## 2017 Vol. 36, No. 4

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[Abstract](594)
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2017, 36(4): 1-12. doi: 10.1007/s13131-017-1011-5
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According to a series of important historical maps, i.e., the Location Map of the South China Sea Islands, the Nansha Islands, Zhongsha Islands, Xisha Islands, Yongxing Island and Shidao Island, and Taiping Island (archived by the Territorial Administration Division of the Ministry of Interior of Republic of China in 1946), and the Administration District Map of the Republic of China published in 1948, the dashed line surrounding the South China Sea Islands represents China's sea boundary in the South China Sea at that time. It was both connected with, and an extension of, the land boundary of China. At that time the dashed line was used to represent the waters boundaries while the solid line was used to represent the land boundary-a universal method used in maps that was then recognized internationally. The above observation provides historical and scientific evidence of China's sea boundary in the South China Sea that is useful for the international maritime delimitation over the South China Sea area.
2017, 36(4): 13-20. doi: 10.1007/s13131-017-0973-7
[Abstract](1191) [PDF 4582KB](1046)
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A buoy of 10 m in diameter was used to record the current speed and direction in a vertical profile in the offshore area of the Changjiang (Yangtze River) Estuary (with an average water depth of 46.0 m) for one year. The results include:(1) the currents rotate clockwise and the current direction is consistent in a vertical profile without clear seasonal variations. (2) The horizontal current speeds are generally high, with a maximum of 128.5 cm/s occurring in summer and 105.5 cm/s appearing in winter commonly close to the surface. The average current speeds in the vertical profile fall in the same range (the differences are less than 8.0 cm/s), with the maximum of 47.0 cm/s occurring in summer and 40.8 cm/s in winter. The average current speed during spring tides is twice that during neap tides (26.5 cm/s). (3) Significant differences of speeds are observed in the vertical profile. The maximum current speed occurs at either surface (spring and winter) or sub-surface (summer and autumn), with the minimum current speed appearing at the bottom. The maximum average current speed of all layers is 57.9 cm/s, which occurs in the 18-m layer during summer. (4) The average speed of the residual currents ranges from 7.5 cm/s to 11.3 cm/s, with the strongest occurring in spring and weakest in winter. The residual currents of all layers are eastward during spring and winter, whereas northeastward or northward during summer and autumn. (5) The currents in the offshore of Changjiang Estuary are impacted collectively by diluted Changjiang River discharge, the Taiwan Warm Current, monsoon and tides.
2017, 36(4): 21-30. doi: 10.1007/s13131-017-1012-4
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Water circulation and sediment transport in the Beibu Gulf are important for its environmental protection and resource exploitation. By employing the Regional Ocean Modeling System (ROMS), we studied the seasonal variation of circulation, sediment transport and long-term morphological evolution in the Beibu Gulf. The simulation results show that the circulation induced by tide and wind is cyclonic both in winter and summer in the gulf and that the wind-driven circulation is stronger in winter than that in summer. The sediment concentration is higher in the Qiongzhou Strait, west of the Hainan Island and the coast of Vietnam and the Leizhou Peninsula. The sediment is transported westwards in winter and eastwards in summer in the Qiongzhou Strait. The west entrance of the Qiongzhou Strait is dominated by westward transport all the year round. The sediment discharged by rivers is deposited near the river mouths. The simulated result demonstrates that the sediment transport is mainly controlled by tidal induced bottom resuspension in the Beibu Gulf. Four characteristics are summarized for the distribution patterns of erosion and deposition. (1) The erosion and deposition are insignificant in most area of the gulf. (2) Sediment deposition is more significant in the mouths of Qiongzhou Strait. (3) The erosion is observed in the seabed of Qiongzhou Strait. (4) Erosion and deposition occur alternatively in the west of Hainan Island.
2017, 36(4): 31-36. doi: 10.1007/s13131-017-1013-3
[Abstract](1128) [PDF 973KB](1092)
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The Dongsha area is one of the most promising target areas for gas hydrate exploration in the South China Sea (SCS). The study of pore water geochemistry has played a key role in Chinese gas hydrate exploration. Br/Cl, I/Cl and δ37Cl in pore water were applied here in tracing gas hydrate occurrence, chemical evolution of pore fluids and water/rock interactions in low temperature sediment environments. The samples were collected from Sites HD255PC and HD309PC in the Dongsha area in 2004. At Site HD255PC, we found the elevated Br/Cl, I/Cl and decreased SO4/Cl at the depth of 4-5 m, suggestive of a laterally migrated fluid probably generated from the gas hydrate occurrence. The range of δ37Cl is -0.54‰ to +0.96‰, and positive δ37Cl at 4-5 m interval should be related with different diffusion rates between 35Cl and 37Cl. At Site HD309PC, a laterally migrated fluid was also found at the depth of 3-4 m, with the Br/Cl two times to that of the seawater and decreased I/Cl, indicating the fluid has no relationship with the gas hydrate. In this site, the chlorine isotopic composition varies from -0.7‰ to +1.9‰. Extra high Br/Cl might relate with the deep generated fluid. At higher temperature and pressure, the Br/Cl of the fluid is elevated during the hydrous silicate formation, while positive δ37Cl is also associated with the same mechanism.
2017, 36(4): 37-45. doi: 10.1007/s13131-016-0918-6
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Coastal lagoon deposits provide evidence for the magnitude and frequency of past tropical cyclones prior to instrumental records and historical documentation. In the present study, we attempt to analyze the sedimentary records containing typhoon information for the northern South China Sea region. For this purpose, sediment cores were collected from two coastal lagoons in the southeastern Hainan Island, and were analyzed in laboratory to derive the data sets about grain size, organic and inorganic carbon contents, and deposition rates. The grain size and organic-inorganic carbon data were used to formulate the proxies of typhoon events. The deposition rates, as calculated using the CRS 210Pb method, are around 0.5 mm/a for both lagoons, on the basis of which an age model is established. Within the cores, sedimentary layers associated with 35 typhoon events have been identified. On such a basis, a 350 year history of local typhoon activities is reconstructed by incorporating the 210Pb dating results, typhoon-induced sedimentation patterns and the historical documents. A comparison of the frequency of typhoon occurrence with the regional climate records indicates that the observed changes in tropical cyclone activity patterns, as revealed by the lagoon sedimentary records, may be related to El Niño, Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), sunspot, and other potential climate drivers that affect the tropical cyclone variability. This study demonstrates that the sedimentary record of storms can be analyzed in combination with historical documents, to provide meaningful information on past storm activities and their long-term variability.
2017, 36(4): 46-53. doi: 10.1007/s13131-017-1053-8
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In this study, about 220 satellite images between 2000 and 2012 were obtained from FY-series, MODIS, CBERS, HJ-1A and HJ-1B to estimate the impact of duststorms on the South Yellow Sea (SYS), which serve as an important source of particles there. The analyzing results from the images support a total occurrence of 88 duststorms (including the locally-generated dusty weather) that affected the SYS during 2000-2012. The annual occurrence was about 4-10 times (10 times in 2000 and 2004; four times in 2009 and 2012), predominantly in March (29%), April (33%) and May (22%). By mapping the distribution of their frequency, the duststorms influencing the SYS were found primarily moving from the northwest (39 times, 44.3%) and west (37 times, 42%) to the study region with only 11 duststorms (12.5%) coming from the north and 1 duststorm (1%) from the southwest. We estimated that an annual amount of 0.5-3.5 million tons of sediment particles was brought to the SYS by the duststorms during 2000-2012.
2017, 36(4): 54-63. doi: 10.1007/s13131-016-0941-7
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The response to the catchment changes of the sedimentary environment of the western intertidal flat of Yalu River Estuary was investigated by analyzing the vertical variations of the grain size of sediment cores, along with the hydrologic data and human activities in the catchment. The results demonstrated a stepwise decreasing trend for the variations of both the sediment load and water discharge into the sea, which could be divided into three stages as 1958-1970, 1971-1990 and 1991-2009. Reservoir construction and the changes of catchment vegetation coverage turned out to be the two predominant contributors to the changes. There are four periods for the variation of the sensitive components of the sediment cores from 1940 to 2010, i.e., 1940-1950, 1951-1980, 1981-1990 and 1991-2010. The vertical distribution of grain size in the cores mainly varied with the changes of vegetation coverage in the catchment and reservoir construction from 1960 to 1980, whereas it varied depending on the intensity of water and soil erosion in the catchment from 1980 to 1990. Despite the further reduction of the water and sediment input into the sea from 1990 to 2009, this period was characterized by coarsening trends for the grain size of sediment in the estuarine intertidal flat and correspondingly, the significantly increased silt contents of the sensitive component.
2017, 36(4): 64-71. doi: 10.1007/s13131-016-0939-1
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An understanding of the sedimentary environment in relation to its controlling factors is of great importance in coastal geomorphology, ecology, tourism and aquaculture studies. We attempt to deal with this issue, using a case study from the Xincun Lagoon, Hainan Island in southern China. For the study, surficial sediment samples were collected, together with hydrodynamic and bathymetric surveys, during August 2013. Numerical simulation was carried out to obtain high-spatial resolution tidal current data. The sediment samples were analyzed to derive mean grain size, sorting coefficient, skewness and kurtosis, together with the sand, silt and clay contents. The modern sedimentary environments were classified using system cluster and principal component analyses. Grain size analysis reveals that the sediments are characterized by extremely slightly sandy silty mud (ESSSM) and slightly silty sand (SSS), which are distributed in the central lagoon and near-shore shallow water areas, respectively. Mean grain size varies from 0 to 8.0Ф, with an average of 4.6Ф. The silt content is the highest, i.e., 52% on average, with the average contents of sand and clay being 43% and 5%, respectively. There exists a significant correlation between mean size and water depth, suggesting that the surficial sediments become finer with increasing water depth. Cluster analyses reveals two groups of samples. The first group is characterized by mean grain size of more than 5.5Ф, whilst the second group has mean grain size of below 3.5Ф. Further, these groups also have different correlations between mean grain size and the other grain size parameters. In terms of the tidal current, the average values of the root mean square velocity (RMSV) are 7.5 cm/s and 6.9 cm/s on springs and neaps, respectively. For the RMSVs that are higher than 4 cm/s, a significant positive correlation is found between the content of the 63-125 μm fraction and the RMSV, suggesting that the RMSV determines the variability of the very fine sand fraction. Based on system cluster and principal component analyses (PCA), the modern sedimentary environments are classified into three types according to the grain size parameters, RMSVs and water depth data. The results suggest the importance of grain size parameters and high-spatial resolution hydrodynamic data in differentiating the coastal sedimentary environments.
2017, 36(4): 72-79. doi: 10.1007/s13131-017-1014-2
[Abstract](1040) [PDF 3106KB](1171)
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The exploration of unconventional and/or new energy resources has become the focus of energy research worldwide, given the shortage of fossil fuels. As a potential energy resource, gas hydrate exists only in the environment of high pressure and low temperature, mainly distributing in the sediments of the seafloor in the continental margins and the permafrost zones in land. The accurate determination of the thickness of gas hydrate stability zone is essential yet challenging in the assessment of the exploitation potential. The majority of previous studies obtain this thickness by detecting the bottom simulating reflectors (BSRs) layer on the seismic profiles. The phase equilibrium between gas hydrate stable state with its temperature and pressure provides an opportunity to derive the thickness with the geothermal method. Based on the latest geothermal dataset, we calculated the thickness of the gas hydrate stability zone (GHSZ) in the north continental margin of the South China Sea. Our results indicate that the thicknesses of gas hydrate stability zone vary greatly in different areas of the northern margin of the South China Sea. The thickness mainly concentrates on 200-300 m and distributes in the southwestern and eastern areas with belt-like shape. We further confirmed a certain relationship between the GHSZ thickness and factors such as heat flow and water depth. The thickness of gas hydrate stability zone is found to be large where the heat flow is relatively low. The GHSZ thickness increases with the increase of the water depth, but it tends to stay steady when the water depth deeper than 3 000 m. The findings would improve the assessment of gas hydrate resource potential in the South China Sea.
2017, 36(4): 80-86. doi: 10.1007/s13131-017-0971-9
[Abstract](1129) [PDF 2006KB](1083)
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Large-scaled reclamation modifies the coastal environment dramatically while accelerating the disappearance of salt marshes, which causes the degradation of the coastal ecosystem and the biodiversity function. In this study, we explored the changes of tidal flat and salt marsh coverage in a small-scale tidal flat with an area of~160 000 m2 in the plain coast of Jiangsu Province, China. Human activities (e.g., the construction of dikes) are a crucial contributor that benefits for the tidal flat accretions and the following changes of salt marsh coverage. Located in the front of the man-made "concave coastline", the study area is suitable for sediment accretion after the dike construction in the end of 2006. On the basis of the annual tidal surface elevation survey from 2007 to 2012, the sedimentation rates in the human influenced tidal flat varied from a few centimeters per year to 23 cm/a. The study area experienced a rapid accretion in the tidal flat and the expansion of the salt marsh, with the formation of a longshore bar, and a subsequent decline of the salt marsh. Breaking waves during the flooding tide brought much sediment from the adjacent tidal flat to the study area, which caused burial and degeneration of the salt marsh. The vertical grain size changes within a 66 cm long core in the study area also demonstrated the above changes in the tidal environment. This study indicates that the responses of small-scale tidal flat changes to reclamation are significant, and the rational reclamation would benefit for the new salt marsh formation in front of the dikes. Further research about the evolution of small scale tidal flat as well as the spatial planning of the polder dike should be strengthened for the purpose to maintain a healthier coastal environment.
2017, 36(4): 87-96. doi: 10.1007/s13131-017-1017-z
[Abstract](1156) [PDF 3632KB](1005)
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To evaluate the controlling factors for coastline change of the Changjiang (Yangtze River) Estuary since 1974, we extracted the mean high tide line from multi-temporal remote sensing images that span from 1974 to 2014 at 2-year intervals. We chose 42 scenes to constrain the changing pattern of the Changjiang Estuary coastline, and implemented GIS technology to analyze the area change of the Changjiang (Yangtze) Subaerial Delta. Runoff, sediment discharge and coastal engineering were withal considered in the analysis of the coastline changes. The coastline has transgressed seaward since 1974, and a part of it presents inter-annual variations. The area of the Changjiang Subaerial Delta increased by 871 km2, with a net accretion rate of 21.8 km2/a. Based on the change of sediment discharge due to the major projects in the Changjiang River Basin, we divided the changing pattern of the coastline into three stages:the slow accretion stage (1974-1986), the moderate accretion stage (1987-2002), and the rapid accretion stage (2003-2014). Liner regression analysis illustrated that there is a significantly positive correlation between the area changes and sediment discharge in the Chongming Eastern Shoal and Jiuduansha. This suggested that sediment load has a fundamental effect on the evolution of the Changjiang Estuary. Construction of Deep Waterway in the North Passage of the Changjiang River (1998-2010) led to a rapid accretion in the Hengsha Eastern Shoal and Jiuduansha by influencing the hydrodynamics in North Passage. Coastal engineering such as reclamation and harbor construction can also change the morphology of the Changjiang Estuary. We defined a contribution rate of area change to assess the impact of reclamation on the evolution of Changjiang Estuary. It turned out that more than 45.3% of area increment of the Changjiang Estuary was attributed to reclamation.
2017, 36(4): 97-104. doi: 10.1007/s13131-017-1016-0
[Abstract](1217) [PDF 1441KB](1715)
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Land has been widely reclaimed in large area in coastal zones all over the world to relieve the pressure of land shortage, promoting social development and economic growth. Asia has become a focus of land reclamation with the rapid industrialization and urbanization. From the Binhai New Area of Tianjin to the Caofeidian New Area of Tangshan, the undergoing project of land reclamation on the northwest coast of Bohai Bay, China, is the largest in the world. To clarify the environmental issues and benefit sustainable development of the coastal zone, we conducted both retrospective and predictive assessments of the ecological cost caused by land reclamation on the northwest coast of Bohai Bay, China. We calculated the ecological costs of ten aspects of the four ecosystem services, i.e., supply, regulation, support and culture, with the monetary estimate approach. The results indicate that the ecological cost of the new land reclamation is US$971.9 million from 2000 to 2010 and that the cost will be US$702.1 million from 2010 to 2020. The costs of gas regulation and marine food supply account for the greatest parts of the total value. Suggestions for land reclamation oriented to sustainable development in the study area are put forward, including a rational planning based on the comprehensive evaluation, reducing the amount of land reclamation area, optimizing the structure of the reclaimed land, reclaiming land with the concept of "low impact development" and implementing ecological compensation mechanisms, etc.
2017, 36(4): 105-113. doi: 10.1007/s13131-017-1015-1
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The invasions of the alien species such as Spartina alterniflora along the northern Jiangsu coastlines have posed a threat to biodiversity and the ecosystem function. Yet, limited attention has been given to their potential influence on greenhouse gas (GHG) emissions, including the diurnal variations of GHG fluxes that are fundamental in estimating the carbon and nitrogen budget. In this study, we examined the diurnal variation in fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from a S. alterniflora intertidal flat in June, October, and December of 2013 and April of 2014 representing the summer, autumn, winter, and spring seasons, respectively. We found that the average CH4 fluxes on the diurnal scale were positive during the growing season while negative otherwise. The tidal flat of S. alterniflora acted as a source of CH4 in summer (June) and a combination of source and sink in other seasons. We observed higher diurnal variations in the CO2 and N2O fluxes during the growing season (1 536.5 mg CO2 m-2 h-1 and 25.6 μg N2O m-2 h-1) compared with those measured in the non-growing season (379.1 mg CO2 m-2 h-1 and 16.5 μg N2O m-2 h-1). The mean fluxes of CH4 were higher at night than that in the daytime during all the seasons but October. The diurnal variation in the fluxes of CO2 in June and N2O in December fluctuated more than that in October and April. However, two peak curves in October and April were observed for the diurnal changes in CO2 and N2O fluxes (prominent peaks were found in the morning of October and in the afternoon of April, respectively). The highest diurnal variation in the N2O fluxes took place at 15:00 (86.4 μg N2O m-2 h-1) in June with an unimodal distribution. Water logging in October increased the emission of CO2 (especially at nighttime), yet decreased N2O and CH4 emissions to a different degree on the daily scale because of the restrained diffusion rates of the gases. The seasonal and diurnal variations of CH4 and CO2 fluxes did not correlate to the air and soil temperatures, whereas the seasonal and diurnal variation of the fluxes of N2O in June exhibited a significant correlation with air temperature. When N2O and CH4 fluxes were converted to CO2-e equivalents, the emissions of N2O had a remarkable potential to impact the global warming. The mean daily flux (MF) and total daily flux (TDF) were higher in the growing season, nevertheless, the MF and TDF of CO2 were higher in October and those of CH4 and N2O were higher in June. In spite of the difference in the optimal sampling times throughout the observation period, our results obtained have implications for sampling and scaling strategies in estimating the GHG fluxes in coastal saline wetlands.
2017, 36(4): 114-120. doi: 10.1007/s13131-017-0972-8
[Abstract](1020) [PDF 2671KB](1237)
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Knowledge of coastline changes and vulnerability is of great importance to local government departments that are responsible for the management and development of coastal zones. To study the nature of change and vulnerability along the coasts of the Hainan Island, we collected a large number of sediment samples through the last few years, and reconstructed the changes of the coastline by combining the data of sediment grain-size analysis and the nautical charts/TM RS imaginary. Contrary to being almost free from erosion (as expected from the findings that the coastlines are in a relatively stable state), four major cities in Hainan (i.e., Haikou, Wenchang, Sanya and Changjiang) turned out to be suffered from a moderate coastal vulnerability primarily because of the large populations that impose considerable pressure on the coastlines. Thus, the assessment methodology utilized in this study, including both anthropogenic and natural factors, serves as a useful tool to obtain a comprehensive understanding of coastline vulnerability for local government, in terms of coastal management and adaptation.