Astrochronology for the Early Cretaceous Jehol Biota in northeastern China

The Early Cretaceous Jehol Biota in northeastern China provides an evolutionary window for 'feathered' dinosaurs, primitive birds, insects and early flowering plants. It also provides critical information for the biodiversity changes of the Early Cretaceous terrestrial ecosystem. Here we report a time series analysis for the 11.2-m-thick, fossil-bearing lacustrine deposits at the Sihetun section in western Liaoning, northeastern China on the basis of high-resolution magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) measurements. A hierarchy of sedimentary cycle bands of 120-260 cm, 50-67 cm and 18-42 cm was recorded in the MS and ARM series. With available radioisotope age constraints from the same section, sedimentary cycles of 120-260 cm, 50-67 cm and 18-42 cm were interpreted as Milankovitch cycles of short eccentricity (130 and 95 kyr), obliquity (36.6 and 46 kyr), and precession (22.1, 20.9 and 18 kyr), respectively. The 100 kyr-tuned 'floating' astronomical time scale indicates that the duration of the 11.2-m-thick section is ~ 0.67 Myr and the average depositional rate is ~ 1.70 cm/kyr. The duration of the 1.8-m-thick, main fossil-bearing interval that contains 8 beds of 'feathered' dinosaur/primitive bird fossils can be estimated as short as 150 kyr. The results suggest that climate fluctuations manifested in paleobotanical, sedimentological and geochemical records of the Yixian Formation may have been controlled by orbital forcing during Early Cretaceous.