Recognition Of Milankovitch Cycles In Late Cretaceous Nenjiang Formation From The Songliao Basin And Paleoclimate Response

The research of Cretaceous continental sedimentary records is contributed to our understanding of how the terrestrial geological and ecological systems responded to the past climate changes under greenhouse conditions and our ability to predict future climate change.The Songliao Basin of Northeast China is one of the largest and long-lived Cretaceous continental basins in the world,thick lacustrine sediments in the basin preserves a unique geological setting of long-term continuous Cretaceous succession.The Cretaceous Continental Scientific Drilling Program(CCSD)in Songliao Basin has provided a unique opportunity for detailed analyses of Cretaceous continental climate,and is conductive to carrying out detailed scientific researches.Here we present high-resolution elemental data of the Member 1 of Nenjiang Formation(1182.74~1249.32m)and the Member 2 of Nenjiang Formation(1086~1145.24m)from SK-2 East core as acquired using nondestructive X-ray fluorescence(XRF)core scanning.The aims are to recognize the Milankovitch cycles preserved in the sedimentary records and to discuss how did climate respond to the orbital forcing during the Late Cretaceous.Changes in terrigenous detrital elements and K/Ti,Ti/Al,Zr/Rb,Rb/Sr ratios indicate that during the deposition of Nenjiang Formation,the climate changed from humid to semi-arid and semi-humid.By using the Multitaper Method of Spectral Analysis(MTM),Evolutive Harmonic Analysis(EHA),Average Spectral Misfit(ASM),and band pass filter of Rb/Sr,K/Ti,Ti/Al,and Zr/Rb ratios,we identified the well-preserved Milankovitch cycles in the sedimentary sequences of the Nenjiang Formation.The results show that the climate was stable during the deposition of K2n1,and the effects of eccentricity and obliquity on climate were obvious.However,the climate fluctuated significantly during the deposition of K2n2,which was mainly controlled by eccentricity and precession.The modulation effect of obliquity was relatively poor and the more it went to the later stage of the deposition of Nenjiang Formation,the weaker its modulation was.The paleoclimate response mechanisms in the Songliao Basin during the late Cretaceous were mainly characterized by:(1)During the deposition of K2n1,the climate was mainly affected by the monsoon circulation,especially when the obliquity increased significantly and the precession's amplitude increased,the seasonal differences increased,which led to the increase of summer monsoon intensity and rainfall.(2)During earlier period of the deposition of K2n2,the climate was still humid.But in the process of transition to the medium and late deposition stage,the modulation effect of the obliquity was weakened.On the contrary,the control of precession is enhanced.Changes in astronomical orbital parameters reduced seasonal rainfall and increased the intensity of mean meridional circulation during its southward in the winter of northern hemisphere.At the same time,the westerly wind that from the North Atlantic enhanced and the monsoon intensity decreased.The climate began to turn into semi-humid and semi-arid.During the northward shift of the mean meridional circulation in the summer of northern hemisphere,the strong downdraft made the water vapor difficult to condense,which resulted in an arid climate,compared with wet periods.