes depending on how warm the water was when the animal was alive. By measuring these ratios and comparing them to what is known about the chemical makeup of ancient seawater, the team was able to calculate the exact water temperatures for each layer of rock. The results revealed a dynamic thermal history. The water temperatures in this region ranged from a relatively cool 27.31 degrees Celsius at the top of the formation to a scorching 38.44 degrees Celsius in a specific layer of brown shale. Most notably, the data showed two distinct spikes in heat. One peak reached 32.25 degrees Celsius in a layer of rock formed by a massive storm that piled up shells, while the other, even hotter peak of 38.44 degrees Celsius, was found in a layer of mud that also contained evaporite minerals, suggesting a time when the water was shallow and highly saline.

These temperature spikes are interpreted as the local signature of the Middle Eocene Climatic Optimum, the global warming event mentioned earlier. What makes this discovery particularly interesting is the timing. Other studies have suggested that this global warming event began around 40.5 million years ago. However, the rock layers in the Harudi Formation where these heat spikes were found are dated to be slightly older, straddling a boundary around 41.6 million years ago. This suggests that the warming associated with the Middle Eocene Climatic Optimum may have started earlier in this part of the world than the global average suggests, or at least that the signal arrived in the western Indian Ocean before it was fully recorded in other parts of the globe. The researchers also noted that the hottest temperatures coincided with environments that were stressful for life, such as the storm-swept shell beds and the salty, evaporating lagoons. This alignment makes sense, as extreme heat often leads to evaporation and changes in salinity, creating conditions where only the hardiest organisms could survive.

The study is presented as a preliminary step rather than a final conclusion. The team emphasizes that while the six samples provide the first direct temperature record for this formation, the small number of data points means the full picture is still being assembled. The extreme heat values, particularly the 38.44 degrees Celsius reading, are striking, but the researchers caution that more samples are needed to confirm these trends with statistical certainty. They also noted that some of the chemical signals in the shells were unusual, hinting that local environmental factors might have influenced the data in ways that are not yet fully understood. Despite these caveats, the work successfully establishes a foundation for future research. It proves that the Harudi Formation holds a readable climate record and suggests that the warming of the middle Eocene may have had a more complex and earlier onset in this region than previously thought, inviting further investigation into how this ancient greenhouse world truly functioned.