A new palynological assemblage of the Nantun Formation in the Huhehu Sag, Hailar Basin, NE China: Implications for paleoenvironments

Abstract

The Cretaceous period, recognized as one of the warmest in Earth's geological timeline, is crucial for understanding paleoclimatic conditions. Palynological analyses, focusing on spores and pollen, play a vital role in reconstructing paleoenvironmental. This study emphasizes the relationship between climate and plant distribution through detailed palynological analyses. It reports the first discovery of abundant and diverse palynomorphs from the lower Nantun Fm in the Hailar Basin, Inner Mongolia, China. The identified palynological assemblage, termed the Protoconiferus flavus—Pinuspollenites sp.—Quantouendinium sp. (PPQ) assemblage, provides significant insights into the Early Cretaceous flora of northeastern China. The PPQ assemblage can be chronologically attributed to the Hauterivian–early Barremian interval, consistent with the LA–ICP–MS zircon U–Pb isotope ages of the Nantun Fm volcanic rocks. Paleovegetation reconstruction reveals a prevailing dominance of coniferous vegetation in the region represented by the PPQ assemblage, indicative of a characteristic of mid-to high-latitude mountain coniferous forest landscape. The climatic was mainly humid or semi-humid. This conclusion is supported by the abundance of freshwater dinocyst and green algae fossils, suggesting that sedimentation occurred in a shallow, freshwater hydrostatic environment with an estimated water temperature of approximately 20–25 °C. The environment was also nutrient-rich, which facilitated the proliferation of green algae such as Scenedesmus, Pediastrum, and Tetraedron. Identifying this assemblage is important for determining the age of the lower Nantun Fm, reconstructing paleovegetation and paleoclimate, and analyzing the lacustrine water conditions during the sedimentary period in the Hailar Basin.