Oyster fossils break the myth of weak seasonality in the greenhouse climate

An international research team studying fossilized oyster shells revealed an annual substantial variation in sea water temperature at the start of the Cretaceous. The discovery reverses the hypothesis that the land greenhouse periods are marked by universally warmer and uniformly stable temperatures.

The researchers used fossils with the oyster shell of the Neo-Tethys ocean as well as high-resolution climatic models to rebuild seasonal fluctuations in sea surface temperatures during the greenhouse in greenhouse of the stadium of the early Cretaceous valanginian, which lasted from 139.8 to 132.9 million years.

The team was led by Professor Ding Lin of the Tibetan Plateau Institute 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 United Kingdom and the University of Antananarivo in Madagascar.

Although the traditional vision of greenhouse climates supports “low seasonality and rare glacial activity”, this study questions this perspective by revealing significant seasonal temperature variations and periodic glacial fusion events. The results are published in the journal Scientific advances.

“Accecretion organizations and oysters act as space-time bridges between the spheres of the earth, meticulously recording the interaction between climatic rhythms and ecological changes. They inspire us to seek the future of our civilization in the depths of deep time,” said Professor Ding, corresponding author of the study.

Similar to trees, shells of accretion organisms such as oysters develop lights of light and dark growth growth. In summer, rapid growth at warmer temperatures leads to porous “light bands”, while slower and denser growth in winter creates “dark bands”. Based on this principle, the researchers launched a method in 2014 which used isotopic oxygen seasonal signals in the shells of Ostracod to recalibrate paleoaltimetry, revealing that the Gangdeen mountains are prior to Himalayas.

The researchers have precisely identified the growth bands in large shells of Rastellum oyster and led high-resolution micro-samples. Thanks to petrographic analyzes (including electron scanning microscopy and microscopy with cathodoluminescence) and geochemical tests (such as those analyzing the isotopes of strontium, manganese and iron content), they have confirmed the virgin preservation of shells, exemption from digestic alteration and extract from seasonal climate signals.

Using the HADCM3 global climate model, the researchers simulated sea surface temperatures, sea water Δ¹⁸O, and salinity under different co₂ Levels to validate the data obtained from the carbonate agglomerated isotopic thermometer.

The results showed that during the cooling phase of the Weisset event, winter sea temperatures at latitude half -hint in the southern hemisphere were 10 to 15 ° C lower than summer temperatures – similar to modern seasonal variations with comparable latitudes. Seawater fluctuations Δ¹⁸O indicated a seasonal influx of fresh water from the ice cream, similar to the dynamics of the contemporary Greenland glacial cap.

Although current global warming is often simplified as a simple “increase in temperatures”, this study highlights the non-linearity and complexity of the earth’s climate system. High concentrations of greenhouse gases can amplify seasonal extremes rather than leading to uniform warming. The team hypothesizes that the Valangins glacial impulses have been motivated by the comments of the volcanism of Paraná-Etendeka and the orbital cycles.

“Even in the world of global warming today, regional geological events coupled with human activities could trigger unexpected cooling,” noted the co-corner author, Dr. Wang Tianyang.

This study is based on the previous work of the team on the evolution of the continental glacial cap, which estimated that the volume of Valanginian ice has reached half of the today’s antarctic ice cap (around 16.5 million km³). The new discoveries deepen the understanding of the dynamics of the greenhouse climate and the Earth-Ocean interactions.

“This research opens a new window on the ancient climate of the earth, breaking the monolithic story of the stability of the greenhouse to reveal the hidden seasonal rhythms of the planet and the icy echoes,” said the co-author of Professor Andreas of the Senckenberg Biodiversity and Climate Research Center.