Consistent vegetation and climate deterioration from early to late MIS3 revealed by multi-proxies (mainly pollen data) in north-west China

Pollen data are useful palaeoecological archives for understanding vegetation and associated climatic changes. In this paper we present new pollen data and other proxies (lithology, grain size, loss-on-ignition (LOIorg at 550 °C)) obtained from a sediment core (BLK11A) from Balikun Lake, north-west China, to explore the vegetation and climate history of 60.4-27.3 ka (1 ka = 1000 calendar years before AD 1950), covering Marine Oxygen Isotope Stage (MIS) 3. Our three main results are: 1) a high relative abundance of thermophilous temperate tree pollen (e.g. Betula) accompanied by other warm-wet associated species (e.g. Ranunculaceae, Cyperaceae, Typha) suggesting warm and wet climate conditions during most of the early MIS3 (59.0-51.7 ka) that resemble the “Holocene optimum” with an increase in effective moisture in most parts of arid western China. Watershed bio-productivity was high as represented by the high LOIorg value. A high abundance of Ephedra pollen was assumed to be caused by frequent shifts in lake level, with locally favourable habitats (dry lake beds) forming when lake area shrank. In addition, an interval with deteriorated climate and vegetation was found between 57 and 56 ka, with a sharp decrease of vegetation cover and bio-productivity. 2) The middle to late MIS3 (51.7-27.3 ka) was marked by the development of desert shrubs (e.g. Rhamnaceae, Nitraria, Zygophyllum), together with low pollen concentrations of upland communities, indicating a continuous and gradually cooling and drying climate that resulted in reduced vegetation cover. Deterioration of the climate is also suggested by the presence of halite deposits and a sustained decrease in watershed bio-productivity. 3) The regional vegetation around Balikun was mainly dominated by Artemisia and Amaranthaceae, which are typical desert-steppe and/or desert taxa, indicating a progressive aridification in north-western China in the late Quaternary. Our new interpretations contradict earlier inferences of highest lake level and wettest conditions prevailing during the late MIS3, and we propose that the decline of northern high-latitude summer insolation and the increase of global ice volume from early to late MIS3 have exerted a remarkable influence on the evolution of vegetation communities and lake level in arid western China on orbital or sub-orbital time scales.