Paleoecology and paleoclimate across the Early Cretaceous Xing'anling Group: New palynological and radiometric data from the Hailar Basin (NE China)

The Early Cretaceous marked a dynamic interval in the evolution and distribution of terrestrial floras, coinciding with significant climatic and tectonic reorganization. In China, widespread continental deposits from this period have yielded rich palynofloras that offer insights into vegetation dynamics and paleoclimate patterns across eastern Eurasia. Among these basins, the Hailar Basin in northeastern China is notable for its extensive, continuous sedimentary succession exceeding 6000 m, which preserves a detailed record of terrestrial environments. Well HD4, drilled within the Hongqi Sag of the basin, captures the most complete Early Cretaceous succession documented to date. This section comprises a transition from coarse-grained alluvial and fluvial to fine-grained lacustrine deposits, assigned to the Xing'anling Group (Tongbomiao and Nantun formations). This study presents new palynological data from Well HD4, supplemented by zircon U–Pb ages from adjacent volcanic layers, allowing the differentiation of three palynological assemblages spanning the Berriasian to early Barremian. These biostratigraphic inferences align well with new LA–ICP–MS zircon U–Pb dates from both formations. Changes in floral composition reflect a shift from megathermal, Cycadopites–dominated assemblages to more temperate, conifer-rich communities with increasing proportions of ferns and bryophytes. This transition likely signals environmental stabilization and a slight cooling trend. Despite this turnover, the vegetation throughout the section remained adapted to humid conditions, in line with the expansion of lacustrine systems and progressive basin subsidence during the transition from the Tongbomiao to the Nantun formations.