New study reveals asynchronous land–ocean responses to ancient ocean anoxia

Earth experienced a period of intense, large-scale volcanism during the early Aptian. Around that time, it also experienced widespread ocean deoxygenation during the Oceanic Anoxic Event 1a (OAE1a) as well as the onset of a period of unusual stability in Earth's magnetic field, known as the Cretaceous Normal Superchron (CNS), which lasted about 38 million years.

The prevailing hypothesis has been that rapid, volcanism-driven atmospheric CO₂ emissions triggered an immediate and globally synchronous carbon-cycle perturbation across both land and ocean systems, particularly marked by the onset of OAE1a.

Proving this hypothesis has been challenging, though, since it relies on determining the timing of a brief geomagnetic field reversal, termed magnetochron M0r. This reversal is used to define the Barremian–Aptian boundary and to help constrain the timing of OAE1a, with the termination of M0r also marking the start of the CNS.

Unfortunately, the exact age of M0r has long been debated, with published estimates ranging from 126.3 to 120.2 Ma. This uncertainty has hindered the precise correlation of global geological and environmental events, obscured potential causal links between volcanism and oceanic anoxia, and limited understanding of how marine and terrestrial systems responded to OAE1a.

To address this uncertainty, a research team led by Prof. XU Yigang from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences (CAS) and Prof. DENG Chenglong from the CAS Institute of Geology and Geophysics conducted a study of the Yanshan Scientific Drilling Project (YSDP-4) drill core, with a drill depth of 1497.5 meters. The core was recovered from the lacustrine Jiufotang Formation in northeastern China.

The study integrated high-resolution methods of dating the core using analysis of geomagnetic polarities recorded in rocks as well as analysis of orbital climate cycles preserved in the rock record. It precisely determined the termination of M0r—corresponding to the onset of the CNS—to 121.26 ± 0.38 Ma. This refined age improves the Early Cretaceous geomagnetic polarity timescale and establishes a more robust chronological framework for correlating global geological and environmental records.

The findings were published in Science Advances on March 4.

Using this updated timescale, the researchers compared carbon-isotope records from the Jiufotang Formation with marine archives of OAE1a from the same time interval. In marine sections, the negative carbon-isotope excursion that marked the onset of OAE1a appeared 0.3–0.66 million years after the end of M0r. In contrast, the equivalent shift in the terrestrial record began approximately 1.24 ± 0.40 million years after M0r ended.

This temporal offset demonstrates that terrestrial carbon-cycle responses lagged significantly behind marine changes at the onset of OAE1a.

By precisely determining the end of M0r and documenting the asynchronous land–ocean responses to OAE1a, the study provides a more precise timeline for comparing geological records worldwide. It also offers new insights into Early Cretaceous geodynamics and how marine and terrestrial carbon cycles evolved independently.

The research was conducted in collaboration with scientists from the CAS Guangzhou Institute of Geochemistry, the CAS Institute of Geology and Geophysics, the CAS Institute of Vertebrate Paleontology and Paleoanthropology, Peking University, Chengdu University of Technology, and Purdue University. It was supported by the National Natural Science Foundation of China.

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