| The Longfugong Cave (31°38’N, 110°46’E,564m above sea level) is located at Mt.Shennongjia, Hubei Province, China. The mean annual precipitation at the study site is between 1500mm and 2000mm, most of which falls in May to October. The relative humidity and temperature in the cave is about 100% and 180C, respectively.We present a stalagmite (No.LFG10) collected from this cave, which is 2040-mm-long, with a diameter of 150 to 200 mm. And few lithological changes can be observed from the sample along the growth axis. Age model for the stable isotopic sequences (δ18O and δ13C) is established by linear interpolation between individual dated points. Fourteen 230Th dates with typical analytical errors (2σ) ranging from 3 to 72 yrs, were conducted at the Isotope Laboratory of Institute of Global Environmental Change, Xi’an Jiaotong University, China. The dating results are in stratigraphic order. A total of 1410 stable isotope samples drilled along the growth axis with 0.5-mm-diameter carbide dental drill. The analysis was performed by Finnigan-MAT 253 mass spectrometer fitted with a Kiel Carbonate Device, at the Isotope Laboratory of College of Geography Science, Nanjing Normal University. It provides a detailed record of East Asian monsoon variability between 6.60 ka B.P. and 0.45 ka B.P. When compared with high-resolution and precisely-dated stalagmite DA, DAS, HS4 and SB43 generated from East Asian monsoon site, the general well correlation of the record with other cave records in the contemporary growth period, suggesting that variations of the stalagmite LFG10 δ18O is reliable and can reflect broadly regional changes in variations of East Asian monsoon circulation.Although the record generally follows boreal summer insolation, it is punctuated by several weak monsoon events (5.3 ka BP,4.2 ka BP,2.8 ka BP,1.9 ka BP,0.9 ka BP and LIA). These century-scale abrupt monsoon events probably relate to distinct decrease in the solar activities, and link to the North Atlantic Ocean ice rafting events. In addition, their inner characteristics were agreed, especially our high resolution δ18O record (YX57) reveals a striking weak summer monsoon event (2.8 ka event) with δ18O progressively increase at onset and sudden decrease at the end. It displays a double peak and three plunging structure, similar to that of 8.2 ka event recorded by stalagmite H4 from Heshang Cave in Hubei. The internal structure of the 4.2 ka event recorded from stalagmite LFG10 is as well similar to that of DA. Interestingly, the period of ancient culture and civilization decline in our country well accords with the weak monsoon event, indicating climate change may play an important role in the development of human society.Speleothem δ13C used in paleoclimate reconstruction is limited by multiple controls associated with site-specific processes and lack of reproducibility between regional records. The δ13C fluctuations in our record remains stable, around its mean value, and is characterized by persistence of centennial cycles like other two stalagmite δ13C record (DA and DAS); while precessional-to millennial-scale changes are observable in the δ18O signal. Despite of the disagreement in low-frequency pattern, but when detrended, centennial-scale variations of the residual δ18O (δ18Od) signals are well mirrored in the δ13Cd. This tight linkage indicates that centennial-scale changes of both isotopic signals may have the same forcing mechanism, and/or the climatic and environmental changes influence them in the same direction. Moreover, the magnitude of δ13C changes is about twice as large as that of the δ18O signal. These observations suggest that the soil humidity balance associated with regional hydrological cycles is important to control centennial 813C changes, and the ecosystem may amplify the climate signal. Thus, it is likely that the speleothem δ3C can be a reliable indicator to track centennial-scale climate changes through the climatically-controlled soil processes.Additional 2370 isotopic data covering 980 AD-1504 AD, were collected by striking off on the polished surface of LFG10 between 0-225mm, with a resolution approaching seasons. It shows that the Medieval Period is warm and arid. The signal of isotope rapidly increases, implying the beginning of the LIA. This high resolution record depicts twelve extreme arid events and thirteen extreme wet events which last for one to four years. The amplitudes, on thirty years climate average state, reach 0.5‰～0.7%o when these events occur, which are a half to two thirds that of the 4.2 ka event. The δ18O oscillations on decadal to interannual time scale are positive correlation to PDO and inverse correlation to ENSO variance. They all have 60-70 years cycle in common, demonstrated that PDO and ENSO may affect the precipitation of monsoon region in China associated with the Asian summer monsoon intensity. |
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