Land and Sea Didn't React to a Cretaceous Climate Crisis at the Same Time
One hundred and twenty-one million years ago, Earth was a different planet. Massive volcanic eruptions were pumping carbon dioxide into the atmosphere. The oceans were losing their oxygen in what geologists call Oceanic Anoxic Event 1a - a crisis that killed marine life across vast swaths of the global ocean. And somewhere underground, the planet's magnetic field was undergoing its last reversal before entering a 38-million-year stretch of stability.
These three events are connected, at least in theory. The prevailing hypothesis has been that volcanic CO2 triggered a rapid, globally synchronous upheaval in the carbon cycle - hitting land and sea simultaneously. A new study drilled deep into a Chinese lake bed to test that assumption, and the results complicate the picture considerably.
The chronology problem
Correlating events from this period across different rock archives has long been hindered by uncertainty over the precise timing of a brief geomagnetic reversal called magnetochron M0r. This reversal marks the boundary between two geological ages - the Barremian and the Aptian - and its termination is used to define both that boundary and the onset of the Cretaceous Normal Superchron (CNS), the long stable period that followed. Published estimates for the end of M0r ranged across six million years, from 126.3 to 120.2 million years ago. That six-million-year window is too wide to determine whether a volcanic event and an ocean anoxia crisis were cause and effect, or just rough contemporaries.
A research team led by Prof. XU Yigang from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and Prof. DENG Chenglong from the CAS Institute of Geology and Geophysics took aim at that uncertainty using an unusually well-preserved record: a 1,497.5-meter drill core from the Jiufotang Formation, a lacustrine (lake-bed) sequence in northeastern China that is also famous for its exceptionally preserved Early Cretaceous fossils.
Nailing down 121.26 million years ago
The team applied two independent dating methods to the core. The first analyzed the record of Earth's shifting magnetic field preserved in the rock - geomagnetic polarity stratigraphy. The second looked for the rhythmic signatures of Earth's orbital cycles - variations in its tilt and distance from the sun that drive long-term climate patterns - encoded in sediment layers. By combining both approaches in high resolution, they pinned the termination of M0r to 121.26 million years ago, with an uncertainty of plus or minus 0.38 million years.
That precision is substantially better than anything achieved before and is published in Science Advances as part of findings dated March 4, 2026.
Land lagged the ocean by more than a million years
With that anchor in place, the researchers compared the carbon-isotope record from the Jiufotang lake beds with marine records of OAE1a from around the world. Carbon isotopes shift in characteristic ways when large amounts of carbon enter or leave the ocean and atmosphere - they serve as a chemical fingerprint of perturbation to the global carbon cycle.
In marine sections, the negative carbon-isotope excursion marking the onset of OAE1a appeared 0.3 to 0.66 million years after the end of M0r. In the Chinese lake bed record, the equivalent shift in the terrestrial carbon record began approximately 1.24 million years after M0r ended - with an uncertainty of 0.40 million years. After accounting for uncertainties, the lag between marine and terrestrial responses was statistically real.
The conclusion is that the global carbon cycle did not respond uniformly to whatever triggered OAE1a. Land ecosystems absorbed or responded to the perturbation significantly later than marine systems - possibly because they are buffered by soil, vegetation, and freshwater hydrology in ways that dampen or delay the response to atmospheric CO2 changes.
Why this matters for reading Earth's past
The finding has practical implications for how geologists correlate rock records across different environments. If the carbon-cycle signal that defines OAE1a arrives more than a million years later on land than it does in the ocean, then matching continental and marine archives using that signal alone will produce chronologies that are systematically off. Future correlations of Early Cretaceous records will need to account for this temporal offset.
More broadly, the result raises questions about how other ancient climate perturbations propagated through Earth's systems. The assumption of simultaneous, globally synchronous responses to volcanic forcing may need revisiting for other events in deep time. The Cretaceous is particularly relevant because its warm climate, high sea levels, and unusual CO2 concentrations are sometimes invoked as an analogue for near-future conditions - with all the caveats that a hundred-million-year gap entails.
The study was conducted in collaboration with scientists from multiple Chinese institutions and Purdue University, and supported by the National Natural Science Foundation of China.