Throughout Earth's expansive history, every rock and fossil has stood as a silent witness, preserving intricate records of climatic transformations within its structure. Over 170 years ago, Charles Robert Darwin eloquently captured this essence in his seminal work, On the Origin of Species, writing: "The geological record is an imperfect book, yet each page whispers a poem of life dancing with its environment." Today, geologists employ interdisciplinary scientific approaches as their interpretive tools, unraveling the hidden seasonal rhythms embedded within this ancient poetic narrative.
On May 2, a team led by Prof. Lin Ding from the Institute of Tibetan Plateau Research (ITP), Chinese Academy of Sciences (CAS), in collaboration with researchers from the Senckenberg Biodiversity and Climate Research Centre (Germany), the University of Bristol (UK), and the University of Antananarivo (Madagascar), published a groundbreaking study in Science Advances titled “Back to an Ice-Free Future: Early Cretaceous Seasonal Cycles of Sea Surface Temperature and Glacier Ice”. By analyzing fossilized oyster shells from the Neo-Tethys Ocean and integrating high-resolution climate model, the team reconstructed seasonal fluctuations in sea surface temperatures during the greenhouse Earth of the Early Cretaceous Valanginian stage (139.8–132.9 million years ago). The study revealed significant seasonal temperature variations and periodic glacial melt events, challenging the traditional view of "weak seasonality and rare glacial activity" during greenhouse climates. This discovery illuminates the complexity and dynamics of Earth’s ancient greenhouse climate, offering fresh insights into planetary climate evolution (Fig. 1).
"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. Lin Ding, corresponding author of the paper.
Fossils as "Climate Probes": How Do Oyster Shells Encode Ancient Climate?
Similar to tree rings, the shells of accretionary organisms like oysters form alternating light and dark growth bands annually. In summer, rapid growth under warmer temperatures produces porous "light bands," while slower, denser growth in winter creates "dark bands." Building on this principle, Ding’s team pioneered a method in 2014 using seasonal oxygen isotope signals in ostracod shells to recalibrate paleoaltimetry, revealing that the Gangdese Mountains predate the Himalayas (Ding et al., 2014).
In this new study, the team precisely identified growth bands in large Rastellum oyster shells and conducted high-resolution micro-sampling. Through petrographic analyses (scanning electron microscopy and cathodoluminescence microscopy) and geochemical tests (strontium isotopes, manganese, and iron content), they confirmed the shells’ pristine preservation, free from diagenetic alteration, and extracted high-resolution seasonal climate signals (Fig. 2). Coupled with global climate model (HadCM3), simulations of sea surface temperatures, seawater δ18O, and salinity under varying CO2 levels validated data from the carbonate clumped isotope thermometer. Results show that during the Weissert Event cooling phase, mid-latitude winter sea temperatures in the Southern Hemisphere were 10–15°C lower than summer temperatures—comparable to modern seasonal variations at similar latitudes (Fig. 3). Fluctuations in seawater δ18O suggested seasonal freshwater influx from glacial melt, akin to modern Greenland ice sheet dynamics.
"Early Cretaceous greenhouse Earth was like a symphony, its warm melodies occasionally punctuated by brief glacial notes," analogized Dr. Songlin He, the first author of this paper.
While present global warming is often simplified as "rising temperatures", this research underscores the nonlinearity of Earth’s climate system. Elevated greenhouse gas concentrations may amplify seasonal extremes rather than 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 by co-corresponding author Dr. Tianyang Wang.
This study is based on the team’s prior work on continental ice sheet evolution (Wang, He, et al., 2023), which estimated that Valanginian ice volume reached half of today’s Antarctic ice sheet (16.5×106 km3). The new findings deepen 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 by co-author Prof. Andreas Mulch of the Senckenberg Biodiversity and Climate Research Centre.
The study was supported by Excellent Research Group Program for Tibetan Plateau Earth System (continuation grant), and the National Natural Science Foundation of China (42102021, 42402223). It also leverages the team’s self-developed clumped isotope laboratory, a product of relentless technical innovation. Over the past five years, the team has published landmark studies in Science, Nature Reviews Earth & Environment, Science Advances, and National Science Review, driving frontier research in Earth System Science.
Link to article: https://doi.org/10.1126/sciadv.adr9417