One million years of seasonal seesaw in East Asian monsoon winds

Glacial-to-interglacial variations in East Asian summer and winter monsoon are widely ascribed to orbital and/or global ice-sheet forcing. However, the relative impact of orbital and millennial-scale factors on Pleistocene variations in East Asian monsoon still remain controversial. To better constrain the differential response of seasonal monsoon winds over the last million years we present paired records of siliciclastic silt grain sizes, pollen, minerals, and geochemical tracers obtained from high-sedimentation rate deposits at ODP Sites 1144 and 1146 in the northern South China Sea. The proxy records show that loess-style dust supply of winter monsoon was dominant and fluvial input reduced during peak glacials over the last 900 kyr, moreover, during Heinrich stadials, while fluvial mud marked interglacial regimes as result of enhanced summer monsoon, then completely superimposing the weakened dust input of winter monsoon. A dominating superposition of fluvial mud on top of eolian dust, however, also prevailed during the initial part of most glacial stages during and after the Mid-Pleistocene Transition (MPT), in part possibly modulated by long-term groundwater reserves and/or unknown climate forcings linked to the southern Hemisphere. Prior to the MPT, during glacial stages 24–32, prolonged groundwater reserves and/or a more limited extent of northern-Hemisphere ice sheets, or unknown southern Hemisphere forcing may have controlled an ongoing interglacial-style humid climate in East Asia. In summary, our findings suggest that variations of sediment signals of seasonal East Asian monsoon variability in part may have been more sensitive to secondary factors of groundwater storage, plant cover as well as to the redistribution of insolation energy amongst various climate subsystems than to direct orbital and/or northern ice-sheet forcing.