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The expression of the abrupt cooling of the 8.2 ka event in the tropics is poorly understood. In this study, we applied lithostratigraphy, loss on ignition, grain size and pollen analyses to sedimentary sequences from Thale Noi, which is located in the west coast of the Gulf of Thailand, in order to reconstruct the regional environmental history during the 8.2 ka event. Our results, based on sediment. This characteristics and pollen records, indicated that the study area was likely to have flooded prior to c. 8300 cal. a BP. The fall in sea-level coincided with a high precipitation, inferred from grain size, between c. 8250 and 8100 cal a BP. The increased precipitation can be attributed to the strengthening of the northeast (winter) monsoonal rains in the western Gulf of Thailand. The timing of the sea-level fall and strengthen stronger winter monsoon is consistent with the abrupt decrease in the temperature records emanating from the northern hemisphere. Subsequently, we observed a sea-level rise from c. 8100 to 7950 cal a BP, which potentially continued till c. 7650 cal. a BP. The possibility of coastal erosion was detected by a hiatus in the sedimentary sequences dated between c. 7950 and 7650 cal a BP. Then, a standstill of the sea-level occurred from c. 7650 to 7100 cal a BP. This transgression is suggested to have caused coastal erosion that resulted in a hiatus in the sedimentation process between c. 7950 and 7650 cal a BP. Correspondingly, fluctuations in the monsoon strength resulted in an overall decreased precipitation from c. 8100 to 7450 cal a BP, although punctuated wet intervals were observed at c. 7950 and 7575 cal a BP.

Fifteen native luminescent bacteria were isolated from the Gulf of Thailand, and their sensitivity for the detection of toxicity of crude oil and its aromatic components was investigated. Of these isolates, Vibrio campbellii strain FS5 was one of the two most highly inhibited bacteria at all crude oil concentrations. This bacterium showed a decrease in luminescence intensity of between 10.7 and 80.2% after a 15-min exposure to 0.0001–10 mg/L of crude oil. The degree of bioluminescence inhibition increased with increasing concentrations of crude oil. The presence of crude oil at all concentrations had negative effects on the log bioluminescence per log number of viable cells after 15- to 105-min exposure. About 10 to 100 times, lower half maximal effective concentration (EC50) values were observed for polycyclic aromatic hydrocarbons (PAHs) than those for benzene, toluene, ethylbenzene, and xylene (BTEX). In the presence of each individual BTEX and PAH, the bioluminescence inhibition increased with increasing exposure time (1–32 h). This indigenous bacterium can be used as a simple and general indicator of oil contamination and its impact on coastal waters as well as for assessing potential toxicity during oil bioremediation.

To improve knowledge of the relationships of human activities with microplastic pollution in the urban estuary in Phuket province, which has a densely populated city on the western coast of Thailand, a total of 463 plastic-like items from 24 sediment samples in the dry and the rainy seasons were identified by micro-Fourier transform infrared spectroscopy. The microplastic abundance ranged in 300–900 and 33–400 items/kg dry weight in the dry and the rainy seasons, respectively, indicating that the estuary is moderately contaminated with microplastics. The most abundant polymer types were rayon and polyester with colored fibers, suggesting that the microplastics deposited in this area originate mainly from washing effluents. Additionally, our findings show that the microplastic distribution is significantly governed by hydrodynamic energy in the estuary. This provides basic information for a better understanding of the fate of microplastics within estuary, and for management actions to address microplastics in urban estuary.

Summary: Acid volatile sulphide (AVS), one of the most reactive phases in sediments, is a crucial link in explaining a dynamic biogeochemical cycle in a marine ecosystem. Research gaps exist in describing the spatial variation of AVS and interconnections with sediment covariates in the Eastern Upper Gulf of Thailand. Measurements of AVS and auxiliary parameters followed the standard protocol. A comparison of ordinary kriging (OK), cokriging (CK), and regression kriging (RK) performance was evaluated based on the mean absolute error (MAE) and root mean square error (RMSE). The concentrations of AVS ranged from 0.003 to 0.349 mg g−1 sediment dry weight. Most parameters contained short range spatial dependency except for oxidation–reduction potential (ORP) and pH. The AVS tended to be both linearly and non-linearly related to ORP and readily oxidisable organic matter (ROM). The RK model, using inputs from the tree-based model, was the most robust of the three kriging methods. It is suggested that nonlinear interactions should be taken into account when predicting AVS concentration, and it is expected that this will further increase the model accuracy. This study helps establish a platform for ecological health and sediment quality guidelines. Keywords: Spatial estimation, Acid volatile sulphide, Sediment, Geostatistical analysis, Gulf of Thailand

The n-alkanes content in the sedimentary sequence from Thale Noi, on the west coast of the Gulf of Thailand, was analyzed and used to reconstruct the past sea-level fluctuations. The results were further compared with those based on sediment characteristics, loss on ignition, grain size distribution, and pollen analysis from the same core to investigate the efficiency of n-alkanes as a paleoenvironmental proxy. The analysis of n-alkanes indicated the likely periods of regression and transgression from c. 8300–7950 cal. a BP and from c 7950–7665 cal. a BP that agrees well with the reconstructed sea-level changes from the sediment characteristics and pollen records. The steady abundance of mangrove pollen taxa in the sediment core points to a standstill in the sea-level from 7650–6870 cal. a BP. However, the results from the n-alkanes analysis further suggested a sea-level fall at c. 7650–7610 cal. a BP and at c. 7400–6870 cal. a BP, and a sea-level rise at c. 7610–7400 cal. a BP. Furthermore, these sea-level changes were intervened by short intervals of regression centered at c.7590 and 7470 cal. a BP.

After global phasing out of leaded gasoline, anthropogenic sources of atmospheric lead (Pb) are dominated by coal combustion emissions, high-temperature metallurgical processes, and vehicle exhausts. Thailand was one of the first countries in Southeast Asia to completely phase out leaded gasoline by the year 1994. This study investigates the sources of atmospheric Pb in Thai aerosols, a quarter-century after phasing out of leaded gasoline using a multiproxy approach, Pb isotopes in conjunction with trace metal composition of PM2.5. Aerosol samples were collected for 1 year from January 2018 to April 2019 from Bangkok and Chonburi to understand the influence of seasonal variation on aerosol chemistry. Bangkok is the only megacity in Thailand and notorious for traffic congestion and Chonburi is a coastal town, 80 km southeast of Bangkok where the inter monsoon winds make landfall. Aerosol Pb concentrations significantly decreased in 25 years from 74 ± 5 ng/m3 in 1994–1995 to 14 ± 13 ng/m3 measured in Bangkok during Northeast (NE) monsoon winds. Concentrations of all the metals are higher in Chonburi compared to Bangkok in the NE monsoon wind, owing to the addition of local emissions from the megacity over the long-range transported metals before the winds reach the coastal town. The Pb isotope ratios have a wide range (206Pb/207Pb range 1.1343–1.1685 and 208Pb/207Pb range 2.4138–2.4450) but the average ratios are not significantly different in Bangkok and Chonburi for all three seasons. However, present day 206Pb/207Pb ratios are more radiogenic than those measured 25 years ago in Bangkok. The Pb isotopes from both locations in all seasons have considerable overlap with unleaded gasoline and diesel used in Southeast Asia indicating automobile exhaust as an important source. Additionally, the NE monsoon winds are influenced by mixing of crustal dust (Al/Pb ratios correlates well with 206Pb/207Pb, r2 = 0.63) with coal combustion emission (strong inter-correlation of Pb, Zn, As and Cd, r2 from 0.72 to 0.94) from China and Vietnam. Pb isotopes of the Southwest (SW) monsoon winds are also influenced by crustal dust and falls on the mixing line between coal combustion and ore processing emission from India in a Pb triple isotope plot. Chemical composition of aerosols in the inter-monsoon winds are similar to SW monsoon winds. After a quarter-century of leaded gasoline banning in Thailand, the atmospheric Pb sources have changed from less radiogenic leaded gasoline exhaust and Chinese ore processing to more radiogenic unleaded gasoline and diesel exhaust and coal combustion.

The high precipitation around Southeast Asia results in abundant freshwater outflow, which transports terrestrial dissolved and particulate material to the world's largest marginal sea, namely, the South China Sea (SCS). To estimate the riverine carbon flux to the SCS, carbonate data from 42 rivers were collected. These results, combined with literature data for 13 rivers, indicate that the concentrations of dissolved inorganic carbon (DIC) are positively correlated with the cation exchange capacity and the bulk density, except for the Low Discharge category. The highest DIC concentration and flux are in the Low and Medium Discharge categories. Negative correlations exist between dissolved organic carbon (DOC) concentrations and the base saturation in the Low and High Discharge categories and between DOC and bulk density in the Low and Medium Discharge categories. However, the correlation between the DOC concentration and the organic carbon content in the soil is only significant in the Medium Discharge category. Once the negative exponential relationships between the particulate inorganic carbon (PIC) or the particulate organic carbon (POC) and the total suspended matter are determined, these can be used to estimate the PIC and POC carbon fluxes. Of the annual riverine carbon flux, 83.0 ± 8.1 Tg C, 40.14 ± 6.11 Tg is DIC, 25.03 ± 4.54 Tg is DOC, 1.83 ± 0.35 Tg is PIC, and 16.03 ± 2.87 Tg is POC. The total discharge amounts to 6.2–10.3% of the global riverine discharge for a sea covering only 1% of the world's ocean surface area and for watersheds covering only 2.2% of the global land mass.

The South China Sea offers an excellent case for studying source-to-sink transport processes of fluvial sediments among the global marginal seas. This study synthesizes existing clay mineralogical and geochemical data from similar to 1500 samples from the seafloor and surrounding rivers, deepwater mooring observation results, and high resolution glacial-cyclic clay mineralogy records from six high-quality sediment cores. Source-to-sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales: modern process and Late Quaternary glacial cycles. The results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the South China Sea, e.g., dominant illite and chlorite in Taiwan, overwhelming majority of smectite in Luzon, dominant kaolinite in South China, and similar amounts of the four clay mineral species in Red and Mekong river systems. The formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the East Asian monsoon climate with warm temperature and high precipitation, and subordinately by tectonic activity and specific lithological character. The basin-wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects. The differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the South China Sea. Through combining clay mineralogical distributions on the seafloor with observed oceanic current systems, the modern transport pathways can be well established, e.g., in the northern South China Sea, illite and chlorite from Taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies. High-resolution sediment dynamic analysis for the Late Quaternary reveals different sediment transport patterns in the northern, western, and southern South China Sea when the land-sea configuration dramatically changed during glacial conditions. In the north, the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport. In the west, the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds. In the south, the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods. The transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern South China Sea. The relative contributions from three provenances (South China, Luzon, and Taiwan) highlight their distinct variability over the last 28 ka, implying that the southward shift of the inter-tropical convergence zone (ITCZ) at 16 Ka BP caused an increased sediment contribution from South China, the intensified influence of the Kuroshio Current intrusion controls more efficient westward transport of Luzon sediments, and the stronger deepwater current transports Taiwan-sourced sediments further westward from the last glaciation to the Holocene. (C) 2015 Elsevier B.V. All rights reserved.