The Holocene Stalagmite Records From The Monsoon Fringe Zone In Northern China

As an important part of the global monsoon system,the East Asian summer monsoon（EASM）plays a crucial role in the global hydrology and energy cycle.The Holocene is the nearest warm period,which is of great significance for human civilization and society development.Study on the Holocene climate changes not only helps us to understand the mechanism of the past climatic and environmental changes,but also provides an important natural reference for the monsoon evolution in the context of global warming.However,there are still various views on the temporal and spatial characteristics of the Holocene East Asian summer monsoon climate change currently.In particular,a hot topic about the Holocene optimum shows distinct temporal characteristics in the East Asian monsoon region.The monsoon marginal area is highly sensitive to the advance and retreat of the East Asian monsoon,and is an ideal area for studying East Asian climate changes.Reconstruction the Holocene climate evolution of the monsoon marginal area is crucial for deciphering the spatiotemporal characteristics and mechanism of the Holocene optimum of the East Asian monsoon.In this paper,four stalagmites from Longfeng Cave,Shanxi Province in the northern China,were selected for high-precision ICP-MS dating,¹⁴C dating,trace elements（Mg/Ca and Sr/Ca）analysis and high-resolution carbon and oxygen isotopic composition analysis to reconstruct high-resolution climatic/environmental changes,and analyzed the climate characteristics and abrupt climate events of the Holocene of East Asian summer monsoon.We obtained 58 ²³⁴U-²³⁰Th ages from 26stalagmites from Longfeng Cave and proposed a cumulative growth frequency record to reconstruct the characteristics of hydrological changes since the deglaciation in northern China.We also present high-resolution stable isotope records of oxygen(δ¹⁸O)and carbon(δ¹³C)spanning the period of 1869-2018 AD from an annually laminated stalagmite LF36.We discussed the climatic significance of carbon and oxygen isotopes on short time scales and revealed the characteristics of climate changes in the last 150 years.The main conclusions obtained are as follows:（1）The stalagmite δ¹⁸O from Longfeng Cave reflects the change characteristics of large-scale monsoon circulation on the orbital timescale.Lowδ¹⁸O value indicates the increase of EASM intensity and EASM precipitation,and vice versa.The Holocene Optimum of the margin of EASM began at 8.8 ka B.P.and ended at 5.5 ka B.P.,lasting about 3300 years,when the EASM was the strongest and the precipitation is abundant,corresponding to the highest northern summer solar insolation.（2）A series of δ¹⁸O variation at millennium-centennial and decadal time scales superimposed on the long-term trend,affecting the variation of the EASM.By using ensemble empirical mode decomposition（EEMD）to analyzeδ¹⁸O time series from Longfeng Cave,Lianhua Cave and Wuya Cave in northern China,Magou Cave and Jiuxian Cave in central China,and Dongge Cave in southern China,we found that on the millennium scale the amplitude of the climate variability in the middle Holocene is small,while the amplitude in the early and late Holocene is large.In addition,the stalagmiteδ¹⁸O also shows the“+﹣+”spatial pattern of EASM precipitation on the millennial scale,that is,the monsoonal precipitation in southern and northern China are in phase,but in anti-phase in central China.（3）The weak monsoon event of 9.3 ka and 8.2 ka are accurately determined by Longfeng stalagmiteδ¹⁸O record and the abrupt collapse of EASM of 9.3 ka event was stronger than that of 8.2 ka in early Holocene which was connected with the decline of the Atlantic Meridional Overturning Current（AMOC）and solar activity.（4）The cumulative growth frequency of Longfeng Cave is a robust climate indicator to reconstruct the hydroclimate changes since the last deglaciation in northern China.Less stalagmites formed from 16-12 ka B.P.and the growth frequency increased from 12-9.5 ka B.P.The growth frequency is highest during the 9.5 ka to 5ka B.P.,demonstrating optimized climate conditions and abundant precipitation during the middle Holocene.After 5 ka B.P.,the growth frequency decreased significantly,indicating that the local climate was relatively drier.The variation pattern is confirmed not only by the similarity of the growth rate of Longfeng Cave,but also by the synchronization with stalagmiteδ¹⁸O and other paleoclimate records from the surrounding area,indicating that the stalagmite growth frequency can reflect the monsoonal precipitation in northern China.（5）The trace elements（Mg/Ca and Sr/Ca）variations are mainly affected by regional hydroclimate-related prior calcite precipitation（PCP）on orbital time scale,whileδ¹³C variation is mainly affected by local vegetation changes.On millennium and shorter time scales,covariations of trace elements andδ¹³C indicate that PCP associated with regional hydroclimate dominates their changes.（6）On a short timescale（the last 150 years）,the speleothemδ¹⁸O of Longfeng Cave reflects monsoon precipitation changes in northern China,related to the Pacific Decadal Oscillation（PDO）on timescale of 20-30 years and 60-80 years.On the other hand,it indicates that there is a close relationship between East Asian monsoon precipitation in northern China and the PDO.Over the last 150 years,the factors controlling theδ¹³C of the stalagmites in Longfeng Cave are complex.Temperature,precipitation and related soil respiration processes may be main drivers.In addition,increase in global CO₂emissions caused by human activities and global temperature rising may have led to a significant negative trend ofδ¹³C and an increase inδ¹³C variability since the 1960s.