Lacustrine varves in the Lower Cretaceous Yixian Formation of western Liaoning, Northeast China: Implications for seasonal to sub-decadal palaeoclimate variability associated with the Jehol Biota and “Dinosaur Pompeii”

The Lower Cretaceous Yixian Formation in western Liaoning, China contains the exceptionally well-preserved Jehol Biota, and includes the famous assemblages known as “Dinosaur Pompeii”. In this study, we focus on the fossil-rich Zhuanchengzi Bed near Yingwoshan, and describe laminated lacustrine successions that show prominent rhythmites to understand subdecadal palaeoclimate variability. We highlight the cyclicity preserved at a millimetre-scale based on analysis of the rhythmic alterations between the light- and dark-coloured carbonate-siliciclastic layers. Coupling petrographic with phase-specific geochemical analyses, we infer these laminated successions represent varves. The light-coloured layers of the couplets, containing dolomite-dominated carbonates, developed during warm seasons as a result of chemical precipitation enhanced by photosynthetic activities and rhythmic changes in the ambient water properties. The dark-coloured layers are mostly composed of siliciclastic and clay minerals representing terrestrial input during cold seasons. Varve thickness variations likely record seasonal climate changes regulated by ancient Asian Monsoons and decade-level cyclicities, such as ENSO-like and solar cycles. Overall, the varve records in the Yixian Formation offer a novel perspective on the palaeoseasonality of the Jehol Biota and provide insights into the dominant climate mode over interannual time scales of the Early Cretaceous greenhouse world.