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• Organic petrology and biomarkers in the Lower Cretaceous Jiufotang Formation from the Kazuo Basin were investigated. • Terrigenous organic matter was diluted by detrital minerals and phytoplankton was the most important organic matter source. • Anoxic bottom water may be necessary for organic matter enrichment in lake systems. • Climate changes controlled the organic matter enrichment model during deposition of the Jiufotang Formation. The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota. [ABSTRACT FROM AUTHOR]

• New nutrient proxy reveals oceanic P peaks during the Hirnantian glaciation and deglaciation. • The HICE was caused by enhanced organic carbon burial linked to more efficient P cylcing. • Persistent volcanisms and less CO 2 consumption via silicate weathering terminated the glaciation. • Post-glacial carbon cycle and climate depend on the duration of intensive volcanisms and oceanic nutrient availability. The Hirnantian Stage of the Late Ordovician coincides with a positive carbon isotope excursion (HICE, ∼+6 ‰), a major glaciation, increased volcanic activity, expanded marine anoxia, and one of the largest mass extinctions of the Phanerozoic. The origin of the HICE is debated, with proposed mechanisms favoring enhanced low-latitude carbonate weathering and/or increased efficiency of organic carbon burial. To test those hypotheses, we assembled new and published δ13C carb and δ13C org data and shale phosphorus and carbonate-associated phosphate concentrations from diverse depositional settings on several continents. We then evaluated these results using an integrated carbon cycle model derived from GEOCARB, and a coupled oceanic carbon and phosphorus cycle model. This approach yielded quantitative tests of the response of the global carbon cycle to changes in volcanic degassing, silicate and carbonate weathering rates, and organic carbon burial, individually and in combination, to determine which forcings yield signals most aligned with observed δ13C carb and δ13C org records. On this basis, the most plausible scenario for the HICE involves a ∼50 % decrease in weathering of silicates and organic carbon at higher latitudes combined with enhanced carbonate weathering (f wcarb = 88 %) at lower latitudes. Synchronous, globally enhanced burial of organic carbon was due to more efficient nutrient cycling caused by stronger thermohaline circulation and increased volcanism that further enriched oceanic dissolved inorganic carbon (DIC) in 13C. The influence of increased input of 13C-depleted carbon from volcanism and metamorphism may have been overwhelmed by these drivers of the positive δ13C shift. The Hirnantian Glaciation was likely initiated by secular enhancement of continental weathering due to the expansion of early plants but was terminated by increased volcanism and decreased CO 2 consumption by silicate weathering during the cooling. This scenario is consistent with the timeline of known geological events and offers insights into the mechanisms responsible for the associated biotic crisis. Specifically, the onset of the Hirnantian Glaciation induced lower global temperatures, resulting in sea-level fall and a loss of habitat space, while its termination led to enhanced organic carbon export and deep-water anoxia, all of which likely contributed to the Late Ordovician mass extinction. [ABSTRACT FROM AUTHOR]