泰国南部石笋记录的晚全新世早期水文气候变化研究

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 the δ¹⁸O and δ¹³C 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. The δ¹⁸O and δ¹³C 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 precipitation δ¹⁸O in this area is mainly influenced by the local convective activity and rainfall amount. Therefore, we suggest that the stalagmite δ¹⁸O can be used to reflect the rainfall amount in this area. The synchronous variation of δ¹⁸O and δ¹³C also suggests that both the δ¹⁸O and δ¹³C can be used to reflect the hydroclimate change in Southern Thailand. The relationship between precipitation and stalagmite δ¹⁸O and δ¹³C is shown as follows,heavy precipitation is correlated with lower δ¹⁸O in precipitation/stalagmite, and lower soil CO₂ δ¹³C, and no prior calcite deposition (PCP) occurs in the aquifer, and thus leading to higher stalagmite δ¹³C, and vice versa for low precipitation. The decreasing trend of δ¹⁸O and δ¹³C 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.