The Abrupt Climate Change Record Of8.2Ka BP Event From East Asian Monsoon Area With Its Characteristics Analysis

The global climate is getting warmer with temperature increasing and ice melting, which would increase the possibility and the intensity of the extreme weather and would lead to huge negative impact on the nature ecosystem and the society. The human being’s attribution to the greenhouse effect has been proved, which worries the mankind more. Although the mechanism of the greenhouse has been explored, the link between the global warming and the abrupt climate change is still not well-understood, which is very important to predicate the future abrupt climate changes. As the8.2ka BP cold event is the most prominent abrupt cooling anomaly during the Holocene, occupying when the climate got warming gradually after the Last Glacial which is similar to today’s, it could provide important evidence for future abrupt climate predication under similar climate backgrounds and is helpful for estimating its impact on human being. Therefore, compared with other abrupt climate changes, there is a great realistic significance to research the8.2ka BP event. In addition, as the abrupt amplitude and transmission rate of the8.2ka BP event is much clearer than other abrupt climate changes, the deep research on its characteristics, such as abrupt amplitude, transmission rate, timing, duration, from different locations could not only provide indispensable direct evidences, but also establish important foundation for its mechanism research. According to the previous δ18O analysis with a resolution of-16a of stalagmite HS-4, collected from Heshang Cave, Qingjiang valley, the middle reaches of the Yangtze River, more clearer single of δ18O anomaly during the8.2ka BP period was caught by higher resolution (-6a) sampling and the analysis section of HS-4covering the8.2ka BP event record was confirmed (235.7～231.7cm). The chronology of the section from depth237.2-228.4cm of stalagmite HS-4was established by the combination of layer counting and U-Th technique with dense sampling carried on MC-ICPMS produced by Nu instrument at Earth Science Department of Oxford University.Then focusing on the section from depth235.7～231.7cm of HS-4, annual growth rate and seasonal grey scale were obtained by using the softwares of Photoshop and Image J. Seasonal δ18O and δ18C samples were obtained by micro-milling at a pace of～40μm on a MicroMill device produced by American New Wave Research and a Thermo Delta V advantage isotope ratio mass spectrometer fitted with a Kiel carbonate device IV at the department of earth sciences of Oxford University. Mg/Ca plotting with a pace of10μm was achieved by a JEOL JXA8800R Electron Microprobe at Material Science Department of Oxford University. In-situ Mg/Ca, Sr/Ca, Ba/Ca analysis with a internal of4μm was conducted on Agilent7500ICP-MS fitted with GeoLasPro UV optical systems produced by Coherent.Based on the seasonal multi-proxies analysis of stalagmite HS-4on the8.2ka BP event, combined with their interpretations, the seasonal and annual variations of the8.2ka BP event in East Asia area have been analyzed in detail. And the high similarity between the δ18O records from stalagmite HS-4and from Greenland ice cores during8.2ka BP period provides a new evidence for the mechanism exploration. (1) Reliable8.2ka BP event records in East Asian monsoon area ware achieved by seasonal multi-proxies. During8.2ka BP period, HS-4δ18O、δ13C values increased by2.5‰and3.5‰respectively. The annual growth rate and grey scale vaules decreased greatly, especially for the growth rate, as the lowest value during8.2ka BP period decreased by9times compared with the peak value before the event. The Mg/Ca, Sr/Ca and Ba/Ca vaules increased as well, and the most anomaly is from Mg/Ca with a average increase by40%. All the proxies indicate that the8.2ka BP event is recorded clearly by stalagmite HS-4.(2) The climate variation in East Asian monsoon area at yearly and seasonal scale during8.2ka BP period was achieved by multi-proxies instead of a single proxy. During the8.2ka BP period, the interpretation from all the proxies match each well with the increase of δ18O indicating the weakening of the East Asian summer monsoon, the increase of Mg/Ca, Sr/Ca and Ba/Ca vaules demonstrating the decrease of the rainfall around the middle reaches of Yangtze River and the strengthening of the wind and dust transferring; the decrease of the growth rate and grey scale vaules with increase of the δ13C vaule suggesting the decrease of the temperature along the middle reaches of Yangtze River. Therefore, all the proxies interpreted that the climate during8.2ka BP period became cold and dry with two extreme conditions lasting for14±2a and the seasonality became weaker compared to the period out of the event.(3) The precise abrupt behavior of the8.2ka BP event in East Asian monsoon area was achieved. Combine all the multi-proxies’ analysis of HS-4, the8.2ka BP event’s abrupt model could be obtained. The whole and the central event duration of the event are152±2a and70±2a respectively with a cold and dry condition accompanied with weak EASM. It takes47±1a and35±3a for entering into and stepping out of the central event respectively. The abrupt change happened at the beginning of the whole event is shorter than it happened at the end of the whole event by1/2times and the shift rate at the beginning of the whole event is higher than it at the ending of the whole event by2times, which indicate that the shift to cold and dry conditions is more striking. Both the abrupt changes at the beginning and at the end of the central event occupied in9±5a with an relative abrupt shift rate of5%/a, that means the abrupt changes for the central event is a bit longer than it for the whole event. A double-peak/plunge structure could be observed during the central8.2ka BP event at seasonal, yearly and decade scales clearly.(4) The synchronous high similarity between the δ18O records from the HS-4in East Asian monsoon area and those from Greenland ice cores provide a new evidence for the interpretation of the8.2ka BP event mechanism. Within dating errors, the δ18O records from stalagmite HS-4, an indicator of the EASM intensity and the δ18O records from Greenland ice cores, an indicator of the temperature, are comparable with each δ18O peak corresponding with another δ18O valley, which suggests that the climate in EAM area could respond to the climate change around North Atlantic area quickly. The highly similarity between the two proxies from the East Asian area and from the North Atlantic area indicate that abrupt anomaly happened in EAM areas was mainly caused by inter-annual scale climate changes. Therefore, the climate teleconnection between the two locations is influenced by the atmospheric circulation, instead of the ocean circulation.This research demonstrates the8.2ka BP event is clearly recorded by multi-proxies of HS-4, a well-dated stalagmite. The climate conditions during8.2ka BP interpreted from all those multi-proxies of HS-4match each well and all of them suggest the climate turning cold and dry during the event. The multi-proxies analysis not only just by one indicator, give a much stronger evidence than other8.2ka BP event records. Furthermore, the U-Th isotope absolute dating combined with relative layer counting results with multi-proxies analysis on seasonal and annual resolution, make it possible to obtain the event’s yearly and seasonal abrupt change behaviours. The methods based on seasonal multi-proxies with a reliable chronology sequence provide a solid evidence for the EAM area responding to the8.2ka BP event and a critical record for deep understanding the mechanism of the8.2ka BP event.