CENOZOIC DEPOSITIONAL SEQUENCE IN THE WEIHE BASIN(CENTRAL CHINA): A LONG-TERM RECORD OF ASIAN MONSOON PRECIPITATION FROM THE GREENHOUSE TO ICEHOUSE EARTH
The onset and evolution of the Asian monsoon system has long been a topic of hot debate. Suggested onset time in past studies include Eocene(ca. 50 Ma),Early Miocene(ca. 22 Ma),Late Miocene(ca. 8 Ma),Late Pliocene and Early Pleistocene (ca. 2.6 Ma) based on different geological records, proxy indicators, as well as numerical simulations. Several factors contribute to such great uncertainties in these results, such as misinterpretation of proxy indices of paleoclimate, incomplete geological records, and poor age constraints for the depositional sequences. Therefore, there is a great need for a reliable long-term and continuous record of Asian monsoon variations. The Weihe Basin is a mid-sized basin formed during Cenozoic in Central China. It is located at the margin of the Asian monsoon region, therefore sensitive to changes in monsoon precipitation. The basin contains more than 7000 meters of unconsolidated fluvial-lacustrine deposit, forming a nearly continuous sequence from mid-Eocene to Holocene. Monsoon precipitation determines the amount of runoff in the Weihe River catchment, which in turn controls the deposition. Therefore the sedimentary sequence is an excellent archive of the long-term variations of the Asian monsoon system. We conducted comprehensive analyses of the depositional facies and stratigraphy, mammal fossil assemblage, pollen, and stable oxygen and carbon isotope composition of the sedimentary records. Our results show that monsoon precipitation probably started during the mid-Eocene, at least ca. 45 Ma, with an annual precipitation of ca. 1200 mm, but the modern monsoon circulation pattern was not established until the Early Pleistocene, ca. 2.6 Ma, with the annual precipitation of 600 ~700 mm. The collision of the Indian and Eurasia Plates at Paleogene e. g. 60 Ma built the Asian topography, and the thermal contrast between the great continent and the oceans at that time could have initiated the Asian monsoon circulation. This monsoon circulation was then further strengthened by each of the following events : close of the para-Tethys Sea during Late Eocene and Early Oligocene (ca. 34 Ma),the orographic growth of Himalayan-Tibetan Plateau during the Early Miocene (ca. 24 Ma) , and the high-latitude cooling in the Northern Hemisphere during the Late Miocene (ca. 8 Ma). However, before the Late Pliocene, ca. 3.4 Ma, the changes in monsoon intensity was small and incremental. The modern monsoon circulation is likely to be finally established at ca. 2.6 Ma, when the great Northern Hemisphere Glaciation pushed the intertropical convergence zone (ITCZ) toward the equator and compressed the monsoon rain belt. The monsoon system evolved through the shift between the greenhouse and icehouse Earth at Early Oligocene with the onset of the Antarctic ice sheet,but the global climatic cooling and the topographic change at that time did not completely set up the modern Asian monsoon circulation. The monsoon circulation was finally established by the northern hemisphere cooling during the Late Pliocene and Early Pleistocene. Moreover, the Asian deformation and surface uplift has contributed to the Asian monsoon evolution during Cenozoic.