Quaternary stratigraphy of Bayan Borehole in eastern Songnen Plain and its Palaeoclimate Significance

The Songnen Plain is a key region for Quaternary stratigraphic research in Northeast China. In this area, the Quaternary strata of the Bayan borehole reached a thickness of 58.00 m, offering excellent potential for detailed investigation. A comprehensive analysis of the borehole was conducted using lithological characteristics, optically stimulated luminescence (OSL), electron spin resonance (ESR), magnetic susceptibility, grain-size distribution, and magnetostratigraphy. Based on these results, a polarity stratigraphic framework was established, subdividing the sequence into four units: the Holocene Series (0–1.50 m), Guxiangtun Formation (1.50–18.80 m), Harbin Formation (18.80–35.50 m), and Huangshan Formation (35.50–58.00 m). Chronostratigraphic and paleoenvironmental interpretations were derived to establish a Quaternary stratigraphic framework for the eastern Songnen Plain. Grain-size end-member analysis was applied to the Bayan borehole using the Analysize method to extract three effective grain-size components. EM ₁ (6.72 µm) corresponded to the atmospheric background dust; EM ₂ (21.20 µm) reflected the input from distal dust sources associated with summer monsoon signals; and EM ₃ (45.60 µm) indicated the proximal deposition influenced by winter monsoon intensity. Comparative analysis of magnetic susceptibility, M _z , grain-size end-members, and marine isotope stages revealed strong correlations, identifying multiple climate response events. In the eastern Songnen Plain, the Middle Pleistocene was marked by a transition from dry-cold to cool-moist conditions, with a boundary at approximately 250 ka. During the Late Pleistocene, the region experienced alternating phases of dry-cold and cool-semi-humid conditions. The Holocene was characterized by a warm and semi-humid climate. These findings provide a robust data foundation for refining the Quaternary stratigraphy of the Songnen Plain and offer critical insights into regional paleoclimatic evolution.