A continental record of Early Cretaceous (Aptian) vegetation and climate change based on palynology and clay mineralogy from the North China Craton

Abstract

The Early Cretaceous Aptian Stage (121.4-113.0 Ma) witnessed significant climate changes and environmental perturbations including Ocean Anoxic Event 1a; however, reconstructions of paleoclimate have predominantly relied on marine records. In this paper, we report palynological and clay mineralogical data from a continuous core (YSDP-4) in the Kazuo Basin of Northeast China to produce a high-resolution terrestrial Aptian record that is correlated with the marine record. In the lowermost part of the Aptian succession (similar to 121 to similar to 120 Ma), vegetation types were dominated by bisaccate conifers (>75%), while smectite and illite clay minerals were co- dominant (each similar to 40-50%), indicating cool and semi-humid to semi-arid conditions with a tendency towards aridity. This was followed by an exceptionally dry and hot climatic event, from similar to 120 Ma to similar to 117.5 Ma, marked by a rapid increase in drought-resistant plants including members of the Cheirolepidiaceae (>60%). During the transition from the early Aptian to the late Aptian (similar to 117.5 to similar to 114 Ma), the clay mineral composition exhibited a complex pattern containing chlorite and kaolinite, and the proportion of vegetation representing relatively cool and moist conditions increased, indicating a gradual transition to a humid climate. By the end of the late Aptian (similar to 114 to similar to 112 Ma), climate became cooler and more humid, as indicated by a sharp increase in Taxodiaceae pollen (similar to 50%). Based on the similar patterns of vegetation and sedimentary records observed in the Kazuo Basin and across the Tethyan Realm, we propose that the early Aptian hot/dry climatic event was synchronous with Oceanic Anoxia Event 1a. The findings of our study provide an important terrestrial sedimentary reference section for future integration of Cretaceous climatic events and biological evolution across the marine-terrestrial spectrum