The Jenkyns Event (early Toarcian OAE) in the Ordos Basin, North China

• Published in: Global and planetary change 2020-10, Vol.193, p.103273, Article 103273
• Main Authors: Jin, Xin, Shi, Zhiqiang, Baranyi, Viktória, Kemp, David B., Han, Zhong, Luo, Genming, Hu, Jianfang, He, Feng, Chen, Lan, Preto, Nereo
• Format: Article
• Language: English
• Subjects: Biomarkers; Carbon isotopes; Early Jurassic; Lacustrine; Palynostratigraphy
• ISSN: 0921-8181; 1872-6364
• DOI: 10.1016/j.gloplacha.2020.103273

The early Toarcian (Early Jurassic) Oceanic Anoxic Event (T-OAE) or Jenkyns Event (~183 Ma) was marked by a globally synchronous negative carbon-isotope excursion, large-scale organic carbon burial and widespread oxygen depletion in the oceans. These features have been associated with volcanic activity of the Karoo and Ferrar large igneous provinces. The Jenkyns Event is well investigated in the Tethyan Ocean, but its expression in continental settings outside of Europe are still poorly understood. Here, the lacustrine Anya (AY) section in the NE Ordos Basin, North China, is studied for carbon-isotope geochemistry and palynology. Palynostratigraphy constrains the age of the section to the Pliensbachian-Toarcian transition, and a positive trend in δ13C, interrupted by a pronounced negative carbon-isotope excursion, has been identified in bulk organic matter and long-chain n-alkanes. The negative excursion is the expression of the Jenkyns Event, and coincides with variable total organic carbon (TOC) content in lake sediments. The maximum TOC (~21.5%) values precede the most negative carbon isotopes. A turnover from spore and seed fern-dominated palynological assemblages to Classopollis superabundance is coincident with the isotopic excursion and indicates clearly environmental perturbation. Biomarker data suggest that the organic matter was mainly sourced from higher plants. The magnitude of the Toarcian excursion in the bulk organic matter of this section (~ − 12.5‰) is much larger compared to that typically observed in marine and lake strata. Humidity-related fractionation effects in land plants and changes in plant groups might have contributed to the recorded magnitude. This magnitude is also larger than that of other terrestrial records, and casts doubt on the use of terrestrial C-isotope data to constrain deep time pCO2. Nevertheless, our findings show that the Jenkyns Event is well-recorded in lacustrine facies of the Ordos Basin. •Palynological assemblages indicate the Anya section is late Pliensbachian to early Toarcian in age.•The negative excursions of ~ − 12‰ in bulk rock and of ~ − 10 to −11‰ in long-chain n-alkanes record the global Jenkyns Event carbon-isotope excursion (CIE) in the Ordos Basin.•The negative CIE coincides with variable total organic carbon (TOC) content, but the high values mostly occur before the major negative CIE.•A vegetation turnover, the expansion of lake facies and siderite-rich beds are coincident with the Jenkyns Even