Tectonic And Orbital Timescale Climatic And Environmental Changes In The Qaidam Basin As Revealed By Grain Size Records Of The Huaitoutala Section

Understanding past variations of Asian inland aridification is instructive in better predicting its future variations.Both uplift of Tibetan Plateau and global cooling have a far-reaching impact on the evolution of paleoclimate and paleoenvironment in Asian inland.However,separating their effects is a difficult task.Past research reveals that the late Miocene is a period of important environmental changes in South Asia and East Asia.Furthermore,knowledge of both timing and extent of Tibetan Plateau uplift and global ice volume changes are clearer in this time period.However,little is known climatic and environmental variabilities during this interval at the Asian inland.This is mainly because high resolution environmental change records are lacking in this area.The thick lacustrine deposits in the Qaidam Basin,provide an excellent opportunity to reconstruct climatic environmental variations in Asian inland.Previous studies have established age model and generated environmental magnetic records from one of the well-exposed section(huaitoutala).Here we generate a grain size based paleoenvironmental record during the late Miocene(~9.95-5.95 Ma)from the same section in order to test previous inferences about environmental variations in this area.Our results are in line with environmental magnetic parameter and both records suggest that the interval ~8.6-6.7 Ma was an interval of lake expansion,likely driven by East Asian summer monsoon intensification.This onset of wetting in the Qaidam Basin can be explained by the northeastward expansion of the Tibetan Plateau but can not be explained by ice volume changes because ice volume records did not show apparent changes.We also perform spectrum analysis on the grain size records.We found that the 41 kyr and 100 kyr cycles are both significantly at 9.95-8 Ma.However,the 400 kyr cycles became obviously after 8 Ma.In addition,the 23 kyr cycles are pretty prominent in the entire sequence.Based on this observation,we improved the precision of the age model of this set of sediments by tuning the grain size record to the precession record.In addition to the orbital cycles,we also detected cycles of 66.7?55.6?26.3 and 16.4 kyr which are not corresponding to orbital parameters.We argue that these cycles might come from internally generated scales in autogenic dynamics.