The preservation of C org in various shelf seas from different geological, hydrological and climatological settings is compared. Most of these shelves have been studied extensively over the last two to three decades, often within the framework of large (inter)national research projects. Various factors (physical oceanography, chemical conditions, sedimentation rates) that could influence the preservation of C org are discussed. Not only recent conditions, but also longer time scale fluctuations (glacials versus inter-glacials, sea level variations) are dealt with. Most (>95%) of the C org introduced onto shelves by primary production and imported from the oceans and continents is mineralized in the water column and sediments. It appears that the role of shelves as sinks for C org is sometimes overestimated. Large areas of the continental shelves do not show any accumulation of organic matter under present day conditions. Only locally, where hydrological and sedimentological conditions are favourable for organic matter accumulation considerable amounts of C org are buried. On longer time scales the role of shelves as sinks for C org is limited, even more than under recent conditions. Continental slopes, canyons and deep-sea fans are thought to be the main sinks for C org .

Organic carbon in shelf seas: sinks or sources, processes and products

To provide baseline information for the biogeochemical studies, the circulation in the three-sea (Bohai, Yellow Sea and East China Sea) system is derived from climatological forcing, using a three-dimensional general circulation model. In the light of extensive literature dealing with similar subjects, the discussion is focused mostly on previously under-represented parts of the circulation, namely, seasonal variations of circulation in the surface layer, Changjiang plume dispersal, sea-level variations, and upwelling–downwelling locations and intensities. In terms of circulation, notable features include the development of China Coastal Current in fall and winter, appearance of a southward coastal jet off the west coast of Korean Peninsula in winter, and northward expansion of Taiwan Warm Current in summer. In response to the seasonally changing circulation, the Changjiang plume dispersal is primarily southward in winter and northeastward in summer. Two upwelling centers near the shelfbreak are worth noting. One is to the northeast of Taiwan and the other is to the southwest of Kyushu. Both are associated with anticyclonic meanders of the Kuroshio Current. Along the Kuroshio axis, a series of upwelling and downwelling centers develops in response to topographic forcing. East of Ryukyu Islands, the model also predicts a persistent upwelling location east of Okinawa Island.

A climatological description of circulation in and around the East China Sea

210Pb, 137Cs and 239,240Pu in East China Sea Sediments: Sources, Pathways and Budgets of Sediments and Radionuclides

As part of the time-series program in the Joint Global Ocean Flux Study (JGOFS), the major objective of the multidisciplinary and multi-institutional SouthEast Asian Time-series Study (SEATS) is to investigate through direct observations the processes that influence the carbon cycle and bioactive elements in the intra-annual to decadal time scale. The SEATS site is situated at 18.3°N and 115.5°E in the tropical oligotrophic northern South China Sea. Among the active time-series stations, it is located at the lowest latitude and it is the only one located in a marginal sea. The major defining environmental characteristics at this site include: oligotrophic water, outside the influence of major rivers, high surface temperature with little seasonality, strong and persistent stratification, thin mixed layer, shallow top of the nutricline depth, deep photic zone, distinct monsoonal seasons, frequent traverses of tropical cyclones, and a high depositional flux of aerosols. Given these environmental conditions, a site-specific hypothesis in SEATS is that the interactions between the upper waters and the atmosphere constitute a major controlling mechanism of the biogeochemistry in the northern South China Sea. The SEATS station has been occupied at approximately seasonal intervals since 1999. The results obtained primarily between 1999 and 2003 in the initial phase of this study are presented here. In contrast to the generally accepted notion that seasonal variations in the tropical waters are minimal, well defined, and regular seasonal patterns were observed in the carbon cycle, the nutrient dynamics and the biological community structure in the northern South China Sea. In the carbon cycle, superimposed on the large seasonal signals were the more subtle inter-annual variations of progressive increases in the mixed-layer CO2 fugacity (fCO2) and total dissolved CO2 (TCO2) that also were observed in other time-series stations. This suggests that the absorption of CO2 by the oceans in response to the increase in atmospheric CO2 in modern times may well be globally manifested. The nutrient dynamics indicate that N2 fixation could play a key role in keeping the nutrient cycle in balance in the northern South China Sea.

The SouthEast Asian Time-series Study (SEATS) and the biogeochemistry of the South China Sea—An overview

Ecological anomalies in the East China Sea: Impacts of the three gorges dam?

The Kuroshio edge exchange processes (KEEP) study is a multidisciplinary study on the internal cycling of material, especially carbon, within the East China Sea Shelf and the exchange of material between this Shelf and its adjoining Kuroshio. The project has been ongoing since 1989. The East China Sea Shelf is a net sink of atmospheric carbon dioxide. Rich supplies of nutrients, mostly from the upwelling of the Kuroshio Subsurface Water and, to a lesser extent, from the riverine discharges, notably from the Changjiang, sustain a high primary production (550 mg C m−2 d−1) on the Shelf and help the draw down of carbon dioxide. The sum of the demands for organic carbon for sustaining the observed bacterial production in the water column and the rate of sulfate reduction in the sediments of this Shelf appears to exceed its primary production. This suggests that a large fraction of the photosynthetically fixed carbon is recycled effectively within the Shelf. However, a comprehensive and definitive carbon budget for the Shelf cannot yet be constructed. Organic particles that survive oxidation within the Shelf and reach the Okinawa Trough are deposited in a belt along the upper northwestern slope of the Trough. A particle-rich mid-depth layer and the very high fluxes of sinking particles off the shelf break northeast of Taiwan suggest active cross shelf transport of particles from the Shelf to the Okinawa Trough. The cyclonic eddy at the shelf edge northeast of Taiwan is an important pathway for the exchange of dissolved and particulate materials between the Shelf and the Kuroshio. Nitrogen fixation may be a significant contributor of combined nitrogen to the oligotrophic Kuroshio Surface Water and the Taiwan Strait Warm Water so that it may support up to 25% of the new production in the Kuroshio Surface Water.

The Kuroshio edge exchange processes (KEEP) study — an introduction to hypotheses and highlights

The Princeton Ocean model was used as a tool to examine the plume dynamics of river effluent and associated sediment dispersal of a small mountain river in southern Taiwan. A module of sediment transport was added to the model to address the advection, settling, and resuspension of sediment grains of a given size. The model was driven by synoptically observed water level variations and wind field. At the river mouth open boundary, sediment-laden river flow is prescribed to initiate the buoyant plume. The model was run for a 12-day period from late July to early August 1996 which includes two typhoon events. In this paper, three 12-h segments of the simulation using medium-grained silt (15.6 m) as the sediment size are presented and discussed. Each of these segments represents plume behavior under the influence of (a) longshore tidal currents alone, (b) additional influence of northerly typhoon winds, and (c) additional influence of southerly typhoon winds. The results show that the dispersal of Tseng-wen River plume is determined by the interplay of the buoyancy of the effluent, the turbulent mixing by tide-dominated coastal flows in the lower part of the water column, and the turbulent mixing by the wind field in the upper part of the water column. The coastal jet related to the plume facilitates the dispersal of river sediment to the coast on the right-hand side of the river mouth. The grain-size composition of the river effluent also plays a role in the plume-related sediment transport because it not only determines the total amount of sediment dispersal, but also the temporal and spatial characteristics of the sediment flux associated with the plume. Other mechanisms that this model does not address, including effects of surface and internal waves should also be considered in the future.

Numerical modeling study of sediment dispersal by a river plume

A comprehensive field experiment was conducted to monitor the plume of the Tsengwen River, a monsoon-regulated small mountainous river located on a wave-dominated micro-tidal coast in southern Taiwan. The field work included the deployment of a tripod near the river mouth that measured the salinity, temperature, tide, waves, current, and turbidity at 1 m above the bed. Shipboard profiling of the salinity, temperature, and turbidity was also conducted. In addition to the ground survey, a Landsat TM image taken during the time of the field work was converted to a map for suspended sediment distribution in the study area. Our results show that the expansion and contraction of the river plume is largely controlled by the river discharge, tidal phase, and the wind field. Horizontal advection, downward settling, and the resuspension are the three mechanisms that control the near-bed suspended sediment concentration near the river mouth. Both forms of hypopycnal and hyperpycnal plumes from the river have been documented. Our study shows the type of plume of small rivers not particularly rich in suspended sediments on wave-dominated coasts have greater variability and is subject to the influence of the suspended sediment dynamics. In the presence of weakened runoff and absence of runoff, stratification can become unstable in the farfield when vertical movements of suspended sediments are present. In summary, the plumes observed in this study represent three types of plumes in terms of relative dominance between the river and nearshore suspended sediment dynamics.

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Shenn-Yu Chao

Papers

The Kuroshio edge exchange processes (KEEP) study — an introduction to hypotheses and highlights

Numerical modeling study of sediment dispersal by a river plume

The Influence of Suspended Sediments on the Plume of a Small Mountainous River