Quantitative reconstruction of precipitation changes in the Mongolian Altai Mountains since 13.7 ka

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Holocene climate changes in the Altai Mountains and the surrounding areas are well documented, but the timing of the Holocene precipitation maximum remains controversial. We obtained 17 accelerator mass spectrometry (AMS) 14C dates and 310 fossil pollen assemblages from sediments in Tolbo Lake, which we used to reconstruct the history of vegetation and climate change in the Mongolian Altai Mountains over the past 13,750 years. The results suggest that mean annual precipitation (Pann) was the most significant control on the fossil pollen record at the study site. Based on the fossil pollen record from Tolbo Lake and 469 surface pollen samples within the radius of 1300 km, the reconstructed Pann was lowest during 13.7–11.3 ka (1 ka = 1,000 cal yr BP); relatively low but increasing Pann occurred in the early Holocene (11.3–9.4 ka); and maximum Pann occurred during the middle to late Holocene (after 6.8 ka), possibly because of the high temperatures. A significant climatic and ecological transition occurred in the Altai Mountains at ∼7 ka: steppe and desert steppe vegetation with generally low Pann was present before ∼7 ka, while subsequently there was the expansion of forest vegetation and a marked increase in Pann, possibly in response to the minimum in Northern Hemisphere ice sheets and the increased radiative forcing caused by rising atmospheric greenhouse gases.