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An international research team studying fossilized oyster shells has revealed substantial annual temperature variation in sea water during the Early Cretaceous. The finding overturns the assumption that Earth's greenhouse periods are marked by universally warmer and uniformly stable temperatures.

The researchers used oyster shell fossils from the Neo-Tethys Ocean along with high-resolution climate models to reconstruct seasonal fluctuations in sea surface temperatures during the greenhouse Earth period of the Early Cretaceous Valanginian stage, which lasted from 139.8 to 132.9 million years ago.

The team was led by Prof. Ding Lin from the Institute of Tibetan Plateau Research at the Chinese Academy of Sciences (CAS), in collaboration with researchers from the Senckenberg Biodiversity and Climate Research Center in Germany, the University of Bristol in the U.K., and the University of Antananarivo in Madagascar.

While the traditional view of greenhouse climates supports "weak seasonality and rare glacial activity," this study challenges that perspective by revealing significant seasonal temperature variations and periodic glacial melt events. The findings are in the journal Science Advances.

"Accretionary organisms like oysters act as spatiotemporal bridges between Earth's spheres, meticulously recording the interaction between climatic rhythms and ecological shifts. They inspire us to seek the future of our civilization in the depths of deep time," said Prof. Ding, corresponding author of the study.

Similar to tree rings, the shells of accretionary organisms such as oysters develop alternating light and dark growth bands annually. In summer, rapid growth under warmer temperatures results in porous "light bands," while slower, denser growth in winter creates "dark bands." Building on this principle, the researchers pioneered a method in 2014 that used seasonal oxygen isotope signals in ostracod shells to recalibrate paleoaltimetry, revealing that the Gangdese Mountains predate the Himalayas.

The researchers precisely identified growth bands in large Rastellum oyster shells and conducted high-resolution micro-sampling. Through petrographic analyses (including scanning electron microscopy and cathodoluminescence microscopy) and geochemical tests (such as those analyzing strontium isotopes, manganese, and iron content), they confirmed the shells' pristine preservation, free from diagenetic alteration, and extracted high-resolution seasonal climate signals.

Using the global climate model HadCM3, the researchers simulated sea surface temperatures, seawater δ¹⁸O, and salinity under different CO₂ levels to validate data obtained from the carbonate clumped isotope thermometer.

Results showed that during the Weissert Event cooling phase, mid-latitude winter sea temperatures in the Southern Hemisphere were 10–15°C lower than summer temperatures—similar to modern seasonal variations at comparable latitudes. Fluctuations in seawater δ¹⁸O indicated seasonal freshwater influx from glacial melt, akin to the dynamics of the contemporary Greenland ice sheet.

While current global warming is often simplified as merely "rising temperatures," this study underscores the nonlinearity and complexity of Earth's climate system. Elevated greenhouse gas concentrations may amplify seasonal extremes rather than lead to uniform warming. The team hypothesizes that Valanginian glacial pulses were driven by feedback from Paraná-Etendeka volcanism and orbital cycles.

"Even in today's warming world, regional geological events coupled with human activities could trigger unexpected cooling," noted co-corresponding author Dr. Wang Tianyang.

This study builds upon the team's prior work on continental ice sheet evolution, which estimated that Valanginian ice volume reached half of today's Antarctic ice sheet (approximately 16.5 million km³). The new findings deepen the understanding of greenhouse climate dynamics and land–ocean interactions.

"This research opens a new window into Earth's ancient climate, shattering the monolithic narrative of greenhouse stability to reveal the planet's hidden seasonal rhythms and icy echoes," remarked co-author Prof. Andreas Mulch from the Senckenberg Biodiversity and Climate Research Center.