Astronomical timescale and palaeoclimatic implication of stacked 3.6-Myr monsoon records from the Chinese Loess Plateau

Magnetic susceptibility and grain size records from two continuous red-clay and loess–palaeosol sequences on the Chinese Loess Plateau have been generated to investigate the evolution and variability of the East Asian monsoon (EAM) during the late Pliocene and Pleistocene. Tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess–palaeosol sequence (0–2.6 Ma), and an extended age model for the upper red-clay formation (2.6–3.6 Ma). This timescale indicates older ages for a number of the magnetic polarity boundaries, consistent with lock-in depth offsets in loess sequences. Good site-to-site correlation enables generation of stacked grain size and susceptibility records spanning the last 3.6 Myr. These records indicate that monsoon evolution since the late Pliocene can be subdivided into three phases: 0–1.25, 1.25–2.72 and 2.72–3.4 Ma; each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon (magnetic susceptibility) and winter monsoon (grain size). Spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity (410- and 100-kyr), obliquity (41-kyr) and precession (23- and 19-kyr) bands over the past 3.4 Myr, implying a non-linear response of the long-term EAM evolution to orbital and glacial forcing.