Late Cretaceous Environmental Dynamics in the Northern Songliao Basin: Geochemical and Palynological Evidence from the Yaojia-Nenjiang Formations

The Santonian–Campanian transition of the Late Cretaceous, a period of profound global climatic reorganization marked by the final major Oceanic Anoxic Event (OAE 3), is relatively underrepresented in terrestrial proxies. To address the scarcity of coeval palynological and geochemical evidence from continental sequences, we present detailed high-resolution datasets from the Yaojia–Nenjiang succession in the HLD-1 borehole (Songliao Basin, NE China). Analysis of palynomorphs, major and trace elements, and total organic carbon from the mid-Santonian Yaojia Formation and the late Santonian–early Campanian Nenjiang Formation records a marked vegetation change. The prevalence of xerophytic taxa (Classopollis, Schizaeoisporites) in the lower unit transitioned to a dominance of hygrophytic ferns and conifers in the upper unit, indicating a climate shift from semi-arid to warm-humid subtropical conditions. Geochemical proxies further support this climatic transition, revealing significantly intensified chemical weathering (CIA values 69.02–77.36) and escalating oxygen depletion in the water column. Declining Se/Mo (4.99 to 3.05) and Ge/Mo (3.72 to 2.34) ratios, alongside elevated Mo concentrations (up to 2.34 ppm), document a progression from oxic/suboxic to persistently anoxic bottom waters. These findings collectively elucidate a “weathering–productivity–anoxia” positive feedback loop. Elevated nutrient inputs from enhanced chemical weathering under greenhouse conditions stimulated primary productivity. Subsequently, thermal stratification during peak lake transgression led to bottom-water anoxia, promoting exceptional organic matter preservation. This terrestrial archive provides a robust demonstration of how continental lake systems accurately capture and amplify global climate perturbation signals, establishing a crucial new land–sea correlation for understanding OAE 3.