Palynological Record of the Aalenian–Bajocian Cooling Event from the Santanghu Basin, Northwest China

Abstract

The Aalenian–Bajocian (early Middle Jurassic) cooling event (ABCE) was a significant global climate disturbance during the Jurassic. Our analysis of sporopollen fossils from 18 mudstone and silty mudstone samples, collected from the lacustrine-terrestrial succession Xishanyao Formation in the Santanghu Basin, Northwest China, revealed a total of 191 species belonging to 53 genera. We identified an assemblage, the Cyathidites–Deltoidospora–Osmundacidites–Cycadopites assemblage, which dates to the Aalenian–Bajocian (early Middle Jurassic). This assemblage can be further divided into three subassemblages in stratigraphic order: the Cyathidites–Osmundacidites–Cycadopites subassemblage, the Cyathidites–Cycadopites–Psophosphaera subassemblage, and the Cyathidites–Deltoidospora–Osmundacidites subassemblage. We applied the Sporomorph EcoGroup (SEG) model to interpret the paleoclimate features. The sporopollen fossils indicate that the Santanghu Basin underwent a shift in vegetation types, from ground cover vegetation as the dominant form to canopy trees and then back to ground cover vegetation as the primary vegetation during the Aalenian–Bajocian. The SEG model analysis demonstrates that the CCP subassemblage is characterized by a low lowland SEG/upland SEG ratio, low wetter/drier ratio within the lowland SEG, and a low warmer/cooler ratio within the lowland SEG. These characteristics reflect the vegetation’s response to the ABCE in the Santanghu Basin.