A Study On Variability Characteristics And The Driving Mechanism Of 2.8 Ka BP Event In East Asian Monsoon Region

With global warming,the frequency of extreme climate events has increased significantly,causing serious economic losses and human casualties,which should be concerned rightly.Understanding the variations and mechanisms of extreme climate events in the past with high-resolution paleoclimate carriers and well-proxied indicators is of great significance for predicting extreme climate changes in the future.The 2.8 ka BP event is a global extreme climate event that occurred in the Late Holocene,which is considered to be related to the historical changes in Western Zhou period.Conducting an in-depth study of this event can be important not only for future climate change prediction,but also for understanding the relationship between climate change and human historical changes.However,studies on this event at present exist highly controversial,especially on the timing,characteristics and driving mechanisms due to the differences in interregional climate change and those of resolution and dating errors among different carriers.Stalagmite is an ideal carrier for studying past climatic extremes and is widely used for the study of 2.8 ka BP event,but a single stalagmite oxygen isotope record is always hardly to precisely and completely capture the variations of 2.8 ka BP event in a large scale,as it’s affected by multiple factors,which limits the investigation of 2.8 ka BP event and its dynamical mechanisms.Here,we present an annually-resolution stalagmite（HS4）δ¹⁸O record from 3.11 ka BP to 2.44 ka BP based on HS4 stalagmite with clear chronological layers from Heshang Cave,Qingjiang,Hubei.Combining with seven additional stalagmiteδ¹⁸O records in East China monsoon region with a resolution of less than 10 years,we extracted an ASM record(δ¹⁸O_syn-7)within the 3.11～2.44 ka BP period following a data synthesis method of principal component analysis（PCA）.The timing,characteristics,the driving mechanisms as well as the impact on the development of human society of the 2.8 ka BP event were explored to some extent.The main conclusions are as follows:（1）During the 3.11～2.44 ka BP period,theδ¹⁸O record shows a gradually increasing trend with some significant interdecadal-centennial scale variations.Theδ¹³C record remains a relatively constant change but has several obvious positive bias phases internally.Theδ¹³C andδ¹⁸O records have a strong correlation with correlation coefficient of 0.51（n=669,p<0.01）,indicating they might have some same driving factor.Wavelet analysis and Redfit spectrum analysis showed～130,～120,60～80,5～32,and 2～9-year cycles of them,indicating that theδ¹³C andδ¹⁸O records might be related to solar activity,AMO,PDO,ENSO,etc.（2）Together with HS4 and another 6 stalagmiteδ¹⁸O records in the East China,a highly-resolution ASM record(δ¹⁸O_syn-7)was extracted during the period 3.11～2.44 ka BP based on principal component analysis.δ¹⁸O_syn-7shows a gradual weakening trend of the ASM during the period 3.11～2.44 ka BP,accompanied by 5 weak monsoon and 5strong monsoon periods with an interval of about 120 years,indicating that the ASM has experienced a phase weakening and strengthening.（3）Theδ¹⁸O_syn-7 record reveals that the 2.8 ka BP event began and ended at 2.96±0.3 ka BP and 2.65±0.3 ka BP the whole event lasting about 310 years,with a"W"pattern of three peaks and two valleys.The whole event consists of two asymmetric weak monsoon phases,corresponding to 2.96±0.3～2.80±0.3 ka BP and 2.80±0.3～2.65±0.3 ka BP,respectively.There was a phase of extreme monsoon weakening within each of the two weak monsoon periods,with the time of 2.84±0.3～2.81±0.3 ka BP and 2.72±0.3～2.68±0.3 ka BP,and the duration of 23 and 45 years,respectively,the center of the two phases are 2.82±0.3 ka BP and 2.70±0.3 ka BP,respectively.（4）Comparing theδ¹⁸O_syn-7record with the total solar radiation（TSI）and Greenland temperature records（NGRIP and GISP2）reveals that the monsoon weakening during the2.8 ka BP event is driven by North Atlantic cooling instead of solar activity,but the solar activity strengthened the monsoon anomaly weakening within the second phase of the 2.8ka BP event,suggesting that both internal variability and external forcing may have affected the 2.8 ka BP event.The weakening of AMOC is the most likely internal driver of the 2.8 ka BP event,which is closely related to the reduction of North Atlantic Deep Water（NADW）and the Holocene Atlantic cooling event.Reconstructed benthic carbon isotopes indicate a significant decrease in NADW during 2.8 ka BP,suggesting a weakening of AMOC.The weakening of AMOC around 2.7 ka BP is thought to be associated with weak subpolar gyre（SPG）.Combining theδ¹⁸O_syn-7record with ancient documents,it is suggested that the development of agricultural economy,population,dynastic changes and cultural changes in the late Western Zhou period are closely related to the climate change during the 2.8 ka BP event.