Multiscale monsoon variability during the last two climatic cycles revealed by spectral signals in Chinese loess and speleothem records

The East Asian Monsoon (EAM) exhibits a significant variability on timescales ranging from tectonic to centennial as inferred from loess, speleothem and marine records. However, the relative contributions and plausible driving forces of the monsoon variability at different timescales remain controversial. Here, we spectrally explore time series of loess grain size and speleothem delta O-18 records and decompose the two proxies into intrinsic components using the empirical mode decomposition method. Spectral results of these two proxies display clear glacial and orbital periodicities corresponding to ice volume and solar cycles, and evident millennial signals which are in pace with Heinrich rhythm and Dansgaard-Oeschger (DO) cycles. Five intrinsic components are parsed out from loess grain size and six intrinsic components from speleothem delta O-18 records. Combined signals are correlated further with possible driving factors including the ice volume, insolation and North Atlantic cooling from a linear point of view. The relative contributions of components differ significantly between loess grain size and speleothem delta O-18 records. Coexistence of glacial and orbital components in the loess grain size implies that both ice volume and insolation have distinctive impacts on the winter monsoon variability, in contrast to the predominant precessional impact on the speleothem delta O-18 variability. Moreover, the millennial components are evident in loess grain size and speleothem delta O-18 records with variances of 13 and 17 %, respectively. A comparison of the millennial-scale signals of these two proxies reveals that abrupt changes in the winter and summer monsoons over the last 260 kyr share common features and similar driving forces linked to high-latitude Northern Hemisphere climate.