Holocene Climate Change Over The Northwest Mongolia Recorded By Lake Tolbo

The arid central Asia(ACA)is one of the largest non-zonal arid regions in the world,with a fragile ecosystem and sensitive response to climate change.Investigations on the climate history in this region are of great significance to the protection of the regional ecological environment and in-depth understanding of the relationship between cultural exchange and climate change.However,the research on climate change in this region since Holocene is still controversial,and the records are mainly concentrated in Xinjiang and its adjacent areas,there are few high-resolution paleoclimate records with reliable chronology in northwest Mongolia.In this paper,Tolbo Lake,an alpine lake located in northwest Mongolia,was selected,and sediment cores(TB19-A and TB19-B)were drilled to determine the AMS¹⁴C age of bulk organic matter and assess the reservoir effect(RE).Based on establishing reliable chronology,the history of climate change in this region since the late last deglacial is reconstructed by using multiple proxies such as magnetic susceptibility,carbon and oxygen isotope of fine carbonate,carbonate content and mineral composition change,and the possible driving mechanism is discussed by comparing with the climate records in neighboring regions.The results show that the RE of Tolbo Lake sediments is about 1440 years,and the bottom age of TB19-A is13.75 cal kyr BP.The magnetic susceptibility and the carbonate content were used as indicators of inflow runoff and effective humidity(precipitation/evaporation),respectively.The dolomite content and the oxygen isotopes of fine carbonates(?¹⁸O_carb)reflect the change in regional summer temperature and evaporation intensity.According to the reconstruction results of multiple proxies and regional comparative analysis,the paper draws the following conclusions:(1)From the late last deglacial(13.75 cal kyr BP)to 10.8 cal kyr BP,the lake is principally replenished by the melting water from snow or glaciers;During the early-middle Holocene(10.8?4.8 cal kyr BP),the lake was in a stable state,and the recharge mainly consisted of summer precipitation.On the whole,the climate is relatively humid during this period;In the late Holocene(after 4.8 cal kyr BP),multiple proxies changed obviously,the climate was characterized by long-term warm and dry.(2)The comparison of moisture records since Holocene between northwest Mongolia and adjacent areas at the same latitude shows that the regional climate around 4.8 cal kyr BP has shifted to drier conditions,and the moisture pattern has regional consistency,but it is obviously different from the‘persistent wetting'pattern in Tianshan area.As far as temperature changes are concerned,the temperature in northwest Mongolia is on the rise after a cooling period of about 5.0 cal kyr BP.(3)From the perspective of regional cultural development,in the late Holocene,northwest Mongolia shifted to drier,while the humidity in the Tianshan area increased,which provided an impetus for the migration of population and culture to the south.At present,the northwest Mongolia and Tianshan areas are controlled by the mid-latitude Westerlies.The difference in humidity change between the two places during the Holocene is mainly since they are located at different latitudes in the ACA,and are regulated by the amount of water vapor carried by Westerlies on different water vapor transport paths affected by different North Atlantic Oscillation(NAO)phase changes.The increase of temperature in the late Holocene also increased evaporation and decreased effective humidity in northwest Mongolia.