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Understanding marine sedimentary process and the land-sea interaction is vital to human production and life. Gulf of Thailand is a hotspot of such research due to the strong erosion, transportation, and deposition processes under the control of the monsoon climate in the region over the last deglaciation. Based on comprehensive analyses of lithology, grain size, and element index of core BT-7, we revealed that the sedimentary history of this region could be divided into three stages: (I) 13.5–7.5 cal ka BP, continental deposition period, when the central Gulf of Thailand was completely exposed to the surface; (II) 7.5–6.2 cal ka BP, land-sea interaction period, when the material source varied abruptly and the weathering intensity changed significantly, corresponding to frequent climate fluctuations between cold and warm, along with intermittent sea-level still stands; and (III) after 6.2 cal ka BP, period of stable marine sedimentary facies, when the seawater inundated the central Gulf of Thailand, the weathering intensity was slowly weakened with no significant change, and the climate fluctuated with a trend from warm to cold. Notably, since 3.5 cal ka BP, the weathering intensity has been rapidly weakened, entering a small ice age, and the relatively stable sea level has led to a consistent sedimentary environment in the Gulf of Thailand into modern times. Therefore, this study reveals the evolution of sedimentary environment at the interaction of the Indian Ocean and the Pacific Ocean over the last deglaciation. Based on the robust evidence, our findings could help improve the understanding of the sedimentary patterns and land-sea interactions in history.

To investigate the evolution of sediment provenances and transport processes in the central Bay of Bengal (BoB) since the Last Glacial Maximum, we present a comprehensive study on the grain size, clay minerals, rare earth elements, and accelerator mass spectrometry (AMS)14C analyses of the core BoB-24 sediments. Six AMS14C dates were obtained from planktonic foraminiferal Neogloboquedrina dutertrei (N. dutertrei) extracted from the core to reconstruct a reliable age model. The assemblage of clay minerals and the (La/Yb)UCC–δEu relationship indicated that the core contained a mixture of sediments from the Himalayas and Indian Peninsula. These results showed that the Himalayas were the main source of erosional materials during 25–6.5 ka BP, whereas those from the Indian Peninsula increased since the mid-Holocene. Between 25 ka BP and 12 ka BP, the sea level was relatively low, and a large amount of terrigenous detrital material were directly transported from the Himalayas and the Tibet Plateau to the study area by submarine canyons, forming a deposition center on the Bengal Fan. Between 12 ka BP and 6.5 ka BP, the sea level gradually rose and the Indian summer monsoon gradually strengthened, whereas the sedimentation rate and the amount of material transported from the Ganges–Brahmaputra River decreased greatly; the deposition center transferred from the Bengal Fan to the shelf of the BoB. Since 6.5 ka BP, the sea level has risen, reaching its present position. A high sea level and a strengthened East Indian Coastal Current resulted in an increase of material contribution from India, while the deposition center was on the shelf of the BoB. Overall, this study confirmed that both sea level fluctuations and variations of the Indian summer monsoon controlled the evolution of sediment provenances in the central BoB and transfer of the deposition center from the Bengal Fan to the continental shelf. This typical sedimentary pattern observed in the northeastern Indian Ocean improves our general understanding of past land–sea interactions.

Whether the Indian Summer Monsoon (ISM) Maximum started in the early Holocene or middle Holocene remains a controversial issue. Grain-size measurements were conducted on 157-cm-long core ADM-C1 from the Andaman Sea; the core spanned the last 11.2 kyr BP. Four end-members (EMs) were unmixed using lognormal parametric end-member modeling. Genetic analyses of the grain-size EMs suggested the EM1 and EM2 may correlate with suspension in the upper layers and transportation in the benthic nepheloid layer, respectively, while the EM3 and EM4 were tied with deposition affected by monsoonal currents and sedimentation under extreme events, respectively. The EM3 was dominated by ISM intensity and transported by summer monsoon currents, with increase in the proportion of EM3 reflecting increasing ISM intensity, and vice versa. Variations in EM3 were used to define three stages of ISM evolution, as follows: (1) During 11.2–9.1 kyr BP, EM3 was relatively low, indicating a weak ISM; (2) during 9.1–4.5 kyr BP, EM3 increased substantially, indicating a strengthened ISM; (3) during 4.5-0 kyr BP, EM3 decreased gradually, suggesting a gradual weakening of the ISM. The Holocene ISM Maximum started at ~9.1 kyr BP rather than in the early Holocene, which may have been partly due to the slowdown of the Atlantic meridional overturning circulation (AMOC) during the early Holocene that resulted in a decreased land-sea thermal contrast between the landmass and the Indian Ocean. An additional cause may have been the remnant Northern Hemisphere ice sheets that impeded the northward shift of the Intertropical Convergence Zone (ITCZ). Seven relatively brief decreases in the ISM intensity (events 7, 6, 5, 4, 3, 2, 1) occurred at ca. 10.3, 9.7, 7.3, 5.7, 4.2, 2.4, and 0.8 kyr BP, respectively. They corresponded, within the age uncertainties, to the increased supplies of ice-rafted detritus to North Atlantic sediments, implying that changes in the ISM intensity on millennial scale were dominated by climatic processes in northern high latitudes.

Understanding the development of shallow seas is essential, as they provide a major environmental and economic resource. An investigation of the Holocene development and the present conditions of the Andaman Sea shelf was carried out based on hydroacoustic surveys and sedimentological sampling. The results show that the relative sea level in the offshore Phang Nga province (Thailand) was at a present-day water depth of approximately 63 m at 13 cal ka BP. This agrees with the sea level development of the Sunda Shelf. Since that time, the Andaman Sea continental shelf developed as a sediment-starved environment, with less than 2 m thickness of sediment deposited during the Holocene on the crystalline basement over large areas between 60 m and 20 m water depth. Between 28 and 17 m water depth, a series of moribund asymmetrical sand ridges exist. These ridges were formed around 9.0 ka cal BP. They strike oblique to the coastline. On the seaward side of the sand ridges, small NW-SE directed submarine dunefields developed, shaped by monsoon-induced currents.

To understand the modern sedimentation and land-sea interaction processes in the Asian margin seas, we analyze rare earth element contents (REE, i.e., La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and grain sizes, detrital minerals from 158 surface sediment samples collected from the western Gulf of Thailand (GoT). The results reveal that the REE contents ranged from 25.42 μg/g to 201.21 μg/g, with an average value of 137.21 μg/g, and that the GoT is characterized by higher accumulations of LREE relative to HREE, together with negative Eu anomalies. Based on the spatial distribution characteristics of REE contents, δCe and δEu, the western GoT can be classified into three geochemical provinces. The Province I covers the northern and northwestern coastal zones of the GoT, including the entire upper GoT; its coarse size fraction and distribution pattern of hornblende- and quartz-dominated minerals caused REE contents to be the lowest, and these sediments were mainly derived from the rivers around the upper GoT, including the Chao Praya River and the Kea Klong River. The Province II is located in the central GoT and has the highest REE concentrations due to finer sediment composition and higher contents of zircon, authigenic pyrite, and biogenetic limestone. The Province II has two accumulation centers around Samui Island and Koh Chang Island, with REE spatial distribution influenced by the seasonal setting and mixed derived from multiple provenances around the GoT. The Province Ш is located in the lower GoT, close to Malaysia. The sediment type in this region is similar to that of Province I except for different detrital mineral compositions, especially the presence of siderite that is responsible for higher REE contents. The sediments in this region exhibits similar distribution patterns to those of terrestrial materials from the Malaysian peninsula such as that from the Kelantan River. The transportation processes of sediments in this region are mainly controlled by the coastal current.

The Andaman Sea (AS) is characterized by surprisingly weak stratification in its deep basin in drastic contrast to the nearby Bay of Bengal (BoB), presumably due to strong diapycnal mixing fertilized by the dissipation of internal tides. Here, we report on the first estimates of tidal mixing in the AS using idealized numerical simulations resolving the generation and evolution of low-mode internal tides. The estimation is based on a diagnostic analysis of the energetics of the simulated internal tides of both local and non-local origins. The results highlight the dominant role of the internal tides generated in the channels along the western boundary island chain in energizing diapycnal mixing in the entire AS. Tidal dissipation and mixing in the deep basin of the AS are found to be highly elevated, with the depth-integrated dissipation rate and the diapycnal diffusivity being O(10(-2)-10 ) W m(-2) and O(10(-3)-10(-2)) m(2) s(-1), respectively, both of which are 1-2 orders of magnitude larger than those in the deep BoB. These model-based estimates of tidal mixing are in general consistent with those from in situ conductivity-temperature-depth (CTD) measurements using the fine-scale parameterization. Our results suggest that the oft-used tidal mixing parameterization considering only local internal tide generation would underestimate the depth-integrated tidal dissipation in the AS by several orders of magnitude, highlighting therefore the necessity of considering contributions of non-local internal tides in parameterizing tidal dissipation and mixing in marginal seas like the AS.

Lithogenic grain-size, geochemical elements of core sediments from the central Bay of Bengal (BoB) were analyzed to identify sediment provenance and explore the regional “source-sink” processes since 25 ka. Based on the geochemical compositions of core BoB-88, relative contributions of three end-member sources (Himalayan, Myanmar, Indian Peninsula) were calculated and support the general understanding that Himalayan sources were dominant since the last glacial period, which could reach 70% on average in our study. However, sediments from the Indian Peninsula and Myanmar also contributed nonnegligible materials to the central BoB since 25 ka, especially the former shows an obvious increase since 7.5 ka, which was neglected in the previous studies. To trace the sedimentary processes of different provenance terrigenous materials and reveal their response to climate changes, four typical stages with obvious provenance percent variation, including the Last Glacial Maximum period (LGM), Heinrich 1 (H1), Early Holocene Climate Optimum (EHCO), and mid-late Holocene period, were highlighted and discussed. Sea-level change played a dominant role in the glacial-interglacial scale by controlling the transition of deposition center between the shelf/subaquatic delta and the Bengal Fan, especially for sediment from the Himalayan source, exerting influence on contributions from different provenance end-members between the Holocene and the last glacial period. The opposite directions of the southwest and northeast monsoon current in the BoB have a significant influence on the transfer of the Indian Peninsula and Myanmar materials to the studied core, which led to an increase of up to 11% than the average of the Indian Peninsula and a decrease of up to 9% than the average of the Myanmar source during the Holocene. The balance between sea level (limited transport to the deep sea) and ISM (improved erosion production and output to the BoB) during the mid-late Holocene period maintained the stability of Himalayan source percent. Due to the control of the ISM variations, the Himalayan source showed a decrease of up to 20% and 21% than average during the LGM and H1 period, respectively, and an increase of up to 14% than average during the EHCO. The Myanmar source exhibited fluctuations around the average value during the LGM period, but an obvious increase of up to 21% during the H1 period and a decrease up to 8% during the EHCO, while the Indian Peninsula source percent showed little variations during the LGM and H1, but a decrease of up to 6% during the EHCO. Obvious 1/4 processional and millennial periodic changes (5.0 kyr and 1.1 kyr) of contribution proportions from three end-members provide additional evidences for the climate control on the regional source-sink processes and proved the controlled mechanisms of the orbital parameters and possible influence from the solar activity on millennial scales.

The diel vertical migration (DVM) behaviors of zooplankton and micronekton vary among species in response to diverse ambient conditions. Based on the observed mean volumes of backscattering strength (MVBS), we studied the mean features and seasonal variations of DVM in the Andaman Sea. Nocturnal-ascent DVM pattern prevailed throughout the observation period, with a mean downward (upward) migratory speed of 3.85 (3.81) cm/s. The DVM in this area exhibited a 3-layer vertical structure. Between the surface nighttime residence and the daytime residence below 300 m, there was a weak subsurface daytime residence probably formed by the assemblage of migrants avoiding aerobic predators. The variations in lower margin depth of surface nighttime residence and the upper margin depth of subsurface daytime residence were consistent with the variation in oxycline depth, which is closely related to thermocline variations. Compared with the premonsoon season, the MVBS was weaker, but the DVM was more active during the summer monsoon season, corresponding to the water clarity trend. Overall, equatorial Kelvin waves and monsoon winds presumably dominated the seasonal variations in DVM by modulating sea water properties such as oxygen concentration and transparency.

Grain size, clay mineralogy, 87Sr/86Sr, eNd, and AMS14C analyses of deep-sea sediments cored in the central Bay of Bengal are used to reconstruct the evolution of provenances and climate control since the last glacial period. Clay minerals, 87Sr/86Sr ratios and eNd values indicate a mixture of sediment from the Himalaya source, Indian Peninsula and Indo-Burman Ranges/Irrawaddy River. These analyses show that the Himalaya Mountains and Indo-Burman Ranges/Irrawaddy River are the main suppliers before the Holocene period, while the inputs of erosion materials from the Indo-Gangetic floodplain and the Indian Peninsula increase during the Holocene period, accompanied by reduced inputs of the Indo-Burman Ranges and Irrawaddy sediments. The regional “source-sink” process is significantly controlled by the climate. Warm and wet climate conditions and enhanced Indian monsoon intensity during the Holocene period are responsible for the increase of sediment input from the Indo-Gangetic Plain and the Indian Peninsula by improving erosion in the source area, input from the floodplain to the river channel and the intensity of the southwest monsoon current. In contrast, due to the decrease of the northeast monsoon current during the Holocene period, sediment input from the Irrawaddy and Indo-Burman Ranges are constrained. Deposition center transition between the submarine fan and shelf during the last glacial period and the Holocene period occurs in response to the sea level change, which controls the sedimentary mode and thus significantly influences the transportation and deposition processes in the Bay of Bengal (BoB).

The characteristics of currents and tidal currents in the Andaman Sea (AS) are studied during the second half of 2016 using observed data from a moored acoustic Doppler current profiler (ADCP) deployed at 8.6°N, 95.6°E. During the observation period, the mean flow is 5–10 cm/s and largely southward. The root mean square and kinetic energies of the low and high frequency flows, which are divided by a cutoff period of 5 d, are at the same level, indicating their identical importance to the total current. A power spectrum analysis shows that intraseasonal oscillations, a tidal-related semilunar month signal, a semidiurnal tidal signal and periods of 3–4 d are prominent. The barocliny of an eddy kinetic energy is stronger than the mean kinetic energy, both of which are the strongest on the bottom and the weakest at 70 m depth. Residual currents are largely southward (northward) during the summer (winter) monsoon season. Two striking peaks of the southward flow cause the 80 d period of meridional currents. The first peak is part of a large-scale circulation, which enters the AS through the northern channel and exits through the southern channel, and the second peak is part of a local vortex. The 40 d oscillation of the zonal current is forced by geostrophic variations attributed to local and equatorial remote forcing. The tidal current is dominated by semidiurnal constituents, and among these, M2 and N2 are the top two largest major axes. Moreover, astronomical tidal constituents MM and MSF are also significant. Diurnal constituents are weak and shallow water tides are ignorable. The aims are to introduce the new current data observed in the AS and to provide initial insights for the tidal and residual currents in the Andaman Sea.

The Bengal Fan is world famous for its developed turbiditic system. This study identified turbidite records in a gravity core (BoB-104) taken from near a currently inactive channel (E4) based on lithology, geochemical and mineralogical parameters. This core contained turbidite and hemipelagic layers in this core. This study identified 21 turbidite layers which exhibited consistent provenance with the hemipelagic layers, showing a two end-member mixing pattern and indicating consistent sources since the last glacial period. Typical turbidite layer analysis was used to study the possible turbidite deposition mechanism. Features such as the lithology break interface, normal graded sequence, a consistent pattern of a sharp increase and gradual decrease in the variation of the mean grain size (Mz), and median grain size (D50) and Si/Al ratios commonly implied a continuous deposition in accordance with variations in densities and particle sizes. This study identified long-term evolution of turbidite deposition in the eastern middle Bengal Fan that appears to be the last and indispensable part of the systematic response of the “Bengal depositional system” to the climate. Four stages were identified from the turbidite records and all the identified intervals showed synchronous changes with the entire system. The Indian summer monsoon (ISM) played a dominant role in the system interactions. The sea level affects the turbidity system by regulating the transition of the deposition center between the deep-sea and the shelf, whereas the migration of the main channel to the “Active Valley (AV)” between 14.5 ka to 10.4 ka almost halted the deposition of turbidites along the E4 channel. Thus, turbidity activity in the eastern middle Bengal Fan is not an isolated incident but part of a comprehensive system resuling from global and regional changes at different scales.

The Holocene evolution of the Indian Ocean Monsoon (IOM), especially the spatiotemporal nature of its maximum precipitation period, remains controversial. Here we reconstruct high-resolution local δ18Osw and sea surface temperature (SST) values by pairing Mg/Ca values and the δ18O data from planktonic foraminifera Globigerinoides ruber (white) in the southern Andaman Sea in order to understand IOM rainfall variability during the Holocene. Local δ18Osw values show that local maximum rainfall occurred between 10.6 ka and 8.6 ka in the southern Andaman Sea, before declining markedly during the 8.6–5.0 ka period, and then decreasing slightly after 5.0 ka. The Mg/Ca-SST was relatively low during the 10.3–8.6 ka period, before increasing slightly between 8.6 ka and 4.0 ka; it declined thereafter. But the sedimentation rate at the site of Core ADM-C1 relating to monsoonal precipitation of the source region of sediments exhibited its highest value during the 8.6–5.5 ka period. After making further comparisons with other published records for this region, we would propose that maximum IOM precipitation during the early Holocene has exhibited a clearly meridional migration: it was located in the tropics around 7°N during the 10.6–8.6 ka period, migrated northward to the subtropics during the 8.6–5.5 ka period, before migrating southward again after 5.0 ka. We would suggest that the northward shift of maximum precipitation during the Early Holocene in the IOM region was in response to the progressively northward migration of the Intertropical Convergence Zone (ITCZ) and the change of El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) activities.

Changes of marine paleoproductivity could reflect the history of marine biogenic cycle process, and exploring the evolution and mechanism of marine productivity are important for understanding the global biogenic factor cycle and its roles in past, present and future climate change. Based on the analysis of biogenic silica (BSi) in core ADM-159 and core BoB-56 from the northeastern Indian Ocean, high-resolution curves of paleoproductivity evolution were obtained. The results show that the BSi contents in these two core sediments are less than 1.8%, mainly affected by the low supply of silica skeleton and the high dissolution and dilution of terrigenous materials. Distinct fluctuations in BSi contents could still be reflected, and their relative values were controlled by chemical weathering processes and terrigenous material supply. The power spectrum analysis of BSi content shows reliable periodicities of 5.86 kyr, 1.95 kyr, 1.35 kyr and 1.09 kyr, suggesting that millennial-scale BSi value might be affected by solar activity. Based on the BSi mass accumulation rates (MAR), lower paleoproductivity during the glacial period and higher level during the Holocene were revealed in our records, which matched well with the global climate changes. Temperatures controlled by low latitude insolation changes could influence the nutrient substance supply, further promote productivity variations at the glacial–interglacial scale. Some millennial events, such as the Heinrich stadial 1 (HS1) and Younger Dryas (YD) periods, showed relatively low paleoproductivity, while the Bolling/Allerod (B/A) period and the middle Holocene showed higher paleoproductivity. These millennial paleoproductivity fluctuations may be due to the changes of nutrient supply caused by Indian summer monsoon (ISM). In addition, the BSi MAR of core ADM-159 and core BoB-56 are well correlated with the isotopic compositions of Greenland ice core and Chinese speleothem, providing robust evidence for the link between northeastern Indian Ocean paleoproductivity and global climate change.

To better understand the process of hydrographic and temperature changes in the Indian Ocean, we present Glycerol dialkyl glycerol tetraether data on core BoB-56 from the Bay of Bengal to reconstruct the sea surface temperature (SST) variability during the last glaciation. The results indicate that the TEX86-derived SST ranged from 22.00 to 28.30 °C with several fluctuations since 40 ka BP and that the evolution pattern matched well with the marine isotope stage (MIS). During MIS 3, the SST kept around 24.60 °C with some small amplitudes. The SST showed more significant fluctuations during MIS 2 and increased gradually with a ~3 °C cooling during the Last Glacial Maximum (LGM). After the Younger Dryas (YD) stage, Holocene SST values exhibited miniscule variations. During the last glaciation climate variability process, the cold signals of the YD and Heinrich events were recorded well in our core SST series, revealing a regional response to global climate changes. The warming trend during these millennial-scale cold events in the northern Indian Ocean was also identified, and SST increased in the range of 2 °C. A comparison of SST records and atmospheric CO2 concentration shows that the warming process during the YD, LGM, and Heinrich (1–4) events matched the gradual increase in atmospheric CO2 concentration in the northern Indian Ocean. Hence, this study implies millennial-scale hydroclimate change linked with atmospheric CO2 forcing that could improve our knowledge of the role of atmospheric CO2 in triggering abrupt Indian Ocean climate changes.

Paleoclimatic teleconnections between monsoon-dominated regions and the northern Atlantic has been revealed in the tropical Indian Ocean. Millennial-scale climate variability, as registered in Greenland ice cores, was transferred to the low-latitude regions by changes in the Atlantic meridional overturning circulation (AMOC). However, there is still some disputes regarding the mechanism of climatic variation in the tropical Indian Ocean. To explore the interplay responses to regional monsoons and global in the northeastern Indian Ocean, here we present new evidence of trace elements (Mg/Ca and Ba/Ca) and stable isotopes measured from the planktonic foraminifera Globigerinoides ruber (white) in core ADM-159, which was collected from the central Andaman Sea. Twelve accelerator mass spectrometry (AMS) radiocarbon dates of the planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate gradual surface warming with several short-lived fluctuations since ~42 kyr ago. The SST during the Last Glacial Maximum (LGM) was ~3 °C colder than the present SST, and a sharp increase in warming beginning at 18.5 cal ka BP indicated the transition from the LGM to the Holocene; these matched well with the warming trend in the Antarctica and the Southern Ocean and the rise in atmospheric CO2 levels. The seawater δ18O record (defined as δ18Osw, presumably salinity-driven) and the Ba/Ca record, which is a measure of the continental riverine runoff from Myanmar, revealed a prominent millennial-scale variability pattern controlled by the Indian summer monsoon (ISM) as seen in speleothem records. The higher salinity values that suggested weakened ISM precipitation and a decrease in freshwater output from the Irrawaddy-Salween rivers were seen at 41.1 cal ka BP–39.9 cal ka BP, 34.9 cal ka BP–32.8 cal ka BP, 29.3 cal ka BP–28 cal ka BP, 25.7 cal ka BP–23.8 cal ka BP, 19.3 cal ka BP–15.8 cal ka BP, 13 cal ka BP–11.4 cal ka BP, and 4.0 cal ka BP–2.3 cal ka BP, reflecting strong correspondence with cold events in North Atlantic. Thus, this study provides evidence that climatic changes in the tropical Indian Ocean responded well to both regional monsoon and global climate changes, and during that atmospheric CO2, solar insolation, and AMOC played the most important roles in those responses.

The clay mineral contents in 110 surface sediment samples collected from the middle of the Bay of Bengal were analyzed by X-ray diffraction (XRD) to investigate the provenance and transport patterns. The illite content was highest, followed by chlorite, kaolinite and then smectite, with average weight percent distributions of 52%, 22%, 14% and 12%, respectively. Illite and chlorite had similar distribution pattern, with higher contents in the northern and central areas and lower contents in the southern area, whereas smectite showed the opposite distribution pattern. Kaolinite show no obvious higher or lower areas and the southern “belt” was one of the highest content areas. Based on the spatial distribution characteristics and cluster analysis results, the study area can be classified into two provinces. Province I covers the southwestern area and contains high concentrations of illite and smectite sediments. Province II covers most sites and is also characterized by high concentrations of illite, but the weight percent of smectite is only half of that of province I. According to a quantitative estimate using end-member clay minerals contents, the relative contributions from the Himalayan source and the Indian source are 63% and 37% on average, respectively. Integrative analysis indicates that the hydrodynamic environment in the study area, especially the turbidity and surface monsoonal circulation, plays an important role in the spatial distribution and dispersal of the clay fraction in the sediments. The sediments in province I are mainly from the Indian source transported by the East Indian Coastal Current (EICC) and the surface monsoon circulation with minor contributions from the Himalayan source while the sediments in province II are mainly from the Himalayan source transported by turbidity and surface monsoonal circulation with little contribution from Indian river materials.

Studying marine sedimentary archives from the Indian Ocean have provided great opportunities to assess the forcing/response relationship between the Indian monsoons, millennial-scale climate variability as revealed in Greenland ice cores, and possible roles of the Atlantic meridional overturning circulation (AMOC) changes that are most likely responsible for driving both. Here we present new evidence of sea surface temperature (SST) estimates and δ18O of seawater (δ18Osw, presumably salinity-driven) from the paired data of Mg/Ca and δ18O measured from planktonic foraminifer Globigerinoides ruber (white) in core BoB-24 collected from the central Bay of Bengal (BoB), tropical Indian Ocean. Eleven accelerator mass spectrometry (AMS) radiocarbon dates on planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate a gradual surface warming with several short-lived fluctuations since ~42 kyr ago, and a ~3 °C cooling during the Last Glacial Maximum (LGM) and a ~3 °C warming from the last glacial to the Holocene. Our δ18Osw revealed an increased salinity during the Marine Isotope Stage (MIS) 2, which is likely attributable to a weakening of Indian Summer Monsoon (ISM) precipitation and associated river discharge in the BoB. Our record also indicates clearly a phase of strengthening ISM during the mid-MIS 3 (37.5–32 ka) as evidenced by low foraminiferal δ18Osw values of ~0.2‰. Lower than present δ18Osw values during the Bolling/Allerod (B/A) event (14.5–12.6 ka) and the early Holocene (10–5 ka) in core BoB-24 indicate lower salinity that in turn suggest enhanced ISM precipitation and increased freshwater output from the Ganges-Meghna-Brahmaputra-Irrawaddy (GMBI) rivers together with peninsular rivers and Irrawaddy-Salween rivers from Myanmar. This study brings data evidence that implies weakened ISM was linked with Northern Hemisphere cooling events, especially during the MIS 2. Furthermore, the millennial-scale hydroclimate changes in the BoB presented in this study support the dynamic role of AMOC in driving a teleconnection between the North Atlantic and Indian Ocean monsoon regions.

In this study, we analyze major and trace elements (SiO2, Al2O3, Fe2O3, CaO, K2O, MgO, Na2O, TiO2, P2O5, MnO, Cu, Pb, Ba, Sr, V, Zn, Co, Ni, Cr, and Zr) and grain size of 157 surface sediment samples from the western Gulf of Thailand (GoT). On the basis of the space distribution characteristics, the study area can be classified into three geochemical provinces. Province I covers the northern and northwestern coastal zones of the GoT, including the whole upper GoT and thus the sediments from the rivers in the area. It contains high contents of SiO2. Province II is located in the middle of the GoT and has similar geochemistry composition as the South China Sea (SCS). It contains sediments that are characterized by higher contents of Na2O, TiO2, Ba, Cr, V, Zn, Zr, and Ni. Province Ш is located in the lower GoT, close to Malaysia. Major and trace elements in this area showed complex distribution patterns, which may be due to terrestrial materials from Malay rivers combining with some sediments from the SCS in this province. The results also indicate that grain size is the controlling factor in elemental contents, and that the hydrodynamic environment and mineral composition of the sediments play an important role in the distribution of these elements. The anthropogenic impact of heavy metal introduction (especially Cr, Zn, Cu, and Pb) can be seen in surface sediments from the nearshore region of Chantaburi province and north of Samui Island.

The coastal margins around Southeast Asia (SE Asia) may serve as an ideal location to study the source sink process of sedimentary black carbon (BC) because SE Asia has been identified as one of the major BC emission source regions in the world. This study provides an extensive picture of recent regional-scale sedimentary BC sequestration in the Gulf of Thailand (GOT), a tropical marine system in SE Asia. Generally, the sedimentary BC concentrations (0.07-3.99 mg/g) were in the low to moderate ranges of those obtained in other coastal sediments around the world. Regional variability of the BC and its correlation with the sediment grain size and total, organic carbon (TOC) content indicated a general hydrodynamic constraint on BC occurrence in the lower Gulf in contrast to the upper Gulf with a more source dependence due to the direct land-based input. BC/TOC% values and the varied BC components (char and soot), as well as their correlations suggested that char was the predominant constituents of sedimentary BC both in the upper and lower Gulf, which could be mainly derived from biomass burning and entered into the nearshore region through direct fluvial transport and surface run-off. The estimated BC burial flux (similar to 212 mu g/cm(2)/y) and mass inventory (similar to 200 Gg/y) in the GOT on the hundred-year timescale were of the same order of magnitude compared with other oceanic margins, and thus the tropical shelf sediments from SE Asia could serve as an important sink of land-emitted BC. (C) 2015 Elsevier Ltd. All rights reserved.

Downcore geochemical composition of sediment core retrieved from the central Andaman Sea were measured to help understanding the potential connection between chemical weathering and monsoon climate since the last glacial period. The age model was confirmed based on twelve accelerator mass spectrometry (AMS) 14C dates on planktonic foraminifera Neogloboquedrina dutertrei. Provenance discrimination results distinguished by Rare Earth Elements pattern and Sc/Th-(La/Yb)UCC diagram revealed that terrestrial sediment from the Irrawaddy River are the dominant input in the central Andaman Sea. Chemical weathering indexes, including chemical index of alteration (CIA), weathering index of Parker (WIP), αAlNa, and a stack of these data were reported and the terrestrial input was assessed using Ti/Ca ratios. These indexes showed general synchronicity with the regional monsoon precipitation and temperature record, the chemical weathering intensity and monsoon precipitation/temperature were stronger/higher during MIS 3 and MIS 1 and weaker/lower during MIS 2, indicating existence of a general “climate-driving weathering” pattern on glacial-interglacial time scales. However, certain anomalies occurred during 18–13 cal. kyr BP and 6–3 cal. kyr BP, when the chemical weathering intensity showed higher and lower values, respectively. The chemical weathering intensity enhanced during 18–14 cal. kyr BP, due to increased isolation and markedly increased temperatures, as well as lower erosion production and sediment transfer rate caused by lower precipitation, whereas the opposite conditions prevailed during 14–13 cal. kyr BP. Indeed, for the weak chemical weathering intensity during 6–3 cal. kyr BP, decreased isolation, relatively high terrestrial sediment supply, slightly lower precipitation and markedly lower temperatures were suggested to be the primary reasons, indicating the presence of a “climate-driving weathering” pattern on millennial time scales. Our findings support the hypothesis that the chemical weathering evolution in the north-eastern Indian Ocean was synchronous with both global and regional climate conditions, representing distinct changes controlled by Indian summer monsoon on different time scales.

Organic carbon cycling in SE Asia is data-sparse and poorly studied, although high sediment yield and organic carbon intensity occur in the tropical region. In the present study, we evaluated the role of tropical monsoon pertaining to fluvial discharge, sediment load, coastal current and water stratification on seasonal organic carbon dynamics during four sampling campaigns in the Upper Gulf of Thailand (UGoT). This study demonstrates that particulate organic carbon (POC) is closely correlated with the river influx of suspended sediment, which is generally regulated by the local rainfall. Higher POC is found near the large estuarine section (Chao Phraya River, CHAO) during southwest monsoon period and the small estuarine section (Mae Klong River, MK) during November 2013 when tropical cyclones impacted. POC in the estuarine sections is more significantly influenced by the seasonal shift compared with the coastal sections. Land-derived organic matter prevails in the small estuarine and coastal sections, while marine-derived organic matter dominates in the estuarine sections impacted by CHAO and MK. Anthropogenic inputs and dam regulation also influences the river export of organic matter during the high precipitation periods. Total organic carbon (TOC) however displays less significant seasonal monsoon variations than POC. Further, TOC tends to accumulate in the sub-silt fraction of sediments, which mainly occurs in the small estuarine and eastern coastal sections and is more obviously influenced by marine-derived factors. TOC in sediment of the CHAO and MK influenced sections however displays more seasonal variations with prevailing river input evidenced by coarser sediment and higher C/N ratios. Moreover, the almost year round water stratification across the region acts as the barrier in retaining organic carbon in the estuaries and their vicinities from dispersal into the lower portion of Gulf of Thailand. High sedimentation rate (~1.1 cm·yr−1) further facilitates the organic carbon burial in the study area. The delivery, dispersal and burial of organic carbon are closely associated with the climate-controlled precipitation, and thus the tropical monsoon climatology under the global warming in particular is an important factor influencing the organic carbon in the UGoT.

In recent years, Thailand and Bangladesh have been drawing more attention due to groundwater pollution, but there is rarely a comparative study on hydrochemical evolution and quality assessment of groundwater in the areas In this paper, end-member mixing and Gibbs model are used to study the evolution of groundwater hydrochemistry based on the 74 water samples. Due to the seawater intrusion, the evolution path of groundwater in Bangkok Bay is Ca/Mg-HCO3 → Na (Ca/Mg)-HCO3 and Na (Ca/Mg)-Cl → Na-Cl, which led to increased Na+, K+, and HCO3– concentrations in groundwater and enrichment of fluorine in groundwater caused fluorine pollution. In contrast, although the evolution path of groundwater in southern Bangladesh shows a similar pattern as Ca/Mg-HCO3 → Na (Ca/Mg)-HCO3 and Ca/Mg-Cl → Na-Cl, the principal influence factors are residential, agricultural and industrial pollutants. The scope of seawater intrusion is only limited to estuary areas of the southern estuary, whereas most groundwater is received recharge of polluted surface water, which resulted in the enrichment of Fe and Mn. In addition, compared with shallow groundwater, deep groundwater is updated slowly and evolved to higher degree, and is in the Na (Ca/Mg)-HCO3 and Ca/Mg-Cl stage. For improving groundwater conditions in southern Bangladesh, enhancing the supply capacity of surface potable water and significantly reducing groundwater exploitation are priority. Strictly controlling the generation, storage and treatment of domestic waste, industrial and agricultural pollutants are also important. In Bangkok Bay, reducing the groundwater exploitation and dispersing the exploitation area is required.

This study investigated the distribution and characteristics of microplastics in surface sediments of the Gulf of Thailand (GoT), and discussed the correlation between sediment grain size and microplastic content. The results indicate the abundance of microplastics is 150.4 ± 86.2 pieces/kg dry weight, representing a medium microplastic pollution level compared to other sea areas. Small microplastics (0.5–1 mm) take up >70% of total microplastic numbers. Fibrous microplastics are the dominant component of microplastics. According to micro-Fourier transform infrared spectroscopy, rayon (37%) and polyester (PES: 16%) are the most typical polymer types found in sediments. The results imply that secondary microplastics are the dominant pollutant, while fibrous microplastics are mainly from municipal sewage discharge. We also find that inconspicuous correlation between grain size and microplastics, which is caused by the multi-sources and different flow field. This study deepens our understanding of the environmental risks posed by microplastics to marine ecosystems in the GoT.

Paleoenvironment and paleoclimate change in the Andaman Sea during the last 26 ka were reconstructed from high-resolution records of grain-size, major elements and Sr–Nd isotopes in core ADM-9. The values of eNd(0) and 87Sr/86Sr were in good agreement with those of Irrawaddy River sediments, indicating a common source of origin. Two sensitive grain-size intervals (3.4–7.5 and 16.8–21.2 μm) were identified; the former was controlled primarily by sea-level change, whereas the latter was related to Irrawaddy River discharge and South-west Current transport driven by the Indian Summer Monsoon (ISM). Proxies of chemical weathering (K/Al) and terrigenous input (Ti/Ca) coupled with sensitive grain-size interval (16.8–21.2 μm population) revealed that the ISM was weak during ∼15–26 ka BP and then strengthened gradually to a maximum during ∼7–9 ka BP; subsequently, the ISM exhibited a generally declining trend to ∼2 ka BP. The variation in the ISM recorded in this work is consistent with ISM variations observed in an open area in the northern Indian Ocean and adjacent continents, implying the evolution of the Asia Summer Monsoon since 26 ka.

Generation, morphology, and distribution of authigenic minerals directly reflect sedimentary environment and material sources. Surface sediments were collected from the western Gulf of Thailand during 2011–2012, and 159 samples were analyzed to determine detrital minerals. Authigenic minerals, including siderite, pyrite, and glauconite, are abundant whereas secondary minerals, such as chlorite and limonite, are distributed widely in the study area. Siderite has a maximum content of 19.98 g/kg and appears in three types from nearshore to continental shelf, showing the process of forming-maturity-oxidation. In this process, the MnO content in siderite decreases, but Fe2O3 and MgO content increase. Colorless or transparent siderite pellets are fresh grains generated within a short time and widely distributed throughout the region; high content appears in coastal area where river inputs are discharged. Translucent cemented double pellets appearing light yellow to red are mature grains; high content is observed in the central shelf. Red-brown opaque granular pellets are oxidized grains, which are concentrated in the eastern gulf. Pyrite is mostly distributed in the central continental shelf with an approximately north–south strip. Pyrite are mainly observed in foraminifera shell and distributed in clayey silt sediments, which is similar to that in the Yangtze River mouth and the Yellow Sea. The pyrite in the gulf is deduced from genetic types associated with sulfate reduction and organic matter decomposition. Majority of glauconite are granular with few laminar. Glauconite is concentrated in the northern and southern parts within the boundary of 9.5° to 10.5°N and is affected by river input diffusion. The distribution of glauconite is closely correlated with that of chlorite and plagioclase, indicating that glauconite is possibly derived from altered products of chlorite and plagioclase. The K2O content of glauconite is low or absent, indicating its short formation time.

A high density sampling program during two joint China–Thailand scientific cruises in 2011–2012 included collection of 152 gravity box cores in the Gulf of Thailand (GoT). Samples from the top 5 cm of each core were analyzed by X-ray diffraction for clay mineral content. Several systemic analytical approaches were applied to examine the distribution pattern and the constraint factors of clay minerals in the surface sediments of the western GoT. The clay minerals mainly comprise illite, kaolinite, chlorite and smectite, having the average weight percent distributions of 50%, 34%, 14% and 2%, respectively. Based on the spatial distribution characteristics and statistical results, the study area can be classified into three provinces. Province I contains high concentrations of smectite, and covers the northern GoT, sediments in this province are mainly from rivers discharging into the upper GoT, especially the Chao Phraya and Mae Klong Rivers. Sediments in Province II are characterized by higher values of illite, located in the central GoT, where fine sediments are contributed by the Mekong River and from the South China Sea. Province Ш, in the coastal regions of southwestern GoT close to Malaysia, exhibits a clay mineral assemblage with complex distribution patterns, and may contain terrestrial materials from the Mae Klong River as well as re-suspended sediments. Results of integrative analysis also demonstrate that the hydrodynamic environment in the study area, especially the seasonal various circumfluence and eddies, play an important role in the spatial distribution and dispersal of clay fraction in sediments.

The effects of pH reduction on the settlement and development of the coral Pocillopora damicornis were investigated. Three different pH treatments (pH = 7.6, 7.9, and 8.1) were used. In addition, water quality (temperature, salinity, total alkalinity) around the study site was monitored. The results showed significant differences in the settlement rates of Pocillopora damicornis larvae between pH treatments (p ≤ 0.05). A decrease in pH levels caused a strong decline in larval settlement rate. In addition, at pH 7.6 and 7.9, all larvae were unable to complete metamorphosis, and metamorphosis delay was observed. Field monitoring showed low fluctuation of all seawater parameters within 24 hours, and there was no difference between seasons. From this study, a strong negative effect of pH reduction on P. damicornis larvae was observed. Although the function of physiology is still not clearly understood, correlations are likely to exist.