An astronomically tuned 8.1 Ma eolian record from the Chinese Loess Plateau and its implication on the evolution of Asian monsoon

Abstract

The Chinese loess-paleosol sequences have led to a great understanding of the evolution of the East Asian monsoon (EAM) in the Quaternary. However, evolution of the EAM in the late Miocene and Pliocene is quite blurry, hindering our understanding of the mechanisms driving the EAM. Thus, we obtained high-resolution grain size records of 8.1 Ma from the thickest eolian Red Clay sequence at Chaona on the Chinese central Loess Plateau, and astronomically tuned the grain size records to the orbital variation with a time resolution of 1.5 ka to 5.8 ka. The variability of coarse size fraction, a recognized proxy for the East Asian winter monsoon (EAWM), indicates a persistent but stepwise intensification of the EAWM at ca. 5.5 Ma, 3.6 Ma, 2.6 Ma and 1 Ma. Filtering of this proxy record at 100, 41, and 21 ka bands was conducted to trace the evolution of the EAWM in a frequency domain over 8.1 Ma. Temporal variations of these three cycles show a remarkable simultaneous enhancement at ca. 5.5 Ma, indicating a synchronous intensification of high and low latitude signals in the EAWM record. We suggest that the closure of Panama Seaway to form the North Atlantic thermohaline circulation and the high latitude cooling during this time might collectively trigger such a response. Copyright 2011 by the American Geophysical Union.