The Ordovician Period stands as a critical chapter in Earth's geological history, with carbon isotope records serving as both a key tool for stratigraphic correlation and a vital archive to unravel the coevolution of ancient climates and biospheres. For decades, however, prior research has largely focused on carbonate carbon isotope (δ^¹³C_carb) data, leaving organic carbon isotope (δ^¹³C_org) records significantly understudied.

To address this gap, a research team led by Prof. WU Rongchang from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), in collaboration with international partners, has presented a comprehensive δ^¹³C_org sequence spanning the upper Lower to lower Upper Ordovician. Derived from the Hule-1 core in the Jiangnan slope of South China, the study fills a critical knowledge void by documenting the Middle Darriwilian Isotopic Carbon Excursion (MDICE) in the δ^¹³C_org record.

Published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, the research sheds new light on the MDICE in deep-water depositional environments and its implications for global carbon cycle fluctuations. It also offers new insights into the environmental and biological coevolution that unfolded during the Middle Ordovician.

The MDICE is a globally recognized positive carbon isotope excursion event in the Ordovician. Historically, it has been primarily documented in δ^¹³C_carb data, with only scattered, incomplete records in organic matter. This new study marks the first time a fully preserved MDICE signal—with an amplitude of approximately 1.1‰—has been clearly identified in δ^¹³C_org records. The signal, extracted from fine-grained siliciclastic sediments of the Hule-1 core, Jiangnan slope, South China, is supported by a well-constrained graptolite biostratigraphy. It includes three distinct phases: a rising limb, a peak interval, and a falling limb, spanning the middle to late Darriwilian epoch.

Notably, the finding challenges a traditional view that only the rising limb of the MDICE is preserved in South China, due to supposed sedimentary hiatuses. Instead, the research demonstrates that this event is also well recorded in deep-water settings.

Through intercontinental correlation of both δ^¹³C_carb and δ^¹³C_org datasets, this study confirms the global synchronicity of MDICE and suggests that it likely reflects a significant perturbation of the global carbon cycle during the Darriwilian, which may be closely linked to coeval climatic cooling, oceanic oxygenation, and the biological radiation.

Beyond documenting the MDICE, the study provides a reliable δ^¹³C_org chemostratigraphic standard for Ordovician correlation. It also serves as a case study for advancing understanding of environmental-biological coevolution during the Darriwilian epoch.

The research was conducted in collaboration with scientists from Lund University (Sweden), Friedrich-Alexander Universität Erlangen-Nürnberg (Germany), and the Institute of Geology and Geophysics, CAS. It received financial support from the Strategic Priority Research Program of CAS and the National Natural Science Foundation of China.

Paleogeographic location of the Hule-1 core in the Jiangnan region, South China. (Image by NIGPAS)

δ^¹³C_org record and TOC content of the upper Lower–lower Upper Ordovician in the Hule-1 core. (Image by NIGPAS)

The Hule-1 core δ^¹³C_org record and compiled global δ^¹³C_carb trend, with comparison to paleoenvironmental factors and biological changes during the late Early–early Late Ordovician. (Image by NIGPAS)