New Drill-Core Study Reveals Asynchronous Land–Ocean Responses to Early Cretaceous Oceanic Anoxic Event 1a

A March 2026 Science Advances paper has upended ᴀssumptions about one of the most dramatic global climate perturbations of the Cretaceous: Oceanic Anoxic Event 1a (OAE1a, ~120 Ma). Led by Gui-Ming Lu and colleagues, the team analysed the YSDP-4 drill core from the South China Sea and produced the most precise chronology yet for magnetochron M0r, revealing that marine and terrestrial carbon-cycle responses to OAE1a were strongly asynchronous.

By integrating high-resolution magnetostratigraphy, carbon-isotope stratigraphy, and radioisotopic dating, the researchers show that the negative carbon-isotope excursion marking the onset of OAE1a in marine records began 0.3–0.66 million years after the end of M0r. In contrast, the equivalent terrestrial excursion—recorded in plant-derived organic matter—lagged by approximately 1.24 ± 0.40 million years. This multi-hundred-thousand-year offset demonstrates that the land biosphere and carbon cycle responded far more slowly than the oceans to the same underlying trigger, most likely mᴀssive volcanic CO₂ release from the Ontong Java Plateau.

The findings challenge the long-held view of near-instantaneous, synchronous global carbon-cycle perturbations during OAEs. Instead, they highlight decoupled terrestrial and marine systems: oceans rapidly absorbed and buried carbon, while continental weathering, vegetation, and soil carbon pools adjusted on much longer timescales. The refined M0r chronology also тιԍнтens the placement of the Barremian–Aptian boundary and provides a new global correlation tool.

“This study proves that the Earth system did not respond uniformly,” Lu stated. The discovery forces a re-evaluation of how ancient greenhouse crises cascaded across land and sea, with major implications for understanding modern climate feedbacks. After decades of ᴀssuming тιԍнт synchrony, OAE1a has revealed a far more complex, time-lagged world.