<p>Gypsum and its fluid inclusions hold potential for reconstructing paleo-fluid compositions, paleo-seawater temperatures, paleogeography, and paleoclimate. This study focuses on continuous gypsum beds within the Eocene–Oligocene Yehucheng Formation of the Lanzhou Basin, northeastern Tibetan Plateau. By analyzing the gas composition and carbon isotopes of primary fluid inclusions in gypsum, we investigated early Paleogene atmospheric characteristics and the principal controls on aridification in Asia, demonstrating the significant potential of gypsum and its primary inclusions as novel proxies for paleoclimate change. Microscopic examination of gypsum samples reveals extensive secondary alteration in the Yehucheng Formation, dominated by dissolution–reprecipitation processes. Differences in optical properties between primary and secondary gypsum components allow us to propose a petrographic method for distinguishing them under optical microscopy. Quadrupole mass spectrometry analysis indicates that CO₂, N₂, O₂, and Ar dominate the gas composition of primary inclusions, while carbon isotopic signatures suggest a biogenic origin for CO₂ and CH₄. Correlation among gases concentrations confirm that gypsum crystals formed at or near the air–water interface captured traces of ancient atmosphere. After normalization to remove biogenic influences, the average paleo-atmosphere composition during the Yehucheng deposition is estimated to be N₂ = 80.25%, O₂ = 18.84%, and Ar = 0.91%. Stratigraphic profile of gas content and carbon isotopes from the Yehucheng Formation shows CO₂ volume trends responding clearly to two global climate events during the late Eocene–early Oligocene, indicating that aridification in Asian was primarily controlled by global climate change.</p> Graphical abstract <p></p>