Stalagmite Carbon And Oxygen Isotopic Records Over The Past 130 Ka,from Zhenzhu Cave And Bailong Cave

Knowledge of past climate change covering different timescales provides a precious foundation for understanding past and modeling future states of regional and global climate.Over the past several decades,scientists have obtained a lot of valuable paleoclimatic information from various geological archives,such as ice cores,marine sediments,loess-paleosoil sections,lacustrine sediments,tree rings,and so on.During the past two decades,depended on the development of high precision U/Th dating technique,taking the advantages of multi-proxy,high resolution,continuous or semi-continuous deposition,and wide distribution,stalagmites have been increasingly adopted to reconstruct paleoclimatic change history.However,the indicative significance of oxygen isotopes of stalagmite(?¹⁸O_s)in the monsoonal regions of China remains controversial,and there are few long-term records of stalamite that covering glacial/interglacial have been published in monsoonal North China.Here,two stalagmites,namely ZZ1 and BY39(collected from Zhenzhu Cave in Pingshan County,Hebei Province,and Bailong Cave in Kang County,Gansu Province,respectively),have been studied.Based on precisely ²³⁰Th age data(50 and 27 dates respectively),the high resolution stalagmite carbon isotoic(?¹³C_s)and?¹⁸O_s records(2490 and 593 data pairs respectively)covering the past 130 ka have been obtained.For the interpretation of the obtained stalatmite?¹³C_s and?¹⁸O_s records,modern observations covering a period of two years(April 2012 to April 2014)have been conducted in and out Zhenzhu Cave.Finally,the following conclusion recmarks have been summarized,based on the above-mentioned works.(1)The monitoring results from the outside of Zhenzhu Cave demonstrated that,on monthly and inter-annual timescales,the correlations between oxygen and hydrogen isotopes of local precipitation(?¹⁸O_p and?D_p)and corresponding temperature and precipitation amount were not significant,impling both the“temperature effect”and“precipitation amount effect”were not applicable for the interepretation of local?¹⁸O_p and?D_p variations.(2)The monitoring results from Zhenzhu Cave demonstrated that there were no apparently seasonal variations in the oxygen and hydrogen isotopes of drip water(?¹⁸O_d and?D_d)in Zhenzhu Cave.Over the period of two years,the?¹⁸O_d and?D_d data in Zhenzhu Cave were nearly constant,with the absolute values close the average values of local?¹⁸O_p and?D_p data in the outside of Zhenzhu Cave,impling that after a relatively long-term(for example,severarl years)“buffering or smoothing effect”,the local?¹⁸O_p and?D_p signals could be inherited by?¹⁸O_d and?D_d data.(3)Both the temperature and relative humidity in Zhenzhu Cave were nearly constant over the observational period of two years.However,the corresponding concentration of CO₂ in Zhenzhu Cave showed apparently and significantly seasonal variations,demonstrating the CO₂concentration in Zhenzhu Cave responsed sensitively to the outside envrironmental factors,especially the variations of precipitation amount.(4)The?¹³C_s and?¹⁸O_s records from BY39 that covering 27-58 ka BP showed roughly in phase variations with obvious differences in change amplitude.Both the BY39?¹³C_s and?¹⁸O_sdata record 3 Heinrich events(H3,H4 and H5)and 14 DO events.The spatial?¹⁸O_s records comparison between BY39 and other stalagmites from monsoonal China demonstrated similar change patterns with different absolute values and change magnitudes.Specially,in East-West direction(?33°N),the BY39?¹⁸O_s values were more depleted with relatively larger change magnitude,implying the relatively longer transportation path of vapor for Bailong Cave which is located in more inland site.In North-South direction,?¹⁸O_s records from Southwest China are more depleted than those from East Asian monsoonal regions,which is quite possibly resulted from the different major vapor sources for Indian summer monsoonal region and East Asian summer monsoonal region.On the other hand,the BY39?¹³C_s record is consistent with other records from the Chinese Loess Plateau,such as organic carbon isotopic composition,grain size,magnetic susceptibility,which have been widely adopted as indicators of the East Asian summer monsoon intensity or precipitation amount.(5)Similary,the ZZ1?¹³C_s and?¹⁸O_s records showed roughly in phase variations,both characterized by glacial/interglacial variations and abrupt millennial-timescale climate events.The ZZ1?¹⁸O_s record is consistent with the other stalagmite?¹⁸O_s records from monsoonal South China.However,the ZZ1?¹³C_s is more consistent with other records from the Chinese Loess Plateau,such as records of inorganic carbon isotopic compositions,grain sizes,magnetic susceptibilities,which have been widely adopted as indicators of the East Asian summer monsoon intensity or precipitation amount.(6)Generally,over the past 130 ka,the stalagmite?¹³C_s and?¹⁸O_s records from Zhenzhu Cave and Bailong Cave showed roughly in phase variations,but obvious change trends and magnitudes,especially for the records of MIS3 from Zhenzhu Cave.As above-mentioned,due to the stalagmite?¹³C_s records from both Zhenzhu Cave and Bailong Cave are more reliable indicator of East Asian summer monsoon intensity or monsoonal precipitation amount,we therefore concluded that stalagmite?¹⁸O_s records from these two caves cannot be adopted as direct indicator of East Asiam summer monsoon intensity;the in phase variations of stalagmite?¹³C_s and?¹⁸O_s records from these two caves with different change trends and magnititudes revealed the different responses of precipitation amount and precipitation?¹⁸O_p to the adjustments of general atmospheric circulaitons in monsoonal China.