Sea Area Off Research Articles

Winter and summer sedimentary dynamic process observations in the sea area off Qinhuangdao in the Bohai Sea, China

Sedimentary processes in marginal seas play an important role in the biology, physics, and geochemistry as well as ecology of coastal environments and contain abundant information about the material transfer from land to ocean and the regional circulation. Due to the huge sediment discharge of the Yellow River, the Bohai Sea, China is one of the areas with the highest suspended sediment concentration (SSC) in the world. Interestingly, the SSC at the west of the Bohai Sea is low all year round. Thus, it is of great significance to examine the sedimentary dynamic process in this area for better understanding the circulation structure, material exchange and regional environment of the Bohai Sea. Using seabed base observation platform measurements obtained in February 2017 and August 2019, this study examines the winter and summer hydrography and suspended sediment concentration in the sea area off Qinhuangdao located to the west of the Bohai Sea. In summer, the relatively weak residual currents flowed northeastward and showed little correlation with the wind field, especially in the middle layer of the water column. In winter, the residual currents were strengthened, flowing to the northeast during strong wind periods, and predominantly to the southwest during intermittent periods. Moreover, driven by the pressure gradient force associated with the wind-induced sea surface height variations, the winter current was closely related to the wind speed, with correlation coefficients ranging from 0.4 to 0.6 and a time lag of 10 h. The summer SSC was lower and mainly controlled by the tidal current, whereas in winter, owing to the enhanced Reynolds stress and turbulent kinetic energy, strong wind bursts triggered significant sediment resuspension and led to a higher SSC. For the suspended sediment flux (SSF), the advection terms contributed more than 80% in the winter and summer, while the vertical circulation terms contributed 13% in winter, and approximately half that much in summer. Generally, the suspended sediment is transported back and forth, with a little net northward and northeastward motions in winter and summer, respectively. This may explain the low SSC sustaining in coastal Qinhuangdao all year round. These results provide a reference for sedimentary studies conducted in other coastal waters, especially in monsoon-dominating shelf seas.

Sediment resuspension in winter in an exceptional low suspended sediment concentration area off Qinhuangdao in the Bohai Sea

Seawater with low suspended sediment concentration (SSC) and high salinity dominates the sea area off Qinhuangdao all year round, especially in winter, which is an exceptional case since most of the coastal waters in the Bohai Sea are turbid. Being one of the most famous coastal tourist resorts in the area, it is crucial to keep the seawater around Qinhuangdao clear and unpolluted. Three bottom-mooring systems were deployed in the low SSC region from February to March 2017 for up to 22 days to reveal the processes and mechanisms of local sediment resuspension. The observation results showed that during the strongest wind of the whole deployment period, the sediment resuspension process included a turbulent resuspension event that occurred in the high turbulent kinetic energy period, and a continuous resuspension event when the bed shear stress (BSS) was larger than critical. The turbulent resuspension event was characterised by high-turbidity resuspension of sediments near the bed in the form of pulsation, with a depth of influence not more than the thickness of the bottom boundary layer. It was found to be triggered by strong turbulence driven by storm winds. In contrast the continuous resuspension could be attributed to high wave-dominated BSS. During storm winds, high concentrations of suspended sediment in the upper and middle layers appeared to lag the wind speed by about 8–10 h. In contrast, the turbulent resuspension event occurred at the same time as the strong winds. In-situ measurements from all three moorings indicate that it is the local weak sediment resuspension, even under the influence of storm winds in winter, that causes the formation of the low SSC area off Qinhuangdao.

Resilience of the seagrass Posidonia oceanica following pulse-type disturbance

Understanding the response of species to disturbance and the ability to recover is crucial for preventing their potential collapse and ecosystem phase shifts. Explosive submarine activity, occurring in shallow volcanic vents, can be considered as a natural pulse disturbance, due to its suddenness and high intensity, potentially affecting nearby species and ecosystems. Here, we present the response of Posidonia oceanica, a long-lived seagrass, to an exceptional submarine volcanic explosion, which occurred in the Aeolian Archipelago (Italy, Mediterranean Sea) in 2002, and evaluate its resilience in terms of time required to recover after such a pulse event. The study was carried out in 2011 in the sea area off Panarea Island, in the vicinity of Bottaro Island by adopting a back-dating methodological approach, which allowed a retrospective analysis of the growth performance and stable carbon isotopes (δ13C) in sheaths and rhizomes of P. oceanica, during a 10-year period (2001–2010). After the 2002 explosion, a trajectory shift towards decreasing values for both growth performance and δ13C in sheaths and rhizomes was observed. The decreasing trend reversed in 2004 when recovery took place progressively for all the analysed variables. Full recovery of P. oceanica occurred 8 years after the explosive event with complete restoration of all the variables (rhizome growth performance and δ13C) by 2010. Given the ecological importance of this seagrass in marine coastal ecosystems and its documented large-scale decline, the understanding of its potential recovery in response to environmental changes is imperative.

The inclusion of ocean-current effects in a tidal-current model as forcing in the convection term and its application to the mesoscale fate of CO2 seeping from the seafloor

The target seas of tidal-current models are usually semi-closed bays, minimally affected by ocean currents. For these models, tidal currents are simulated in computational domains with a spatial scale of a couple hundred kilometers or less, by setting tidal elevations at their open boundaries. However, when ocean currents cannot be ignored in the sea areas of interest, such as in open seas near coastlines, it is necessary to include ocean-current effects in these tidal-current models. In this study, we developed a numerical method to analyze tidal currents near coasts by incorporating pre-calculated ocean-current velocities. First, a large regional-scale simulation with a spatial scale of several thousand kilometers was conducted and temporal changes in the ocean-current velocity at each grid point were stored. Next, the spatially and temporally interpolated ocean-current velocity was incorporated as forcing into the cross terms of the convection term of a tidal-current model having computational domains with spatial scales of hundreds of kilometers or less. Then, we applied this method to the diffusion of dissolved CO2 in a sea area off Tomakomai, Japan, and compared the numerical results and measurements to validate the proposed method.

Two new bairdiid ostracod species from the early Barremian–Hauterivian of the northern and central North Sea to the Atlantic margin off Norway

Abstract. Two new species of bairdiid Ostracoda are described from the lower Barremian – Hauterivian interval of the Valhall and Åsgard formations in the northern and central North Sea and Atlantic margin off Norway. The new species are Pontocyprella valhalla (lsid:zoobank.org:act:CA6B273F-CFF6-4C38-B9F4-18188225A711, 18 January 2018) and Bairdia asgarda (lsid:zoobank.org:act:4A4DC817-A028-45FB-9287-ABF3794F2FCB, 18 January 2018). These species dominate the ostracod assemblage that occurs abundantly in early Barremian–Hauterivian deep marine sediments of the northern and central North Sea and Haltenbanken area off Norway. Pontocyprella valhalla is restricted to this interval and because of its large size and distinct shape is a useful stratigraphic marker species, its last appearance being within the early Barremian.

3Pb6-3 Sound propagation in boundary region between warm core ring and cold water mass of the east sea area off Tsugaru Straits(Poster Session)

3Pb6-3 Sound propagation in boundary region between warm core ring and cold water mass of the east sea area off Tsugaru Straits(Poster Session)

Geological Structure Investigation of Shallow Layers by the Explosion Seismic Survey Tomographic Technique

The detailed P-wave velocity structure of the surface layers in the sea area off Kyushu, Japan, was analyzed using a three-dimensional (3-D) tomographic method for explosion seismic refraction and reflection surveys. Approximately 1,830 P-wave arrival times received from 15 rounds of explosion seismic surveys were used in the structural analysis of the two-dimensional (2-D) profile. The 3-D pseudo-bending technique, which is a fast algorithm for two point ray tracing, is used in forward modeling. The conjugate gradient (CG) algorithm was used to solve the discrete linearized equation of the travel-time residuals. The velocity structure obtained from the tomographic technique indicates heterogeneity in the lateral and vertical directions for seismic waves within in the study area. The sedimentary layer is thin near the coast and thickens gradually off the coast. The boundary contours of sedimentary layers reveal complicated serpentine curves. Lateral and/or vertical convex and concave structures are prominent in the contours. The scales of some concave and convex anomalous velocity structures are small; anomalous structures on a [Formula: see text] scale were identified. This might imply that a high resolution tomography imaging technique can be introduced in shallow-structure surveys. The explosion seismic survey tomographic technique characterizes the complicated structure, geometry and physical properties of abnormal objects well, and tomographic inversion of the explosion survey is available for detailed geological structure surveys and resource explorations, especially for resources such as oil, gas and minerals in complicated geological conditions. The calculated convergence velocity and stability of the tomography technique are also of a high standard.

The contribution of trace elements from seawater to chimneys: a case study of the native sulfur chimneys in the sea area off Kueishantao, northeast of Taiwan Island

Hydrothermal fluid containing abundant matter erupts from seafloor, meets ambient cold seawater and forms chimneys. So the main matter origins of chimneys are seawater and matter which are taken by hydrothermal fluid from deep reservoir. However, because of seawater's little contribution to the forming of chimneys, it is usually covered by the abundant matter which is taken by hydrothermal fluid. Therefore, chimneys formed in ordinary deep seawater hydrothermal activity, containing complex elements, cannot be used to study the seawater's contribution to their formation. While the native sulfur chimneys, formed by hydrothermal activity near the sea area off Kueishantao, are single sulfur composition (over 99%), and within chimneys distinct layers are seen. Different layers were sampled for trace element determination, with Inductively Coupled Plasma Mass Spectrometry (ICP-MS). By analyzing the data, we consider C-layer (secondary inner-layer) as the framework layer of the chimney which formed early (Fig. 4), and its trace elements derive from hydrothermal fluid. While the trace elements within A, B, D layers have undergone later alteration. A, B layers are affected by seawater and D layer by hydrothermal fluid. The increase of trace elements of A and B layers was calculated using C layer as background. Based on the known typical volume of chimneys of the near sea area off Kueishantao, we calculated the volume of seawater that contributed trace element to chimneys formation to be about 6.37 x 10(4) L. This simple quantified estimate may help us better understand the seafloor hydrothermal activity and chimneys.

国産エネルギー資源としてのメタンハイドレート

Depending on imports for 98.6% for oil and natural gas requirements, it has been a long-cherished dream for Japan to have domestic hydrocarbon energy resources. Methane hydrate, an ice-like white solid composed of water and methane gas, is believed to be widely distributed in the sea area off Japan. Therefore, Japan's Methane Hydrate R & D Program was established by the Ministry of Economy, Trade and Industry in FY2001. The organization established to execute this program is the Research Consortium for Methane Hydrate Resources in Japan (MH21). This program is divided into three phases, extending over 18 years. Phase 1 ended in FY2008. The main results of Phase 1 are as follows: (1) World's first successful continuous production of methane gas from methane hydrate-bearing layers in onshore production tests, (2) For the first time in the world, methane hydrate concentrated zones, which have development potential, were identified in the eastern Nankai Trough area, (3) Experimental in-situ testing methods of artificial and natural core samples of methane hydrate-bearing sediments were established, (4) Probabilistic resource assessment method based on well and seismic data was established, and the amount of methane gas trapped in the eastern Nankai Trough area was estimated, (5) Japan's own simulator dedicated to evaluating the production behavior of methane gas from methane hydrate-bearing layers was developed. Besides the above-mentioned achievements, fundamental studies were conducted on the Environmental Impact Assessment and Methane Hydrate System. The following technological issues have been identified by a sensitivity analysis of the Economic Evaluation: (1) increasing gas production rate (2) improving recovery factor, (3) reducing rate of sand-induced problems, and (4) reducing cost of subsea system. Phase 2 began in April 2009 to resolve these issues. The most important issue in Phase 2 is to conduct two offshore production tests in the sea off Japan. This paper introduces detailed results of Phase 1 and issues to be tackled during and after Phase 2.

Geological records of marine environmental changes in the southern Okinawa Trough

Indexes of sediment grain size, sedimentation rates, geochemical composition, heavy minerals, benthic foraminiferal fauna, indicator species of the Kuroshio Current, paleo-SST and carbonate dissolution of core E017 conformably suggest a great marine environmental change occurring at about 10.1-9.2 cal. kaBP in the southern Okinawa Trough, which may correspond to the strengthening of the Kuroshio Warm Current and re-entering the Okinawa Trough through the sea area off northeast Taiwan. The invasion of Kuroshio current has experienced a process of gradual strengthening and then weakening, and its intensity became more fluctuation during the last 5000 years. Compared to the transition of sediment grain size, geochemical composition and heavy minerals, the foraminiferal faunas show a 900-year lag, which may indicate that the invasion of Kuroshio Current and the consequent sea surface and deep-water environmental changes is a gradual process, and fauna has an obvious lag compared to environment altering. The carbonate dissolution of the Okinawa Trough has had an apparent strengthening since 9.2 cal. kaBP, and reached a maximum in the late 3000 years, which may be caused by the deep-water environmental changes due to the invasion of Kuroshio Current.

A study on the environmental impact of a very large floating structure on marine ecosystem

The environmental impacts of an imaginary very large floating structure on marine environment, especially on carbon circulation around the structure, are examined using numerical simulation by means of coastal ecosystem model. After validation of coastal ecosystem model by comparing predicted results with observed ones, the impacts of the floating structure moored off Oppama or off Haneda are discussed. As a result of several computations, it is revealed that growth rate of sessile organisms adhering to the floating structure are smaller in the sea area off Haneda, where current velocity of sea water is slow, because particulate organic matter and oxygen are not supplied sufficiently. On the other hand, in the sea area off Oppama, seawater flows southward constantly at the speed of about 0.1m/s, and organic matter and oxygen are supplied appropriately, so sessile organisms can sustain their biomass. The biomass of sessile organisms per unit area of the floating structure moored off Oppama is much larger than that in the general tidal flat estuary. Further, the impacts of activities of sessile organisms on carbon circulation in Tokyo Bay are examined. Then it can be said that sessile organisms retain organic matters in the bay, which may otherwise flow out from the bay to the outer sea.

A study on the effects of a very large floating structure on coastal ecosystem in Tokyo Bay

To realize a very large floating structure (VLFS), it is indispensable to examine various effects on marine environment around a VLFS. For an assessment in the physical, chemical, and biological aspects, a numerical study by means of ecohydrodynamic model, which is an ecological-physical coupled model, is conducted. First, to validate the ecohydrodynamic model, numerical simulations of coastal ecosystem in the summer of Tokyo Bay are carried out under the boundary conditions which change every moment, and under those which are kept unchanged for computations. Additionally, the effects of an imaginary VLFS on the surrounding environment are examined in two cases : the VLFS is assumed to be installed in the sea area off Yokosuka or off Haneda in Tokyo Bay. As a result of some computations, it is concluded that predicted change of water quality agreed quantitatively with observation data, and the impacts of the VLFS on marine environment are localized in close vicinity to the floating structure. However the extent of the effects is relatively larger in the sea area off Haneda, where the residual current is weak, than off Yokosuka.

Temperature dependence of growth in Phaeocystis pouchetii (Haptophyceae) in batch cultures

Temperature dependent growth rates ofPhaeocystis pouchetii (Haptophyceae) were investigated in 5–1 batch cultures. The algae had been isolated from the German Wadden Sea area off Sylt. Microscopic cell counts and fluorescense measurements yielded similar results. The growth ofP. pouchetii reveals a typical optimum curve between 7 °C and 20 °C. Maximal growth rates, 3 divisions per day, were obtained at 15 °C. At 5 °C the algae cultures survived, but multiplication of the cells almost ceased. Results of the culture experiments correspond with observations made onPhaeocystis blooms at the German North Sea coast.

Feeding of calanoid copepods in relation to Phaeocystis pouchetii blooms in the German Wadden Sea area off Sylt

Population dynamics of the dominant calanoid copepods Acartia spp. and Tempora longicornis were followed during a dense spring bloom of Phaeocystis pouchetii in the northern Wadden Sea off Sylt. Positive correlations between algal concentration and abundance of the copepod species suggest that P. pouchetii constitutes an important food organism for the copepods in spring and early summer. In laboratory experiments, feeding and ingestion rates of A. clausi and T. longicornis were determined when P. pouchetii colonies were offered as food. Maximum food intake reached 48% of body carbon per day in A. clausi and 87% in T. longicornis, respectively.

Feeding of calanoid copepods in relation to Phaeocystis pouchetti blooms in the German Wadden Sea area off Sylt

Feeding of calanoid copepods in relation to Phaeocystis pouchetti blooms in the German Wadden Sea area off Sylt

Chemical changes and primary production in upwelled water off Northwest Africa

A grid of stations in the sea area off Nouakchott is used for the study of temporal changes in upwelled water after this water has reached the surface. In order to exclude waters which do not belong to the upwelling system of the investigated area, only stations with nearly the same salinity in the surface layer (10 m depth) as was found in the fresh upwelling water are considered. The chemical data of the surface layer are plotted against the corresponding temperatures. The daily increase of temperature in the mixed surface layer of the upwelled water provides the connection between the temperature scale and the desired time scale. The changes of the chemical variables with time are discussed. They look quite similar to the changes which were observed in the open North Sea during the first phase of a spring phytoplankton bloom. From the slopes of the pH, nitrate+nitrite, and phosphate regression lines the net primary production is computed. The result stands in fair agreement with the data published in the literature and with our own14C experiments in the same area. Under the conditions chosen, the chemical changes in the upwelled water at the surface seem to be governed much more by primary (and secondary) production than by vertical and horizontal mixing with the underlying and surrounding waters.

The Submarine Topography and Geological Structure of Sagami Bay, South Coast of Honshu, Japan

Sagami Bay is structurally active area through the geological history, and also the epicenter of the Great Kanto Earthquake (1923) was here. To make clear the features of the topography and geological structure of the active area, it seems to be fundamental step for the earthquake predication today.In April to May 1968, the surveying ship Meiyo of the Japanese Hydrographic Office carried out the sea bottom surveys of Sagami Bay with echo-sounder, seismic profiler and proton-geomagnetometer, including the dredge and coring operation, and took the data from about 3000 kilometers of her traverses.The ship's position was mainly determined by an auto-tape system with the estimated accuracy of ±2 meters. The simultaneous measurement of magnetic intensity, bathymetry and submarine geological structure by air-gun system allows direct comparison of the measurements. Figure 1 is the track chart of the Meiyo and figures 2 and 3 are some profiles based on the collected data.From the analyses of the collected data mentioned above, the topography and geological structure and magnetic anomaly of Sagami Bay can be summarized as follows.1. TopographyFigure 4 shows the bathymetric chart of Sagami Bay. In the eastern part of the bay, many banks and small basins range from NW to SE. Many submarine canyons cross the topography mentioned above and incise deeply, so that especially small basins do not keep their complete shape.The central part of the bay is called Sagami Trough, which is deeper than 1 kilometer. The floor of the trough generally shows low relief with the exception of the eastern part of the trough, which submarine canyons flow into, and gentle slope with the trend of NS at the central part, facing east and reflecting the fault topographically.In the western part of the bay, continental shelf and slope are monotonous, comparing with its eastern part. There are several conical banks between the Izu Peninsula and O Shima Island.2. Geological structureGeological structure based on the seismic profiler is shown in Figure 5. The area surrounded Sagami Bay is relatively uplifted zone in contrast with the trough showing the relative subsidence. Acoustic basement rock outcrops on the uplifted zone and may probably be composed of the Hayama group (Lower Miocene) and partly the lower part of the Miura group (Middle Miocene).The large part of the thinly layered sediment at the upper part of sediment body in Sagami Trough is younger than the age of uplifted zone mentioned above. The layered sediment is wholly tilted to the axis of the trough, and is found the folding in the margin of the trough and also the active fault near surface. Its maximum thickness is found at the northern part of the trough, and is over about 1 second in the reflection time approximately.The area along the Izu Peninsula and around O Shima Island is extensively covered with the volcanic products. Small banks between the Izu Peninsula and O Shima Island are volcanic cones of the effusive body which pierced through the layers probably composed of the Shirahama group (Middle to Upper Miocene).3. GeomagnetismThe resultant magnetic chart, contoured at intervals of 100 gammas, was in Figure 6. The results of the magnetic surveys revealed that there were many magnetic anomalies of about 500 to 800 gammas off shore along the Izu Peninsula in Sagami Bay and a pair of remarkable positive and negative magnetic anomalies of about 1800 gammas in the sea area along the west side of O Shima Island.On the other hand, it was found that there was no remarkable magnetic anomaly in the central part and eastern part of Sagami Bay.From the results of magnetic surveys and submarine geological structure, it can be concluded that the sea area off shore along the Izu Peninsula and around O Shima Island are composed of volcanic rocks of higher magnetic susceptibility, such as basalt or andesite.

Chlorophyll maxima in Kuroshio and adjacent area

Fluorometric determination of chlorophyll a and pheopigments was carried out in the sea area off southern Japan. Maximum concentration of chlorophyll pigments was determined to be at or below the lower limit of the euphotic zone, namely from 50 to 150 m depth. To estimate the activity of phytoplankton in this maximum chlorophyll layer, changes of chlorophyll concentration and number of cells were measured in samples taken from this layer before and after exposure to different light intensities. It was concluded that the growth of shade-adapted phytoplankton and the deterioration of chlorophyll pigments by light are the main factors causing the chlorophyll maximum to occur in a rather deep oceanic layer.