Geophysics Appearance of The South China Sea

<p class="AbstractText">South China Sea (SCS) is underlain by sediments of an average density 2.10 g/cm³ of 2 km thickness at its central part up to 10 km in the margins. The basement rock is the upper and lower crust of densities 2.67 and 2.85 g/cm³ respectively of varying thicknesses. The thinnest crustal rock is at the centre of SCS that is called the South China Sea Basin (SCSB). The Mohorovicic discontinuity is about 15 km depth below the SCSB. Heatflow values in this basin vary from 2 to 3.5 HFU.</p><p class="AbstractText">Lineations of total magnetic anomaly are generally in a west-east direction covering the whole study area. However, an elongated northeast-southwest lineation of dipole anomaly separates the west-east anomaly patterns in the north from those in the south. This feature is also observed in the gravity map. These elongated patterns of the total magnetic features are in coincident with the occurrences of seamounts inferred being remnant of extinct seafloor spreading. Because of this spreading a crustal extension had taken place that separated Kalimantan from the mainland of China to restore its present position. A paleomagnetic study result confirms this hypothesis.</p><p class="AbstractText">The Palawan trench is marked by north-east trending magnetic and gravity anomaly that is inferred being traces of a remnant subduction zone. This anomaly forms a boundary between the Zengmu also called the Sarawak basin and the SCSB. Here, heat flow value is 1 to 2 HFU. This value in coincident with gravity gradient of 2.5 mGal/km also represents an active subduction of the Manila trench north of the Palawan Island. The Manila trench is supposed to be the energizing source of volcanism and earthquake in the Phillippines. Free-air and Bouguer anomaly of the order of 50 to 60 mGal and magnetic of about 100 nT represent the Zengmu basin in the Sunda Shelf. This basin is underlain by sediments of 2.10 g/cm³ of 8 km thickness and also crustal rock which is much thicker than the one underneath the SCSB. Strong topographic relief at the surfaces of sedimentary layer and the crustal rock is very my much associated with normal faulting that may cause fluctuation of the free-air values.</p><p class="AbstractText">The continental margins of Sarawak and the Sunda Shelf are areas of hydrocarbon deposits now still in production, whereas the offshores Vietnam and Hainan are promising target for hydrocarbon exploration.</p>

Sediment Sources and Dispersion on the Western Sunda Shelf, Malay Peninsula, Southern South China Sea

Thirty-nine surface sediment samples collected from the western Sunda Shelf off the Malay Peninsula (WSSMP) in the southern South China Sea (SCS) were analysed for grain size, major and trace elemental compositions, and light/heavy mineral contents to trace the sediment sources and their transport mechanisms in the study area. In the WSSMP, the surface sediments are relatively poorly sorted but transportable. A principal component analysis of 37 elements and grain size fractions indicates that the surface sediments can be grouped into four major assemblages in the study area. Integrating with the light/heavy minerals data in the 63–125 μm fractions of the surface sediment samples, to better trace the sediment sources of the coarse-grained components in the marine environment, the study area can be further divided into four sediment provinces. Province I is located in the northwestern part of the study area. The concentrations of TiO2, Na2O, garnet, siderite, and glauconite in Province I were higher than in the other provinces. The main sediment source for this province originated from the Kelantan River and the Gulf of Thailand transported by the northeastern monsoon current. Province II is located offshore of the Pahang and Endau Rivers. The percentages of TiO2, rare earth elements, Al2O3, quartz, plagioclase, hypersthene, and magnetite in the surface sediments were typically higher in this province than in the other provinces. The Pahang and Endau rivers provide most of the sediments to this province, which are transported by southward coastal currents. Province III is located in the northeastern and eastern parts of the study area, where the coarse-grained sediment fraction had relatively high hornblende and biotite contents. Sediments in this province are mostly transported from the Mekong River during the northeastern monsoon. The other parts of the study area belong to Province IV, where the surface sediment elemental and mineral concentrations were mostly between those of the other three provinces. Therefore, we suggest that Province IV has a mixed source due to inputs from the surrounding rivers.

Spatial Distribution of Total 210Pb in the Marine Sediment of Peninsular Malaysia: Radioactivity Level Variations in The Sunda Shelf Seas

Radiochemical separation techniques were applied to measure 210Pb radioactivity in the samples to determine spatial distribution of total 210Pb radionuclide in surface sediments and water column for the main waters on the Sunda Shelf. The average 210Pb radioactivity level recorded from the Malacca Strait (MS) is significantly higher than the southern South China Sea (sSCS); this is attributed to the supply of input from the Andaman Sea in its northern areas, and massive terrestrial processes from bordering land masses. The results reveal that the decreasing trend of total 210Pb in Peninsular Malaysia’s marine sediment starts with the north maritime zone of Malacca Strait (64.28 ± 5.97 Bq/kg) > south maritime zone of Malacca Strait (50.88 ± 6.15 Bq/kg) > east maritime zone of sSCS (41.01 ± 3.07 Bq/kg) > south maritime zone of sSCS (40.78 ± 3.16 Bq/kg). The Kelantan and Pahang Deltas have been identified as two of the main anthropogenic sources of input for total 210Pb in the sSCS while atmospheric transboundary mobilization affecting total 210Pb in water column. In the Malacca Strait, however, the distribution of total 210Pb might be influenced mainly by in situ processes of the strait’s seafloor and sources origin of sediment.

Remote hydroacoustic-infrasonic detection and characterization of Anak Krakatau eruptive activity leading to, during, and following the December 2018 flank collapse and tsunami

A climactic eruption phase on December 22, 2018, triggered the collapse of the southwest flank and summit of Anak Krakatau stratovolcano, generating a tsunami which struck the coastlines of Sumatra and Java. We employ a selection of remote moored hydroacoustic (H08S, 3307 km; H01W, 3720 km) and infrasonic (IS06, 1156 km; IS07, 3475 km; IS52, 3638 km) stations of the International Monitoring System (IMS) to investigate eruptive activity preceding, during, and after the climactic eruption phase. We observe 6 months of co-eruptive intermittent infrasound at IS06 and powerful infrasound from the climactic eruption on IS06 and IS52. The climactic eruption phase was not detected hydroacoustically, but we observe a ~ 12-day swarm of hydroacoustic signals beginning 24 days before the flank collapse event that we attribute to sustained submarine eruptive activity at Anak Krakatau. We perform hydroacoustic waveform and envelope multiplet analysis to assess event similarity during the hydroacoustic swarm. Hydroacoustic waveforms are not well-correlated, but envelopes with a main pulse duration of ~ 20-s are correlated, with 88.7% of 247 events grouping into two multiplets using a threshold correlation coefficient of 0.75. The repetitive envelopes indicate a repetitive impulsive volcanic process, either underwater submarine explosions or volcanic earthquakes in the solid Earth coupled to the water column from the Sunda Shelf. This study further underscores the potential of remote acoustic technology for detecting and characterizing eruptions at submarine or partially submerged volcanoes.

Song Gede: Situs Gua Hunian Sejak Masa Pleistosen Akhir di Pulau Nusa Penida, Bali

Prehistoric archaeological research in Indonesia recently showed the existence of dwelling caves from the late Pleistocene, starting from Sumatra, Java, Sulawesi to East Nusa Tenggara. The Song Gede site is one of the dwelling caves from the late Pleistocene which was very interesting to be discused, especially when tracing the distribution of early modern humans in Southeast Asia. The location of the Song Gede site is considered to have a very strategic position because it was located at the eastern end of the Sunda Shelf towards Wallacea. The purpose of this paper is to study the chronology of the Song Gede Site and review the results of research at the Song Gede Site as an ocupation site that is on the migration route in the southern part of the Sunda Shelf to the Wallacea region or vice versa. The research method used was stratigraphic analysis, contextual and carbon dating methods. The results showed that the Song Gede Site has been inhabited since the late Pleistocene to the Holocene. The human living in the cave uses various natural resources to meet their daily needs, such as the use of rock materials, and the rest of the animal remains for tools and the use of animal and plants to fulfill their daily consumption.

Macrobenthos diversity along the Exclusive Economic Zone of East Coast Peninsular Malaysia

Aim: To examine the standing stock of macrobenthos along a depth gradient at regional scales in the Sunda Shelf of Malaysian Economic Exclusive Zone (EEZ). Methodology: Macrobenthos was sampled with a Smith–McIntyre grab at 19 stations on the continental shelf of the South-western South China Sea (east coast of Peninsular Malaysia) within the EEZ and was carried out onboard MV SEAFDEC II in May/June 2016. Results: The faunal composition, abundance, and diversity of species, together with environmental parameters were studied. A total of 10,232 individuals comprising 105 families were identified. The dominant macrobenthic group was Mollusca (55.25%), followed by Annelida (26.80%) and Arthropoda (15.36%), while the Echinodermata and Miscellaneous group recorded 1.13% and 1.43% respectively. Based on Bray-Curtis species similarities, five different sample groups (SGs) were distinguished, which were located in different zones and gradients of EEZ. Interpretation: Variations in the macrobenthic community is significantly associated with depth, temperature, and salinity. Further research should be conducted on other factors that contribute to the diversity of macrobenthos along the east coast of Peninsular Malaysia’s EEZ.

Sedimentary Responses to Climate Changes and Human Activities Over the Past 7400 Years in the Western Sunda Shelf

High-resolution records of grain size, major and trace elements, and Sr-Nd isotopes of Core K17 from the western Sunda Shelf were investigated to evaluate the response of weathering and terrigenous input to climatic changes and human activities over the past 7400 years. Sr-Nd isotopic results indicate that the Kelantan River is the main source of sedimentary material in the study core since the mid-Holocene. Chemical weathering levels are represented by the chemical index of alteration (CIA), αAlNa, and K2O/Al2O3 ratios; and geochemical and grain size proxies (including TiO2/CaO, Rb/Sr ratios, and grain size end-member) were used to establish variations of terrigenous input into the study core since 7400 cal yr BP. Based on these records, the evolution of weathering and terrigenous input processes in the western Sunda Shelf can be divided into four stages. During stage 1 (7400–3700 cal yr BP), increasing precipitation and decreasing temperature jointly balanced the relatively stable weathering and terrigenous sediment supply. Dramatically decreasing weathering rates were consistent with less rainfall and lower temperatures during stage 2 (3700–2600 cal yr BP). Heavy rainfall played a more important role than low temperature in controlling weathering and erosion, leading to increasing terrigenous input in stage 3 (2700–1600 cal yr BP). Because of the decoupling between weathering, erosion, and climate in the late Holocene (stage 4, since 1600 cal yr BP), increasing agriculture and related human activities likely dominated weathering and erosion relative to climate changes. Furthermore, the initial time at which human activity overwhelmed natural processes in the southern South China Sea (SCS) is similar to that in the northern SCS. Our results highlight that human activities during the past 1600 years have gradually overwhelmed natural climatic controls on weathering and erosion processes in the western Sunda Shelf.