2019 Vol. 38, No. 3

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Weather induced subtidal flows through multiple inlets of an arctic microtidal lagoon
Li Chunyan, Boswell Kevin M., Chaichitehrani Nazanin, Huang Wei, Wu Renhao
2019, 38(3): 1-16. doi: 10.1007/s13131-019-1361-2
Estuarine processes in the arctic lagoons are among the least studied but important subjects, especially considering the rapid warming of arctic water which may change the length of ice-free period in the summer. In this paper, wind-driven exchange flows in the micro-tidal Elson Lagoon of northern Alaska with multiple inlets of contrasting widths and depths are studied with in situ observations, statistical analysis, numerical experiments, a regression model on the basis of dynamics, and remote sensing data. Water velocity profiles were obtained from a bottom deployed acoustic Doppler current profiler (ADCP) in the northwestern Eluitkak Pass connecting the Beaufort Sea to the Elson Lagoon during a 4.9 day ice-free period in the summer of 2013. The subtidal flow is found correlated with wind (R2 value ~96%). Frequently occurring east, northeast and north winds from the arctic atmospheric high-and low-pressure systems push water from the Beaufort Sea into the lagoon through the wide inlets on the eastern side of the lagoon, resulting in an outward flow against the wind at the narrow northwestern inlet. The counter-wind flow is a result of an uneven wind forcing acting through the asymmetric inlets and depth, an effect of "torque" or vorticity. Under northwest wind, the exchange flow at the northwestern inlet reverses its direction, with inward flows through the upwind northwestern inlet and outward flows through the downwind eastern inlets. A regression model is established based on the momentum equations and Taylor series expansions. The model is used to predict flows in July and August of 2015 and July of 2017, supported by available Landsat satellite images. About 73%-80% of the time the flows at Eluitkak Pass are out of Elson Lagoon for the summer of 2015 and 2017. Numerical experiments are conducted to corroborate the findings and illustrate the effects under various wind conditions. A quasi-steady state balance between wind force and surface pressure gradient is confirmed.
The in situ observation of modelled sea ice drift characteristics in the Bohai Sea
YAN Yu, GU Wei, XU Yingjun, LI Qian
2019, 38(3): 17-25. doi: 10.1007/s13131-019-1395-5
Sea ice drift is mainly controlled by ocean currents, local wind, and internal ice stress. Information on sea ice motion, especially in situ synchronous observation of an ice velocity, a current velocity, and a wind speed, is of great significance to identify ice drift characteristics. A sea ice substitute, the so-called "modelled ice", which is made by polypropylene material with a density similar to Bohai Sea ice, is used to complete a free drift experiment in the open sea. The trajectories of isolated modelled ice, currents and wind in the Bohai Sea during non-frozen and frozen periods are obtained. The results show that the currents play a major role while the wind plays a minor role in the free drift of isolated modelled ice when the wind is mild in the Bohai Sea. The modelled ice drift is significantly affected by the ocean current and wind based on the ice-current-wind relationship established by a multiple linear regression. The modelled ice velocity calculated by the multiple linear regression is close to that of the in situ observation, the magnitude of the error between the calculated and observed ice velocities is less than 12.05%, and the velocity direction error is less than 6.21°. Thus, the ice velocity can be estimated based on the observed current velocity and wind speed when the in situ observed ice velocity is missing. And the modelled ice of same thickness with a smaller density is more sensitive to the current velocity and the wind speed changes. In addition, the modelled ice drift characteristics are shown to be close to those of the real sea ice, which indicates that the modelled ice can be used as a good substitute of real ice for in situ observation of the free ice drift in the open sea, which helps solve time availability, safety and logistics problems related to in situ observation on real ice.
The influence of runoff and wind on the dispersion patterns of suspended sediment in the Zhujiang (Pearl) River Estuary based on MODIS data
OU Suying, YANG Qingshu, LUO Xiangxin, ZHU Fan, LUO Kaiwen, YANG Hao
2019, 38(3): 26-35. doi: 10.1007/s13131-019-1396-4
Cloud-free moderate-resolution imaging spectroradiometer (MODIS) images of the Zhujiang (Pearl) River Estuary (ZRE) taken between 2002 and 2012 are retrieved and used to study the spatial and temporal patterns of suspended sediment concentrations (SSCs) across the estuary under runoff, wind, and tropical storm conditions. Five typical dispersal patterns of suspended sediments in the estuary are defined:Case I shows generally low SSCs under low dynamics; Case Ⅱ shows a river-dominant dispersal pattern of suspended sediments from the outlets, particularly from Modaomen, Jiaomen, Hengmen, and others; Case Ⅲ shows wind-dominant dispersal of high SSCs derived from the west shoal and southwesterly transport under a strong NE wind; Case IV is the combination of relatively large runoff and wind; and Case V is caused by a strong tropical storm with high river discharge and wind, which is characterized by the high SSCs across the entire estuary that are transported eastward by wind-driven and buoyancy currents outside the estuary. Runoff is a dominant factor that controls seasonal and annual SSC variations in the ZRE, with the area of high SSCs being largest in the summer and smallest in the spring. The correlation coefficients between the monthly averaged river-suspended sediment discharge and the area of the high SSCs are approximately 0.6. The wind power over the west shoal increases with a wind speed, which induces more sediment resuspension and shows a close relationship between the wind speed and high SSC area.
The vertical heat transport of internal solitary waves over the continental slope in the northern South China Sea
LIANG Changrong, SHANG Xiaodong, CHEN Guiying
2019, 38(3): 36-44. doi: 10.1007/s13131-019-1397-3
An integrated analysis of internal solitary wave (ISW) observations obtained from two moorings over the continental slope in the northern South China Sea (SCS) leads to an assessment of the vertical heat transport of the ISWs. The clusters of ISW packets are phase-locked to the fortnightly cycle of the semidiurnal tide. The ISWs appear during large semidiurnal tides, and there is a period of 5-6 d when no ISWs are observed. The effect of the ISWs on the continental slope heat budget is observed. The ISWs can modify a local temperature field in which the temperature in the upper layer can be changed by O (100)℃ after the ISWs passed the mooring. Both ISW-induced diffusion and ISW-induced advection contribute to the temperature variation. The estimates imply an average vertical heat flux of 0.01 to 0.1 MW/m2 in the ISWs in the upper 500 m of the water column. The vertical heat transport ranges from 0.56 to 2.83 GJ/m2 with a mean value of 1.63 GJ/m2. The observations suggest that the vertical heat transport is proportional to the maximum vertical displacement.
Wave packet solutions in a bounded equatorial ocean and its interannual and decadal variabilities
ZHANG Dongling, ZHU Juan, LU Xu, ZHANG Ming
2019, 38(3): 45-59. doi: 10.1007/s13131-019-1398-2
Linearized shallow water perturbation equations with approximation in an equatorial β plane are used to obtain the analytical solution of wave packet anomalies in the upper bounded equatorial ocean. The main results are as follows. The wave packet is a superposition of eastward travelling Kelvin waves and westward travelling Rossby waves with the slowest speed, and satisfies the boundary conditions of eastern and western coasts, respectively. The decay coefficient of this solution to the north and south sides of the equator is inversely proportional only to the phase velocity of Kelvin waves in the upper water. The oscillation frequency of the wave packet, which is also the natural frequency of the ocean, is proportional to its mode number and the phase velocity of Kelvin waves and is inversely proportional to the length of the equatorial ocean in the east-west direction. The flow anomalies of the wave packet of Mode 1 most of the time appear as zonal flows with the same direction. They reach the maximum at the center of the equatorial ocean and decay rapidly away from the equator, manifested as equatorially trapped waves. The flow anomalies of the wave packet of Mode 2 appear as the zonal flows with the same direction most of the time in half of the ocean, and are always 0 at the center of the entire ocean which indicates stagnation, while decaying away from the equator with the same speed as that of Mode 1. The spatial structure and oscillation period of the wave packet solution of Mode 1 and Mode 2 are consistent with the changing periods of the surface spatial field and time coefficient of the first and second modes of complex empirical orthogonal function (EOF) analysis of flow anomalies in the actual equatorial ocean. This indicates that the solution does exist in the real ocean, and that El Niño-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) are both related to Mode 2. After considering the Indonesian throughflow, we can obtain the length of bounded equatorial ocean by taking the sum of that of the tropical Indian Ocean and the tropical Pacific Ocean, thus this wave packet can also explain the decadal variability (about 20 a) of the equatorial Pacific and Indian Oceans.
A preliminary study on an upper ocean heat and salt content of the western Pacific warm pool region
YANG Xiaoxin, WU Xiaofen, LIU Zenghong, YUAN Chunxin
2019, 38(3): 60-71. doi: 10.1007/s13131-019-1399-1
On the basis of Argo profile data of the temperature and salinity from January 2001 to July 2014, the spatial distributions of an upper ocean heat content (OHC) and ocean salt content (OSC) of the western Pacific warm pool (WPWP) region and their seasonal and interannual variations are studied by a cyclostationary empirical orthogonal function (CSEOF) decomposition, a maximum entropy spectral analysis, and a correlation analysis. Probable reasons for variations are discussed. The results show the following. (1) The OHC variations in the subsurface layer of the WPWP are much greater than those in the surface layer. On the contrary, the OSC variations are mainly in the surface layer, while the subsurface layer varies little. (2) Compared with the OSC, the OHC of the WPWP region is more affected by El Niño-Southern Oscillation (ENSO) events. The CSEOF analysis shows that the OHC pattern in mode 1 has strong interannual oscillation, with eastern and western parts opposite in phase. The distribution of the OSC has a positive-negative-positive tripole pattern. Time series analysis shows that the OHC has three phase adjustments with the occurrence of ENSO events after 2007, while the OSC only had one such adjustment during the same period. Further analysis indicates that the OHC variations are mainly caused by ENSO events, local winds, and zonal currents, whereas the OSC variations are caused by much more complex reasons. Two of these, the zonal current and a freshwater flux, have a positive feedback on the OSC change in the WPWP region.
Sea-level rise impact on the evolution of a microtidal Mediterranean coastline without human-made structures—a case of the Port aux Princes-Sidi Daoued coastline, Gulf of Tunis, NE-Tunisia
2019, 38(3): 72-77. doi: 10.1007/s13131-018-1331-0
The evolution of the natural and pristine Mediterranean coastline Port aux Princes-Sidi Daoued (Gulf of Tunis, NE-Tunisia) is studied during the period of 1887-2010 on the basis of an ancient minute of bathymetry (1887) and aerial photographs treated by numerical photogrammetric methods. Morphological changes of the coastline shows a general retreat despite the absence of the various anthropogenic actions. Adding to the drift currents and the currents of high energy that are generated by the N-W dominant waves along steeply sub-marine funds, the erosion is mainly due to the sea level rise which increased since the beginning of the 2000s. The Port aux Princes-Sidi Daoued coastline works as a single littoral cell limited by Jbel Korbous to the SW and the fishing harbor of Sidi Daoued to the N-E.
The relative changes of a sea surface temperature in the South China Sea since the Pliocene
BI Dongjie, ZHANG Daojun, ZHAI Shikui, LIU Xinyu, XIU Chun, LIU Xiaofeng, ZHANG Aibin
2019, 38(3): 78-92. doi: 10.1007/s13131-019-1401-y
The reconstruction of sea surface temperature (SST) is a key issue in research on paleoceanography. The recently related studies are mainly concentrated on the Quaternary. The skeletons of a typical species of benthonic foraminifer (Amphistegina radiata) in the top 0-375.30 m interval of Well "Xike-1" reef core, Shidao Island, the Xisha Islands, are uniformly selected. The ratios of magnesium to calcium concentrations and other indicators are analyzed by an electron microprobe analysis (EMPA) with the purpose of estimating the paleo-SSTs since the Pliocene and further investigating the periodic change law of paleoclimate in the South China Sea (SCS). Meanwhile, the geologic significance of paleoclimatic events in the SCS is discussed with global perspectives. The results indicate that the paleo-SSTs reconstructed by the ratios of magnesium to calcium concentrations in the SCS show a general periodic trend of "high-low-high-low" in values changes since the Pliocene. By comparison, the fluctuations of reconstructed paleo-SSTs are much stronger in the Quaternary. Moreover, the variations of the ratios of magnesium to calcium concentrations in the A. radiata skeletons have recorded a series of major paleoclimatic events since the Pliocene, of which the Quaternary glaciation events and the Arctic ice cap forming events during the late Pliocene are more significant. Thus, using the changes of the ratios of magnesium to calcium concentrations in the A. radiata skeletons from Well "Xike-1" reef core to reflect the relative changes of paleo-SSTs is a relatively feasible and reliable way in the SCS, which is also proved by the correlation of drilling cores characteristics in this area.
The spatial distribution of major and trace elements of surface sediments in the northeastern Beibu Gulf of the South China Sea
GE Qian, XUE George Z, YE Liming, XU Dong, ZHAO Jianru, CHU Fengyou
2019, 38(3): 93-102. doi: 10.1007/s13131-019-1402-x
A multi-index analysis including grain size, major and trace elements is performed on the surface sediments from the northeastern Beibu Gulf to trace the sources of the sediments and to understand the controlling factors for elements distribution. The mean grain size exhibits a wide variation ranging from 0.09Φ to 8.05Φ with an average value of 5.33Φ. The average contents of major elements descend in an order of c(SiO2) > c(Al2O3) > c(Fe2O3) > c(CaO) > c(MgO) > c(K2O) > c(Na2O) > c(TiO2) > c(P2O5) > c(MnO), while those of trace elements exhibit a descending order of c(Sr) > c(Rb) > c(V) > c(Zn) > c(Cr) > c(Pb) > c(Ni) > c(Cu) > c(As). On the basis of elementary distribution characteristics and statistical analyses, the study area is divided into the four zones:Zone I is located in the northeastern coastal area of the gulf, which receives large amount of fluvial materials from local rivers in Guangxi and Guangdong, China, and the Qiongzhou Strait; Zone Ⅱ is located in the center of the study area, where surface sediments exhibits a multiple source; Zone Ⅲ is located in the Qiongzhou Strait, where surface sediments are dominated by materials from the Zhujiang River and Hainan; Zone IV is located in the southwest of the study area, where surface sediments are mainly originated from the Red River and Hainan. The statistical analyses of sediment geochemical characteristics reveal that the grain size, which is mainly influenced by hydrodynamics and mineral composition of terrigenous materials, is the leading factor controlling the elementary distribution. Meanwhile, impacts from anthropogenic activities and marine biogenic process will also be taken into consideration.
Sedimentary characteristics comparison and genesis analysis of the deepwater channel in the hydrate enrichment zones on the north slope of the South China Sea
FU Chao, YU Xinghe, DONG Yiis, HE Yulin, LIANG Jinqiang, KUANG Zenggui
2019, 38(3): 103-113. doi: 10.1007/s13131-019-1403-9
Natural gas hydrate (NGH) is one of the important clean energy at present and even in the future. The study of its sedimentary environment and minerogenetic condition has long been a hot issue that has received much concern from geologists all over the world. China has successfully obtained the samples of NGH in Shenhu and Dongsha sea areas in 2007, 2013 and 2015, respectively. From this, the continental slope north of the South China Sea becomes an important test site for the study of NGH sedimentary genesis and minerogenetic condition. NGH has been found in Shenhu, Dongsha and Qiongdongnan areas within the continental slope north of South China Sea, at different depths of water, with different sedimentary characteristics, gas genesis, and minerogenetic conditions. Using a seismic sedimentology theory, combining seismic facies results of each facies, sedimentary facies and evolution of each area are documented in turn establishing a sedimentary model by considering palaeo-geomorphology, sea level change and tectonic movement. The channel system and MTD (Mass Transport Deposition) system among these three areas were compared focusing on the developing position, appearance and controlling factors. Relative location among three areas is firstly defined that Dongsha area in a near-provenance steep upper slope, Shenhu area in a normal gentle slope and Qiongdongnan area in an away-provenance flat plain. Besides, their channel systems are classified into erosional, erosional-aggradational and aggradational channel, and MTD systems into headwall domain, translational domain and toe domain.
Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea
LI Yanlong, HU Gaowei, WU Nengyou, LIU Changling, CHEN Qiang, LI Chen’an
2019, 38(3): 114-123. doi: 10.1007/s13131-019-1404-8
The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations. In situ piezocone penetration tests (CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China's first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments (HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays, the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays, while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.
Miocene paleoenvironmental evolution based on benthic foraminiferal assemblages in the Lufeng Sag, northern South China Sea
XUE Liyuan, DING Xuan, PEI Renjie, WAN Xiaoqiao
2019, 38(3): 124-137. doi: 10.1007/s13131-019-1405-7
The Miocene epoch marks the most crucial period during the Cenozoic cooling trend, characterized by the Middle Miocene Climatic Optimum (MMCO) and a series of short-lived cooling events (Miocene isotope events). To understand the paleoenvironmental evolution along the shallow water shelf in the South China Sea during the Miocene, the benthic foraminiferal assemblage and total organic carbon content (TOC) were analyzed at Hole LF 14 located in the Lufeng Sag, northern South China Sea. Three benthic foraminiferal assemblages (e.g., the Uvigerina spp. assemblage, the Cibicides spp. assemblage, and the Cibicidoides spp. assemblage), corresponding to different watermass conditions, were recognized based on Q-mode factor analysis. Early studies suggested that Hole LF14 was deposited under semienclosed bay, middle to outer shelf or even upper bathyal environment during~18.7-4.53 Ma. The dominant Uvigerina spp. assemblage was characterized by low diversity and shallow infaunal to infaunal species, indicating a warm, low-oxygenation and eutrophic conditions since the Early Miocene to MMCO (~18.7-14.24 Ma). An abrupt sea level drop and significant faunal changes were recorded during 14.24-13.41 Ma, suggesting development of the East Antarctic Ice Sheets, which resulted in a drop of sea level and change in benthic foraminiferal assemblages along the shallow water shelf. Beyond the Uvigerina spp. assemblage, the Cibicides spp. assemblage became important during the middle-late Middle Miocene (14.24-11.54 Ma). This assemblage was dominated by epifaunal species with relative high diversity, suggesting high-energy, high-oxygenation and oligotrophic conditions with episodic supply of organic food. The dominant Cibicidoides spp. assemblage with high diversity, indicates a mesotrophic conditions with relative high-oxygen content during the Late Miocene to Pliocene (11.54-4.53 Ma). The appearance and continuous occurrence of Ammonia spp. and Pseudorotalia spp. since 10.02 Ma, may reflect the influence of the Kuroshio Current.
The B/Ca and Cd/Ca of a subsurface-dwelling foraminifera Pulleniatina obliquiloculata in the tropical Indo-Pacific Ocean: implications for the subsurface carbonate chemistry estimation
DANG Haowen, WANG Tingting, QIAO Peijun, BASSINOT Franck, JIAN Zhimin
2019, 38(3): 138-150. doi: 10.1007/s13131-019-1406-6
Pulleniatina obliquiloculata shells from 16 core-top samples from the tropical Indo-Pacific Oceans are analyzed for the ratios of boron and cadmium to calcium (B/Ca and Cd/Ca). The B/Ca ratios show a very weak positive relationship with[B(OH)4-] and the dissolved carbonate species at the apparent calcification depth of P. obliquiloculata. The boron partition coefficients (KD) between P. obliquiloculata B/Ca and seawater[B(OH)4-]/[HCO3-] distribute around 1.1×10-3-1.3×10-3 with a mean value of (1.19±0.12)×10-3, and are significantly related to the nutrient concentration, especially phosphate. The lack of any clear correlation between the P. obliquiloculata B/Ca and seawater carbonate chemical parameters suggests that the physiochemical controls on boron incorporation are masked by the complexity of natural seawater condition. But the significant dependence of KD on nutrient may likely be explained by a nutrient related growth-rate effect. Cd/Ca of P. obliquiloculata shows significant correlation with seawater phosphate concentration, and its partition coefficients (DCd) are significantly related to temperature. A first-principle methodology of P. obliquiloculata B/Ca is applied, with the aid of Cd/Ca as a phosphate proxy and a constraint on KD, to estimating sea water carbonate chemistry (e.g., pH). The results are fairly promising and allow us to propose the possibility to apply the combination of B/Ca and Cd/Ca proxies (and also Mg/Ca and δ18O for estimating temperature and salinity) for the paleo-reconstruction of seawater carbonate chemistry.
The influence of irregular seafloor topography on the seismic wave field and migration imaging
HAN Fuxing, WANG Kun, SUN Jianguo
2019, 38(3): 151-158. doi: 10.1007/s13131-019-1407-5
One of the problems experienced in marine geophysical exploration is that the layered features in the migration imaging profile are blurred and the seismic energy reflected is weaker in the middle or lower parts. In this study we model the seismic wavefield records in the undulating seafloor when there is both a slight change and significant change in seafloor topography to analyze its influence on the seismic reflection data and migration imaging profiles. We compare and analyze the wave field records collected at the same point on the original and modified velocity models, and the cross-bonding resulting migration imaging profiles. The results show that whether the seismic reflection data collection is performed along the direction of the survey line or against the direction of the survey line, slight changes in the seafloor topography have little effect on the wave field records and the migration profile, while significant changes in the seafloor topography have great effect on both the wave field records and migration profile.
A hydrothermal investigation system for the Qianlong-Ⅱ autonomous underwater vehicle
Wu Tao, Tao Chunhui, Zhang Jinhui, Wang Ao, Zhang Guoyin, Zhou Jianping, Deng Xianming
2019, 38(3): 159-165. doi: 10.1007/s13131-019-1408-4
Qianlong-Ⅱ is a fully autonomous underwater vehicle designed for the investigation of submarine resources, particularly polymetallic sulfides. It was used to successfully explore hydrothermal fields on the Southwest Indian Ridge. Here, we summarized the exploration of hydrothermal systems using Qianlong-Ⅱ, including detailed descriptions of its implementation along with the systems used for data management and fast mapping. We also introduced a method to remove platform magnetic interference using magnetic data while Qianlong-Ⅱ is spinning. Based on hydrothermal anomalies collected by Qianlong-Ⅱ, we developed a rapid method for locating hydrothermal vents. Taking one dive as an example, we systemically demonstrated the process for analyzing hydrothermal survey data to locate hydrothermal vents.