泰国南部石笋记录的晚全新世早期水文气候变化研究

Hydroclimate variability in early stage of late Holocene recorded by stalagmite from Southern Thailand

中晚全新世气候转型期(4.2 ka事件)气候变化对全球多地古文明产生了重要影响,但该事件是否为一次全球性的气候事件,目前仍存在一定的争议。因此,针对该事件有必要开展全球范围的研究工作。文章以泰国南部洞穴石笋为研究对象,通过年代学研究、碳氧稳定同位素测试分析重建了该区域晚全新世早期水文气候变化历史。研究结果显示,该区域水文气候响应亚洲夏季风变化,晚全新世早期夏季风降水呈现逐渐减少的趋势。两次年代际干旱事件(距今3850−3840年和距今3805−3795年)主要受到太阳活动减弱和厄尔尼诺事件的影响。总体而言,该区域水文气候变化受到热带辐合带(ITCZ)位置南北移动的控制。

The climate change during the transition of the middle-to-late Holocene(4.2 ka event)is linked to the collapse of many paleo-cultures worldwide.However,it is still controversial in the following two questions,Is the 4.2 ka event global,and what is the relationship between the climate change and its societal effect?Though it is difficult to identify the relationship,we can construct paleoclimate records from different places all over the world to identify whether the 4.2 ka event is global.In the karst survey in 2019,we found many ancient cultural remains in caves of Southern Thailand,and the time span of many remains covers the 4.2 ka event.However,there are relatively few records on the 4.2 ka event with high precision and high resolution that can reveal the climate and environment in this area.To better understand the 4.2 ka event,we choose the stalagmite record from Phet Cave in Southern Thailand to verify the event in a tropical area.The climate in Southern Thailand is dominated by the tropical monsoon system.The mean values of annual temperature and precipitation are 27.1℃and 2,390 mm,respectively.Precipitation in the rainy season from May to November accounts for 76%of the annual value.The Phet Cave(8°23′36″N,98°46′26″E,54 m a.s.l.)is a dry underground river cave developed along the local fault,and the cave environment is relatively stable.The stalagmite D008-05 is a pure aragonite stalagmite with a length of about 18 cm and a diameter of 5-6 cm.To determine the age of the stalagmite,10 powder subsamples were collected for 230Th/U dating through the Neptune MC-ICP-MS in the Isotope Laboratory of Xi'an Jiaotong University.We got the dating results with relatively high precision-high uranium and low 232Th concentration.The age model was constructed through the StalAge.We confirm that the stalagmite grew in the early stage of the Late Holocene,from 3,738 a B.P.to 3,906 a B.P.,and the mean growth rate was 0.95 mm·a^(−1).We drilled theδ18O andδ13C samples from the top of the stalagmite with an interval of 1 mm,and a total of 160 samples were collected.Then we obtained an annual resolution for the high growth rate.Theδ18O andδ13C were analyzed through a MAT 253 mass spectrometer equipped with a Kiel IV carbonate device at the Institute of Karst Geology,Chinese Academy of Geological Sciences.Previous studies suggest that the moisture source of precipitation in the rainy season of Southern Thailand is mainly from the Bay of Bengal,and the precipitationδ18O in this area is mainly influenced by the local convective activity and rainfall amount.Therefore,we suggest that the stalagmiteδ18O can be used to reflect the rainfall amount in this area.The synchronous variation ofδ18O andδ13C also suggests that both theδ18O andδ13C can be used to reflect the hydroclimate change in Southern Thailand.The relationship between precipitation and stalagmiteδ18O andδ13C is shown as follows,heavy precipitation is correlated with lowerδ18O in precipitation/stalagmite,and lower soil CO2δ13C,and no prior calcite deposition(PCP)occurs in the aquifer,and thus leading to higher stalagmiteδ13C,and vice versa for low precipitation.The decreasing trend ofδ18O andδ13C of the overall stalagmite D008-05 is following the decline of Asian summer monsoon intensity,which is dominated by the decrease of north hemispheric summer insolation in the tropical area.Two decadal-scale drought events were identified between 3,850-3,840 a B.P.and 3,805-3,795 a B.P.The spatial comparison result indicates that these two events happened in many places in the Asian monsoon region.In addition,we found the hydroclimate change in Southern Thailand is dominated by the south-north movement of the Intertropical Convergence Zone(ITCZ),and it is also influenced by the solar activities and El Niño-Southern Oscillation(ENSO)state in the interannual-to-decadal timescales.Strong solar activates will produce more summer monsoon rainfall in Southern Thailand,whereas weak solar activities will induce less summer monsoon rainfall in Southern Thailand.Different from solar activities,the relationship between the ENSO state and summer monsoon rainfall is as follows:in the El Niño state,the summer monsoon rainfall is suppressed,and in the La Niña state,the summer monsoon rainfall is enhanced.We found that the dry trend during the 4.2 ka event had a potential effect on the paleo-culture in Southeast Asia,which caused the change of subsistence pattern from hunting and gathering to crop cultivation and domesticating pigs.As the decrease of precipitation and sea level,more land was outcropped near the river,seacoast,and river delta,which provided more land for rice planting.Besides,the improvement of production tools by culture exchanges also led to life style change in Southeast Asia.