<p>Understanding how the Earth surface responds to hyperthermal events is essential to predict the impact of current global warming. A succession of short-lived hyperthermals triggered long-term warming in the early Eocene. This study focuses on the so far poorly documented impact of hyperthermals on sedimentation and early diagenesis of lacustrine sediments by the integrated grain-size, petrographic, mineralogical (X-ray diffraction), and geochemical (trace elements) analysis of fine-grained sedimentary rocks in the upper member of the Kongdian Formation (Ek₁). Clay minerals (e.g., illite/kaolinite ratio), sporopollen assemblages, and geochemical data (e.g., Sr/Cu, V/Cr, and Sr/Ba ratios) indicate a humid–arid–humid stratigraphic trend for this unit deposited in oxic and brackish-to-saline hydrochemical conditions in the Bohai Bay Basin of NE China. During humid stage I, intensified runoff led to low-salinity and low-oxygen lacustrine conditions. During arid stage II, increased evaporation led to anhydrite precipitation in high-oxygen condition and gravity flows episodically occurred. High-salinity and high-oxygen conditions persisted during humid stage III. These three stages during the early Eocene hothouse showed different carbon-sequestration responses to palaeo-environmental change: light carbon was sequestered in both organic matter and inorganic carbonate minerals during humid stage I, whereas organic carbon was oxidized to carbon dioxide during arid stage II, and extensive carbonate precipitation predominantly accounted for carbon sequestration at the stage III, when oxidation of organic carbon markedly decreased.</p> Graphical abstract <p></p>