Sediment Provenance Evolution In The Gulf Of Thailand Since The Last Deglacial

There is the largest and most complicated“source-to-sink”system on the Asian continent,where a huge amount of terrestrial sediment from the continent is transported to the marginal seas,and then the Pacific and Indian Oceans.Various sedimentary settings forms in the continental margins,which record abundant information of global change.To reconstruct the paleoevnironment evolution in this area is the hotspot and frontier in the global change research.Sr and Nd isotopes were determined in 42 surface samples collected from Gulf of Thailand and adjacent South China Sea.Major and trace elements,Sr and Nd isotopes were measured in a 381-centimeter-long gravity core T93?9.9261°N,100.6442°E?from the western Gulf of Thailand.T93 core was collected by SEAFDEC in May 2012,with a sampling water depth of 59 m.Based on the data of surface and core samples,the modern and historical sediment source of the study area were discussed respectively.For the surface samples.The average value of ⁸⁷Sr/⁸⁶Sr in the surface sediments of the Gulf of Thailand is 0.7241,and average value of ¹⁴³Nd/¹⁴⁴Nd is 0.5121.In the study area,the lowest value of ⁸⁷Sr/⁸⁶Sr is 0.7109 which appears at the mouth area of Kelantan River;the highest ⁸⁷Sr/⁸⁶Sr value is 0.7467,which appears at the mouth area of Mae Klong River.The distribution characteristics of¹⁴³Nd/¹⁴⁴Nd is opposite,lower value of0.5119 appears at the Mae Klong River mouth area,while the highest value is 0.5122,which appears on the west sea area of Ca Mau.According to Sr and Nd isotopic characteristics,the study area can be divided into three main sedimentary areas:?1?The western sedimentary area,including the Upper Gulf of Thailand and western Gulf of Thailand.The sediment has higher value of Sr isotopes?0.7279?and lower Nd isotopes value?0.51206?.Sediments here mainly come from the Mae Klong River.?2?The southwestern sedimentary area,including coastal area from Songka to the rivermouth area of the Kelantan River.The sediment has lower Sr isotopes value?0.7162?and higher Nd isotopes value?0.51211?.Sediments here are mainly from the Kelantan River.?3?The eastern sedimentary area includes sea area located west of Ca Mau.The sediment has high Sr isotopes value?0.7231?and high Nd isotopes value?0.51212?.Sediments here are local materials,mainly from the Mekong River.As for the gravity core.A total of 38 samples were collected at intervals of 10 cm,and the bottom age is about 14640 cal.a B.P.,so the sampling interval is about 375years.The evolution of sediments provenance and sedimentary environment since the Last Deglacial is discussed on the basis of AMS¹⁴C dating,grain-size,color reflectance and element content data.The results show that the process of sedimentation can be divided into four stages:?1?From 14640 cal.a B.P.to 11700 cal.a B.P.,the sea level is basically low and the sediments composed of clayey silt is deposited in a terrestrial environment.The content of alkaline elements such as Na,K,Ca and Mg is extremely low,while the content of Fe,Ti and Ba is high,showing a strong chemical weathering intensity.The average?Eu is 0.60 and?La/Yb?_PAAS is 0.92,respectively,indicating that the sediments are mainly from the Malay Peninsula and partly the Indo-China Peninsula and the South China Sea.?2?From 11700 cal.a B.P.to 8000 cal.a B.P.,the sea level rises rapidly and the central Gulf of Thailand is in a coastal environment.The content of silt and active elements show a slight increase.By contrast,the content of Fe,Ti,and Ba decrease.The average?Eu is 0.58 and?La/Yb?_PAAS is 0.99,indicating the sediments are mainly from the Malay Peninsula.?3?From 8000 cal.a B.P.to 4000 cal.a B.P.,the sea level reaches the maximum.The central Gulf of Thailand become a shelf environment and the sediments consist of silt and clayey silt.The content of Na increases by 50%while the other elements change little,and the chemical weathering intensity becomes weak.The average?Eu and?La/Yb?_PAAS value is 0.61 and 1.07respectively,as a result the sediments mainly derive from the Indo-China Peninsula.?4?The last 4000 years,the elemental and grain-size composition of the sediments remain steady as the sea level slightly decreases.The average?Eu is 0.62 and?La/Yb?_PAAS is0.98,hence the sediments are mainly influenced by material from the Malay Peninsula,and partly affected by sediments from the Indo-China Peninsula.