Using carbon isotopes and organic composition to decipher climate and tectonics in the Early Cretaceous: An example from the Hailar Basin, Inner Mongolia, China

• Published in: Cretaceous research 2021-02, Vol.118, p.104674, Article 104674
• Main Authors: Moore, Tim A., Moroeng, Ofentse M., Shen, Jian, Esterle, Joan S., Pausch, Roman C.
• Format: Article
• Language: English
• Subjects: Carbon isotopes; Cretaceous; Hailar Basin; Inertinite; Laurasia; Palaeoclimate; Structured vitrinite; Thermal sag basin; Yimin Formation
• ISSN: 0195-6671; 1095-998X
• DOI: 10.1016/j.cretres.2020.104674

When conducted in tandem, organic petrography and isotopic analyses can be used to interpret regional tectonics and palaeoclimate within a global context. As an example, a sequence of lignite (vitrinite reflectance average = 0.27%) in the Lower Cretaceous Yimin Formation, Inner Mongolia, China, was investigated. The study area is within the intracratonic Hailar Basin and was situated at ~45°N palaeolatitude. Globally warm temperatures allowed temperate to tropical vegetation such as ferns, cycads and gymnosperms to form thick, laterally extensive palaeomires within the basin. Probabilistic assessment indicates that the lower, thick (~40 m) Seam #16 may have accumulated over a time span between 174 (P10) to 481 (P90) thousand years, whereas the thin (~6 m) overlying Seam #16 Upper took between 20 (P10) and 96 (P90) thousand years. Thermal sag processes were active in the basin during these time periods, with subsidence rates balanced to allow enough accommodation space to accumulate thick peat, but not so fast as to induce flooding and burial by clastics. A high proportion of fossil charcoal (‘inertinite’) in both seams indicate that throughout the peat-forming period, fire was an important part of the mire ecology. This is consistent with global high oxygen content and an in-land continental palaeogeographic position with seasonal precipitation. Stable isotopes of carbon indicate periods of high precipitation but also times of significant relative dryness. The palaeoclimatic conditions are likely to be both seasonal as well as longer term, possibly decadal to millennium in scale and would have occurred over the whole region, not just mire specific. [Display omitted] •In-land Laurasia during the Early Cretaceous was highly susceptible to fire, possibly a result of higher atmospheric oxygen content.•Carbon isotopes indicate the region experienced dry periods, which facilitate degradation of the charcoal (inertinite).•The region was tectonically active allowing accommodation space for thick peats to accumulate.•The initiation and the termination of mire systems were at the mercy of subsidence rates.•Although during a time of angiosperm radiation, they were not yet an integral part of mire eco-systems.