Early Jurassic carbon cycle perturbations recorded in terrestrial sediments of the Sichuan Basin, China

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) is marked in the sedimentary record by a sharp negative carbon isotope excursion, thought to be causally linked to the emplacement of the Karoo-Ferrar Large Igneous Province and the associated release of ¹²C-enriched carbon. The T-OAE coincided with global climate and environmental changes, as well as biotic events, indicating substantial modifications in ecosystems. Recent studies have focused on the evidence of geological responses to the T-OAE in Early Jurassic terrestrial basins in China, particularly the Sichuan Basin. Nevertheless, debate remains over the identification of this event, owing to inadequate age-constraints of many sections, and a lack of robust correlations of the carbon isotope records. Moreover, the long-term evolution of the terrestrial carbon isotope record through the Early Jurassic, and its correlation to marine records, is still not firmly established. In this paper, we present new carbon isotope analyses of carbonate (δ¹³C_carb) from lacustrine carbonates and terrestrial organic matter (δ¹³C_org) from bulk rocks within the Ma'anshan and Da'anzhai members of the Ziliujing Formation from the Dacao 'D' (DCD) section in the eastern Sichuan Basin. Palynological-palynofacies analysis reveals a predominance of Classopollis pollen together with marker taxa such as Ischyosporites variegatus, Contignisporites problematicus, in the palynological assemblage, indicating a Pliensbachian-Toarcian age. A negative carbon isotope excursion (NCIE) is recorded in the organic carbon isotope data at the topmost part of the Pliensbachian Ma'anshan Member, which can be correlated to the Pliensbachian-Toarcian Boundary Event. This is followed, in the Toarcian Da'anzhai Member, by a major NCIE recorded in both organic matter and carbonate carbon isotope data which can be correlated to the T-OAE NCIE. A long-term carbon isotope record spanning the Sinemurian to Toarcian in Sichuan terrestrial sediments is also been reconstructed and its correlation with coeval marine records is proposed. A broader review of δ¹³C data from Chinese terrestrial basins spanning the Pliensbachian-Toarcian highlights a distinct ¹³C-depleted signature in the Sichuan Basin compared to basins at higher latitudes. Changes in latitudinal gradients and organic matters in the lake sediments were likely important factors influencing the amplitudes of the T-OAE NCIE and the carbon isotope values in terrestrial sedimentary records.

Keywords

Toarcian oceanic anoxic event / Lacustrine / Palynology / Carbon stable isotopes / Carbonate diagenesis

Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used Chat GPT in order to improve the language of the manuscript. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

CRediT authorship contribution statement

Xin Jin: Writing - original draft, Visualization, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. Viktória Baranyi: Writing - original draft, Visu-alization, Methodology, Formal analysis. David B. Kemp: Writing - review & editing. Zhiqiang Shi: Writing - review & editing. Hao Zou: Writing - review & editing. Binbing Li: Writing - review & editing, Visualization. Yunwang Zhang: Writing - review & edit-ing. Marco Franceschi: Writing - review & editing, Supervision, Investigation.

Declaration of competing interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appeared to influence the work reported in this paper.15

Acknowledgments

We acknowledge Nereo Preto (University of Padova) for his valuable suggestions and assistance in sample collection during the fieldwork. We also thank associate editor Federico Lucci and editorial adviser M. Santosh, and reviewers Guillaume Suan and Elke Schneebeli-Hermann for their insightful comments which greatly improved the manuscript. The research of VB is conducted in the scope of the internal research project "WEGETA" at the Croa-tian Geological Survey, funded by the National Recovery and Resi-lience Plan 2021-2026 of the European Union - NextGenerationEU, and monitored by the Ministry of Science, Education and Youth of the Republic of Croatia. We are grateful for Dragica Kovačić (Croa-tian Geological Survey) for performing the palynological process-ing. The work is a contribution to IGCP 739.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gsf.2025.102073.

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