Search

In this study, we explored the heavy metal elements in 42 surface sediments from the Malacca Strait in terms of distribution, controlling factors, environmental quality, and primary sources. An analysis of grain size revealed finer sediments near the coast of Malaysia, which gradually thickened toward offshore. In addition, heavy metal elements were abundantly distributed near the coastal area of Malaysia, with a gradual decrease toward the sea; their content increased within waters close to the Perak estuary. Source analysis of heavy metals showed that Cr, Hg, Zn, Cd, and Cu were mostly derived from natural weathering, and their distribution was significantly influenced by sediment grain size. As and Pb were affected by human activities. The environmental quality assessment results showed that Cu, Cr, Cd, and Zn in our study regions were pollution-free. Pb elements showed low-to-moderate pollution, and Hg showed a certain degree of ecological risk due to its high toxicity coefficient. The content of As elements in surface sediments increased significantly when compared to the background value, with several evaluation methods indicating a high-risk index. According to these findings, the area near the mouth of the Perak River is the most polluted, followed by the surrounding coastal areas. • Cu, Cr, Cd, Zn were pollution-free, Pb showed low-to-moderate pollution. • Hg showed a certain degree of ecological risk due to its high toxicity coefficient. • As element within surface sediments showed a high-risk index. • The Perak River mouth is most polluted, followed by the surrounding coastal areas. [ABSTRACT FROM AUTHOR]

The heavy metal contents (Cr, Cu, Zn, Cd, Pb, Hg, and As) of 88 surface sediment samples from the western Sunda Shelf were analyzed to determine their spatial distribution patterns and contamination status. The results demonstrated that high enrichment regions of heavy metals were focused in the Kelantan, Pahang, and Ambat river estuaries, and deep water regions of the study area. These high enrichment regions were mainly controlled by riverine inputs and their hydrodynamic conditions. The enrichment factor (EF), geoaccumulation index (I geo), and potential ecological risk index (PERI) were used to assess heavy metal accumulation. The results indicated that the study area was not significantly contaminated overall at the time of the study; however, Cd, As, and Hg were at levels corresponding to moderate contamination at many stations located in the Pahang River estuary, Kelantan River estuary, and north-eastern region of the study area, primarily because of anthropogenic activities. • The heavy metal contents of surface sediment samples from the western Sunda Shelf were analyzed. • High heavy metal enrichment regions were mainly controlled by riverine inputs and hydrodynamic conditions. • The western Sunda Shelf was not significantly contaminated overall at the time of the study. [ABSTRACT FROM AUTHOR]

To understand the modern sedimentation and land-sea interaction processes in the Sunda Shelf, we evaluated the rare earth element (REE) compositions, the total organic carbon (TOC) contents, grain sizes, and detrital minerals of 39 surface sediments, collected from the western Sunda Shelf. All of the sampled sediments are characterised by a higher content of light REEs (LREEs) relative to the content of heavy REEs (HREEs) with negative Eu anomalies. The total content of REE (ΣREE) in the sediments ranges from 7 μg/g to 193 μg/g, with an average of 119 μg/g. Due to the uneven spatial distribution of REEs in the study area, a Q-cluster analysis is applied to the mean grain size (Mz), ∑REE, δEu, δCe, (La/Yb) N , and (Gd/Yb) N data. On this basis, the study area can be classified into three geochemical provinces. The north to central zone in the study area is designated as province I, which has the highest REE content among the three provinces. This is a result of the finer grain size, dominance of siderite, and higher TOC contents of the sediments in province I. The REE content of province II in the southwest coastal zone is lower than that of province I due primarily to a coarser grain size and high quartz, biodetritus, and plagioclase contents. Covering the central coastal zone, province III has the lowest REE content among the three provinces because of the extremely coarse grain size and very high quartz and biodetritus contents. The upper continental crust (UCC)-normalised REE patterns differ markedly among the three provinces, thereby indicating different provenances and transport mechanisms. Based on the provenance discrimination plot and the UCC-normalised REE patterns of these three provinces and adjacent rivers, we infer that province I is largely fed by the Mekong and Kelantan rivers. The fine-grained sediments from these rivers are transported to north and central zone by the northeast monsoon current. The Pahang River is identified as the main sediment source of province II. The coarse-grained sediments from this river mostly accumulate near the estuary via hydrodynamic sorting, while finer-grained sediments are transported to the south by the southward coastal currents. The sediments in province III are inferred as being primarily the result of coastal erosion due to the strong wave energy in this area, as well as offshore and longshore sediment transport acting, in parallel to the coast line. Our study can serve as a reference for studying the diffusion and transport processes of mountainous river sediment and the evolution of the paleoclimate and paleoenvironment in tropical low-latitude regions. • Surface sediments of the western Sunda Shelf can be classified into three geochemical provinces. • Grain size and heavy minerals play important constraint roles on REE distribution. • Different geochemical provinces have their unique provenances. • Erosion, transport and deposition are mainly controlled by northeast monsoon current, coastal current and wave energy. [ABSTRACT FROM AUTHOR]

Grain size is an important controlling factor of the chemical composition of sediments. To understand the role of grain size in the content of rare earth elements (REE) in tropical mountainous river sediments and to establish suitable provenance proxies, sediments collected from Pahang River and Kelantan River, Peninsular Malaysia, were determined for REE contents in six size fractions (<2 μm, 2–4 μm, 4–8 μm, 8–16 μm, 16–32 μm, and 32–63 μm). The results indicate that the total REE contents (∑REE) from these two rivers are weakly correlated with grain sizes. The normalized REE patterns of various grain-size fractions to <63 μm fractions revealed a flat pattern for light rare earth elements (LREE), but were gradually enriched in heavy rare earth elements (HREE) with increasing grain size. These results indicate that the grain size constraint on REE is very complex. The relationship between REE contents and mineral compositions of size-fractioned sediments further suggests that clay minerals cause high REE contents in clay components (<2 μm). The dilution effect of quartz and feldspars on REE is another controlling factor, leading to low REE content in medium-fine silt components (2–32 μm). In addition, heavy minerals, zircon, monazite and amphibole, play an important role in enriching REE in coarse silt components (32–63 μm), especially zircon as the major contributor to the enrichment of HREE with increasing grain size. By combining the content of REE, the normalized REE patterns and the REE characteristic parameters for different grain sizes, we find that the 4–8 μm fraction is sensitive to REE variation among the six size fractions investigated. Furthermore, the relationship between upper continental crust-normalized δEu and (Gd/Yb) is reliable for distinguishing the sediment characteristics of Pahang River and Kelantan River and can therefore be used as an effective indicator for identifying Malaysian river-sourced sediments in the southern South China Sea. • REE compositions in different grain-size fractionated sediments from two tropical mountainous rivers are different. • Mineral composition is the main controlling factor of the grain size effect on REE. • The 4–8 μm fraction represents REE variation sensitively among six size fractions investigated in these two rivers. • δEu UCC -(Gd/Yb) UCC discriminant diagram can be used to distinguish Malaysian river-sourced sediments in the southern SCS. [ABSTRACT FROM AUTHOR]

The release of hydrogen sulfide by sulfate-driven anaerobic oxidation of methane (SD-AOM) during methane seepage activities has a strong impact on the evolution of marine redox conditions and the cycles of redox-sensitive elements (e.g. Mo and U). Consequently, the sedimentary enrichment or depletion of these elements is often used to trace and reconstruct (paleo-)methane seepage activity. In recent years, tungsten (W) has shown promise as an indicator of marine redox changes, but its applicability to trace methane seepage activity is unknown. This study identified significant differences in the W content of sediments in methane seepage environments compared to suboxic environments not influenced by methane seepage and oxic environments in the South China Sea. The sediments subjected to methane seepage were characterized by low W contents, similar to those of euxinic Black Sea sapropels. It is believed that the sulfidic environment resulting from SD-AOM is the primary cause of W depletion in sediments subjected to methane seepage. By examining W's interaction with its twin element, Mo, we present W Mo enrichment patterns in various environments. In methane seepage environment (sulfidic environments), sediments had high Mo EF (> 10) and low W EF (< 2) with a high Mo/W molar ratio (> 10); in suboxic environments not influenced by methane seepage, sediments exhibited a W Mo enrichment pattern similar to the UCC (the Mo EF and W EF values are predominantly below 1 and 2, respectively, with a low Mo/W molar ratio, <1.5); in oxic environments (enrichment of Fe Mn (oxy)hydroxides), sediments exhibited high Mo EF (2 < Mo EF < 10) and high W EF (> 2), with a medium Mo/W molar ratio (1.5–10). This further indicates that the Mo/W molar ratio has great potential in tracing and reconstructing (paleo-)methane seepage activities. [ABSTRACT FROM AUTHOR]

• The residual sediments were concentrated in the central of the Malacca Strait, while the modern terrigenous sediments were distributed along the coastal side. • The sedimentary environment in the Malacca Strait presented residual, reworked, and modern characteristics. • The sedimentary dynamics restricted the transport of modern terrigenous sediments from the coast to the central region of the Malacca Strait. The Malacca Strait is not only a part of shelf deposition but also a crucial conduit connecting the continental shelf to the deep ocean. Here, we present the grain size characteristics of sediments in the Malacca Strait and regional hydrographic data obtained from global models to delineate its depositional configurations and further identify its sedimentary environments. The results reveal that the residual sediments are concentrated in the central region of the strait, whereas the modern terrigenous sediments are distributed along the coast of Malaysia and the broad northern region of the strait. The central region of MS, ProvinceⅠ, has the coarsest sediments with moderate sorting, indicating a residual sedimentary environment where the sands are residual paleo-river deposits. There are poorly sorted silty sands in ProvinceⅡ, which display finer mean grain size than that of ProvinceⅠ. It suggests a reworked sedimentary environment with variations of sediment sources between the southern and northern regions. It is a palimpsest of the paleo-deltaic deposits in the southern region, which have been intensely altered by modern high-energy hydrodynamic conditions. In terms of the northern part of ProvinceⅡ, it has modern reworked deposits shaped by wave, tide, and monsoon currents. The finer sediments, which correspond to the distribution of higher silt content, are supplied by the surrounding land in ProvinceⅢ indicating a modern environment primarily influenced by seasonally varying sedimentary dynamics, such as monsoon currents, well-developed eddies, and water masses. We thus propose that the depositional configurations in the Malacca Strait are confined by depositional dynamic environments, which restrict modern terrigenous sediments distributed along the coast and scarcely transported into the central region of the strait which exposes residual deposits. This study highlights the importance of both modern and relict sediments in the strait and our findings are important to better understanding modern strait sedimentary systems with high tidal-current-energy. [ABSTRACT FROM AUTHOR]