Geochemical Characteristics and Provenance of Holocene Sediments (Core STAT22) in the Beibu Gulf, South China Sea

The opening of the Qiongzhou Strait during the Holocene was a significant geological event in the Beibu Gulf, profoundly influencing sediment provenance and ocean circulation systems. Due to the scarcity of geological records documenting this event, the understanding of regional Holocene sedimentary evolution has been constrained. To investigate the impact of this event on sediment provenance and ocean currents in the Beibu Gulf, geochemical analyses were conducted on sediment core SO-31 retrieved from the South China Sea. The sediments in core SO-31 were stratigraphically divided into three units based on vertical geochemical profiles, reflecting changes in sea level and shifts in sediment provenance within the study area. The Th/Cr vs. Th/Sc scatter plot for core SO-31 indicate that sedimentary materials primarily originated from the Red River during 11,400–7700 a BP, and a significant change in provenance occurred in the study region around 7700 a BP, characterized by increased contributions from the Qiongzhou Strait and decreased contributions from the Red River. This suggests that the opening of the Qiongzhou Strait significantly influenced the sediment supply to the central Beibu Gulf around 7700 a BP. These findings provide critical geochemical evidence for studying the Qiongzhou Strait opening event and enhance our understanding of Holocene sedimentary evolution and “source–sink” transitions in the Beibu Gulf.

Bauer, A.; Radziejewska, T.; Liang, K.; Kowalski, N.; Dellwig, O.; Bosselmann, K.; Stark, A.; Xia, Z.; Harff, J.; Böttcher, M.E.; Schulz-Bull, D.E., and Waniek, J.J., 2013. Regional differences of hydrographical and sedimentological properties in the Beibu Gulf, South China Sea. In: Harff, J., Leipe, T., Waniek, J.J., and Zhou, D. (eds.), Depositional Environments and Multiple Forcing Factors at the South China Sea's Northern Shelf, Journal of Coastal Research, Special Issue, No. 66, pp. 49–71. Coconut Creek (Florida), ISSN 0749-0208.Analyzing the Beibu Gulf's hydrography and sediment properties is crucial for the understanding of naturally and anthropogenically induced matter and energy fluxes in the South China Sea's north-western coastal regions. For this reason, the present study combines hydrographical (T, S, σt, chlorophyll, nutrients, suspended particulate matter) and sedimentological (grain size, pore water properties, phosphate speciation, foraminifera, plant pigment contents) investigations.On the basis of hydrographical profiles (temperature, salinity and σt) taken at 25 stations, four ecological zones are identified in the study area for the sampling period in September/October 2009. These zones are mainly influenced by riverine input and tidal mixing, water mass transport through the Qiongzhou Strait which also affects the gulf's circulation, and South China Sea waters in the southern Beibu Gulf. The zonation extends from the coastal areas in the northern Beibu Gulf and west of Hainan Island across the central regions to the southern part of the gulf. The study demonstrates that the hydrographical peculiarities of the different zones influence not only the biogeochemical features (chlorophyll, nutrients, suspended particulate matter) of the water column but also the deposition of sediments and their biological (plant pigment contents and foraminifera) and geochemical (pore water properties) characteristics. Both, the near-shore area and the zone in the vicinity of the Qiongzhou Strait show relatively high chlorophyll concentrations and therefore give evidence of enhanced primary production in the entire water column. Whereas the grain size and the foraminifera in the surface sediments follow the hydrodynamically controlled sedimentation conditions, plant pigment contents in the surface sediments additionally follow the productivity pattern in the water column. Depending on the depositional environments with their respective sedimentology and organic matter contents, the geochemical sediment properties reflect the primary production within the water column as well.

A total of 388 surface sediment samples taken from the northern South China Sea (SCS) continental shelf were analyzed to characterize the signature of their rare earth elements (REEs). The average REEs concentration was 192.94 μg/g, with a maximum of 349.07 μg/g, and a minimum of 32.97 μg/g. The chondrite-normalized REEs pattern exhibits a remarkably light REEs accumulation, a relatively flat heavy REEs pattern, and a negative Eu anomaly. We subdivided the study area into three zones using the characteristics of REEs and statistical characteristics. Zone I: continental shelf off western Guangdong Province. Here, the sediment provenance is mainly river-derived from the Pearl River, Taiwanese rivers, and those in the adjacent area. Zone II: Qiongzhou Strait and Leizhou Peninsula. Here, the sediment provenance consists of the Qiongzhou Strait and the Hainan Island. Zone III: Hainan Island and SCS slope sediments are dominated. The REEs compositions are mainly controlled by source rock properties, hydrodynamic conditions, and an intensity of chemical weathering. We reconstructed the sediment dispersal and transport route using the REEs compositions, grain size, and other geochemical characteristics throughout the study area.

Located in northwestern South China Sea (SCS), the Beibu Gulf constitutes an environmentally sensitive region shaped by land-ocean-atmosphere interactions in Asia between the western Pacific and eastern Indian Oceans. This study aims to provide a comprehensive view of the sub-fossil diatom biogeography, distribution pattern and oceanographic environmental controls with support of multivariate methods based on Beibu Gulf core-top samples. Cluster analysis of diatom assemblages divides the distribution pattern into four subclusters. Sea surface salinity (SSS), temperature (SST), trophic state (chlorophyll a concentration in this study) and water depth constrain the diatom distribution pattern through canonical redundancy analysis although only partly support an interpretation of the relationship between these various variables. Chlorophyll a has a strong correlation to diatom distribution, and responds to Paralia sulcata occurrence, while SSS and SST also have significant influence and indicate warm water invasion from the open SCS. Water depth is a subordinate factor in terms of Beibu Gulf diatom distribution. The ca. 25 m water-depth marks the upper extent of Paralia sulcata dominance in the northern Beibu Gulf. A strong mixing area with a complex diatom distribution exists below this water depth in the middle of Beibu Gulf. Coastal currents from north of SCS invade Beibu Gulf through Qiongzhou Strait and south of Hainan Island, as recorded by higher percentages of Paralia sulcata and Cyclotella striata at these sites. Our results provide a selection of evaluation method for a marine ecological red-line definition for sustainable development. This study highlights the perspective relationships between the spatial distribution of sub-fossil diatom assemblages in surface sediments and oceanographic variables, which could serve as a model for paleoenvironmental and paleoclimatic reconstruction in future marginal sea geoscience research for the Beibu Gulf, northwestern SCS.

To understand the evolution of the Qiongzhou Strait and ancient coastlines in the Beibu Gulf - Leiqiong area since the Cenozoic era, and to reveal its implications for regional land-sea pattern changes and global climate change. This article reconstructs the changes of ancient coastline and the evolution process of Qiongzhou Strait in the Beibu Gulf - Leiqiong area since the Cenozoic era based on borehole data. In the Paleogene, the Beibu Gulf formed a NEE trending disconnected fault basin and filled with river lake sedimentary facies. In the late Oligocene, seawater intermittently invaded the ancient Beibu Gulf lake and connected the isolated fault basin;In the Early-Middle Miocene（23.3~10.4 Ma）, the coastline in the northwest of the South China Sea rapidly retreated, and the ancient lake in the Beibu Gulf evolved into the ancient Qiongzhou Strait. In the Late Miocene to Pliocene （10.4~2.58 Ma）, the coastline continued to retreat, forming a wide ancient Qiongzhou Strait, Early Pleistocene regression and volcanic eruptions led to the shrinkage of the ancient Qiongzhou Strait；Frequent climate fluctuations during the late Early Pleistocene to late Pleistocebe controlled the continuous transformation of fjords and land. The significant regression during the last glacial maximum directly led to the transformation of the Beibu Gulf-Leiqiong area from sea to land; Since 15 ~ 12 ka BP, the coastline has rapidly retreated and briefly stopped between 12 and 11 ka BP, and the Beibu gulf has once again transitioned from land to sea, Afterwards, the sea level continued to rise, and the Qiongzhou Strait fully opened from west to east at 11 ka BP. By 6 ka BP, the sea level reached about 2 meters above the current sea level, forming the current sea land pattern. The results indicate that the Beibu Gulf - Leiqiong Area underwent four evolutionary stages in the Cenozoic, including the Paleogene Beibu Gulf ancient lake, the Neogene to Early Pleistocene ancient Qiongzhou Strait, the late early Pleistocene to late Pleistocene fjords, and the Holocene Qiongzhou Strait.

Geochemical investigations of sediments from core GC22 from the Tonkin Gulf, South China Sea, have been carried out in order to reconstruct the palaeoenvironmental evolution of the area during the Holocene. Vertical variations in Al/Ti, K/Al and Mg/Al clearly indicate the degree of chemical weathering in the source area. Zr/Ti and SiO 2 /Al 2 O 3 recorded the history of current velocity changes, and La/Co v. La/Sc, combined with distribution patterns of rare earth elements, suggested that Hainan Island was the main source of the sediments during the Holocene. Based on the results of the analysis, the evolution of palaeoenvironments in the Tonkin Gulf can be divided into four stages: (1) 10.12–6.46 ka BP, the regional climate got warmer, and the sea level of the gulf rose rapidly, which is indicated by rapid declines in Sr/Ba and CaO. (2) 6.46–4.3 ka BP, the gulf had a stable depositional environment, and the local climate became cold and dry. (3) 4.3–3.55 ka BP, the currents and sedimenary provenance in the gulf were significantly influenced by the opening of the Qiongzhou Strait. (4) 3.55 ka BP–present, the regional sea level remained roughly stable.

Based on the three-dimensional ECOM model, the tide, tide-induced residual current, wind-driven and density currents in the Beibu (Tonkin) Gulf and Qiongzhou Strait are diagnostically computed in fine grid. The tides and tidal currents in the Beibu Gulf and Qiongzhou Strait are well reproduced. The model results show that the semidiurnal tidal wave propagates eastward from the Beibu Gulf through Qiongzhou Strait, while diurnal tidal waves enter the strait from both the eastern and western sides and interact on the southeast coast of the strait. The formation processes of the residual currents in Qiongzhou Strait in summer (August) and winter (January) are mainly discussed. It is shown that the total residual currents (coupling effect of wind, tide and density) in the strait are westward in both summer and winter. The water volume transported from the east to west into the Beibu Gulf is 0.026 Sv in summer and 0.116 Sv in winter. Numerical experiments indicate that the dominant factor affecting the residual currents in the strait is not the wind stress but the tidal rectification. The westward tide-induced residual current, driven by the tidal rectification, is rather strong in the strait with water volume transport of 0.063 Sv. The wind in summer only reduces the westward tide-induced residual current to a certain extent, and the wind in winter approximately doubles the westward tide-induced residual current through the strait. The density current contributes little to the total residual current in both summer and winter.

Xia, Z.; Waniek, J.J., and Leipe, T., 2013. Anthropogenic fingerprint in Beibu Gulf (South China Sea) sediments. In: Harff, J., Leipe, T., Waniek, J.J., and Zhou, D. (eds.), Depositional Environments and Multiple Forcing Factors at the South China Sea's Northern Shelf, Journal of Coastal Research, Special Issue, No. 66, pp. 72–90. Coconut Creek (Florida), ISSN 0749-0208.Surface sediments and five cores were chosen to determine grain size, geochemical elements, heavy metals concentrations and age using 14C dating to study sedimentary environment and anthropogenic impact in the Beibu Gulf (South China Sea). The grain size analysis shows differences in the depositional environment amongst the subareas of the gulf. In the north of the gulf, the depositional environment is strong and unstable with complicated hydrodynamics because of the combined influence of rivers, tide, littoral current and the monsoon. In the central part, the depositional environment is weaker and stable, whereas in the south of the gulf (at the entrance), the deposition is higher and influenced mainly by tide. The deposition rate is around 0.3 mm/yr based on 14C dating. The geochemical element analysis indicates different sediment sources in different subareas of the gulf and possible influence of a biogenic source. The terrigenous elements (Al, Fe, Li, Ti, K and Zr) have high positive correlation coefficients, and originate from the northern coast and Hainan Island. The trace elements are mostly enriched within the fine sediments. The enrichment factors (EF) and cultural enrichment factors (CEF) based on aluminum and titanium show that total organic carbon (TOC), As and Hg have high concentrations in the surficial sediments in the north (9.92 mg/kg and 34 μg/kg) and the south (15.5 mg/kg and 22 μg/kg) of the gulf, and are enriched below the surface. Despite the regional differences in heavy metal concentrations in surface sediments, no anthropogenic impact was observed in the center of the gulf according to the results of EF and CEF using the core data. Measured concentrations of the anthropogenic elements were below the evaluation criterion values of the National Standards of GB18668-2002, P. R. China indicating low anthropogenic impact in the entire gulf.

Long-lived radionuclides in marine sediments from the Beibu Gulf, South China Sea: Spatial distribution, controlling factors, and proxy for transport pathway

Tracing the source of Pb using stable Pb isotope ratios in sediments of eastern Beibu Gulf, South China Sea

Baroclinic responses of Qiongzhou Strait throughflow to different forcings

Review of the circulation in the Beibu Gulf, South China Sea

Based on a comparison of synchronized temperature and salinity data collected in the eastern Qiongzhou Strait and at coastal marine stations, this study finds that, in summer, the variation in salinity near the Weizhou Island in Guangxi is similar to that in the eastern and central portions of the Qiongzhou Strait. Additionally, the Beihai Station in Guangxi exhibits a small salinity variation, whereas the Longmen and Bailongwei Stations, both of which are located far from the Qiongzhou Strait, mainly exhibit continental hydrological characteristics in summer. Moreover, a comparison of the multi-year ocean current data from the Qiongzhou Strait and ocean current observations from the Weizhou Island Station and recently installed current-measuring stations shows that the residual current in the Qiongzhou Strait flows westward in winter and summer. The numerical simulation results also indicate that water from the eastern Qiongzhou Strait enters the Beibu Gulf. The characteristics of the temperature and salinity distributions and analyses of the residual currents further confirm that the western Guangdong coastal current is the main source of the westward transport of water in the Qiongzhou Strait. The primary driver of the formation of the western Guangdong coastal current is the westward flow of freshwater from the Zhujiang (Pearl) River. This water enters the Beibu Gulf via the Qiongzhou Strait and enhances the formation of the cyclonic circulation in the northern Beibu Gulf. In summer, the strong influence of the southwesterly wind leads to the formation of a strong northward coastal current along the western coast of the Beibu Gulf. This process promotes the transport of low-salinity diluted water toward the open ocean and the formation of larger-scale cyclonic circulation around Weizhou Island in the eastern Beibu Gulf. The results of this study regarding the effects of the water inflow from the eastern Qiongzhou Strait to the Beibu Gulf on the Guangxi coastal circulation directly challenge conventional conclusions concerning the transport direction of water through the Qiongzhou Strait in winter and summer.

The Beibu Gulf, located in the northwestern South China Sea, is rich in oceanic energy and biological resources. Based on the FVCOM model, a three-dimensional numerical model was adopted in the study, which was validated with the observation data. The results show that Beibu Gulf is a typical diurnal tidal area. The K1 and O1 tide have an amphidromic point near Hue, Vietnam, forming a rotary tidal system. From the amphidromic point, the amplitudes increase gradually, eventually reaching more than 80 cm and 100 cm, respectively. The maximum amplitudes of M2 and S2 tide are about 70 cm and 10 cm, respectively. Meanwhile the tidal current presents a typical rectilinear flow in Beibu Gulf, with strong the diurnal tidal currents. Seasonal changes in the circulation structure of Beibu Gulf are obvious. The Beibu Gulf is generally controlled by a non-closed anticlockwise circulation in winter. In summer, there are two counterclockwise eddies. The tidal residual current flowing through Qiongzhou Strait goes west all year round, with the largest average flow flux of 0.07 Sv in winter. Based on DYE module from FVCOM, pollutants from the Pearl River Estuary pass through Qiongzhou Strait, and reach Weizhou Island after 109 days. This is of great significance to the monitoring and control of red tide, and the protection of marine ecological environment in Beibu Gulf.

The increasing and decreasing water level in Guangxi near the coast is mainly caused by typhoon-induced storm surges. The maximum values of increase and decrease recorded at the Beihai Station over the last 42 years are 1.45 m and -1.87 m. The water level first decreases and then increases in Guangxi near the coast. Apart from the direct influence of typhoons, the westward coastal-trapped wave along the Guangdong continental shelf is also greatly affected. A portion of the westward coastal-trapped wave passed directly through the Qiongzhou Strait into Beibu Gulf, and the other portion passed through the southern part of Hainan Island into the Beibu Gulf. On July 2, 2001, the westward coastal-trapped wave caused by Typhoon Durian induced a strong westward flow along the Guangxi coast, and the geostrophic flow velocity on the surface reached 92cm/s. On July 6, 2001, though Typhoon Utor landed at the northern part of the Pearl River Estuary, it widely induced a 20-cm increase in the water level in Guangxi near the coast on July 8. Apart from the direct and indirect influences of typhoons on the fluctuation of the water level along the Guangxi coast, the particular topography and atmospheric gravity wave also have affect the fluctuation of the water level.