Mechanisms of inertinite enrichment in Jurassic coals: Insights from a Big Data-driven review

Big Data-driven research is thriving in the geosciences, with initiatives such as The Deep-Time Digital Earth (DDE) program (https://deep-time.org/), which is a "big science program" by the International Union of Geological Sciences (IUGS). Northern and northwestern Chinese environments produced a significant number of typically inertinite-rich coals during the Jurassic, which have been extensively researched and represent a high-quality subject for Big Data-driven studies. Inertinite in coals are widely accepted and applied as direct evidence of palaeowildfire with important palaeoclimatic significance, but the enrichment mechanism of inertinite in Jurassic coals have not yet been comprehensively understood. In this research, Big Data methodology and thinking were used to collect 1298 sources of information with inertinite content, and to reconstruct the palaeogeographic distributions of that inertinite. In addition, >300 datasets on polycyclic aromatic hydrocarbons (PAHs) and inertinite reflectance were collected. Based on these data and in combination with palaeoclimatic databases, two enrichment mechanisms for the high levels of inertinite are proposed. The results show that Jurassic coals in China are significantly enriched in inertinite compared to coals in the rest of the world, with an average inertinite content of 18.9% in the Early Jurassic and up to 36.8% in the Middle Jurassic. Combustion-sourced PAHs are widespread in Jurassic coal seams in different basins and are dominated by 4-ring and 5-ring PAHs. Both coal petrology and PAHs characteristics are indicative of a wildfire origin for inertinite, and the inertinite reflectances suggest that the palaeowildfire types were mainly low-temperature fires. It is proposed that the abundant inertinites in Jurassic coals were formed as the result of the multi-factorial coupling of palaeoflora, palaeoclimates, and palaeoenvironmental evolution. The enrichment models for the inertinite in the Early-Middle Jurassic coals indicate that these inertinites were mainly formed during periods of more obvious seasonal contrasts. Based on analysis from a Big Data review of inertinite concentrations in coals, a quantitative classification criterion is proposed to define those with >30% inertinite as "inertinite-rich coal", and those with >50% inertinite as "inertinite coal".