WANG Chengtao¹, CHEN Min^1,3, QI Hongshuai^1,2,3, WICHIEN Intasen⁴, APICHAI Kanchanapant⁴,ZHANG Aimei¹,FNAG Qi¹

( 1.Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, China;
2.Fujian Key Laboratory of Marine Ecological Protection and Restoration, Xiamen 361005, China;
3. The Beibu Gulf Coastal Wetland Ecosystem Field Scientific Observation and Research Station of the Ministry of Natural Resources, Beihai 536015, China;
4.Department of MineralResources, Ministry of Natural Resourses and environment, RatchathewiBangkok 10400, Thailand)

Abstract: AMS14C dating and grain-size analyses were performed on the sediments of three core samples (WLE08, WLE10 and WLE12) in the Welu estuary of Chanthaburi coast, Gulf of Thailand. The results of grain-size analysis show that the lithology of the three core samples in the study area is clay silt. All the core samples contain shells and biological debris. The sorting is poor. Skewness is generally positive. The sediments were formed since the mid-late Holocene. The sensitive grain sizes of sediment have been extracted through the grade-size standard deviation method. Each core sample obtained two sensitive particle sizes, SA1 and SA2, SB1 and SB2, and SC1 and SC2, respectively. Then SA2/SA1, SB2/SB1 and SC2/SC1 were obtained as a proxy of relative intensity change of Asian summer monsoon since the mid-late Holocene. Combined with the dating record, the result shows that the Asian summer monsoon in the study area has experienced a strong-weakening-enhancing-weakening-stable-fluctuating process since the mid-late Holocene. During 8.5-6.5 cal. ka BP, SC2/SC1 value is higher. It means that this was a period of strong Asian summer monsoon. Low and stable SB2/SB1 value of 6.5-3 cal. ka BP shows that Asian summer monsoon belongs to a weaker stable period. The increase in SA2 / SA1 value since 1.5 cal. ka BP indicates that the Asian summer monsoon has strengthened. During the 8.2 cal. ka BP period, the sensitive SC2 / SC1 value decreased sharply, indicating that there was a large weakening period of the Asian summer monsoon. This is consistent with the famous “8.2 cal. ka BP global climate abrupt event”. This is similar to the South Asian summer monsoon recorded by the stalagmite δ18O of Oman, and also has many responses with the East Asian summer monsoon recorded by the stalagmite δ18O of Dongge Cave, reflecting the regional and even global connection of climate change.

Key word: Asian summer monsoon; sensitive grain-size; grade-size standard deviation method; mid-late Holocene; Gulf of Thailand

EARTH AND ENVIRONMENT Vol.49, No.5, Tot No.343, 2021, Page 480-491