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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]