The study investigated seasonal and spatial distributions and biogeochemical processes of dissolved and particulate organic matter in the upper layer of northern South China Sea (SCS). Concentrations of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) in the euphotic zone of northern SCS were in the range of 55-147 £gM, 2.4-9.9 £gM and 0.13-0.38£gM, respectively. A maximum concentration of DOC, DON and DOP, was found in the station close to the Pearl River due to freshwater input. The concentration of DOC decreased generally with distance away from the continent, but the ratio of DOC/TOC increased with distance primarily due to trophic dynamics. Concentrations of DOM were generally the highest in the surface layer and decreased with depth, but their C/N/P ratios increased with depth, indicating that both nitrogen and phosphorus were preferentially decomposed over carbon. Below the mixed layer, DOC degradation contributed only about 16% of AOU (apparent oxygen utilization). Inverse correlation between DOM and density was significant in the upper layer suggesting that the distributions of DOC, DON and DOP were largely controlled by vertical mixing. Inverse correlation was also significant between DOM and AOU, showing the effect of microbial decomposition on DOM in deep water. Concentrations of POC, PON and POP in the euphotic zone were in the range of 1.8-17.7£gM, 0.18-2.45£gM and 10-163 nM, respectively. Relatively high concentrations of POC, PON and POP in the surface water of inner shelf were also likely attributed to the input of the freshwater. Significant correlation between POC abundance and Chl-a suggested that phytoplankton abundance may control the distribution of POC. The ratio of ¡µPOC/¡µPON in the euphotic zone ranged from 4.57 to 7.3, implying various effects of bacteria and planktons on POM. A simple one-dimensional vertical eddy diffusion model was applied to estimate downward fluxes of DOC and POC and upward fluxes of nutrients across the boundary of euphotic layer and/or thermocline base. Total downward fluxes of organic carbon (OC) were compared with total upward nutrient-derived OC fluxes. The results suggested that additional nutrient sources in the euphotic layer were required to balance OC budgets. The ratios of DIN/DIP were much smaller than the Redifield N/P Ratio of 16:1, suggesting a status of N-limitation in the euphotic zone. The DOC/DON ratio, however, was much higher than the Redfield ratio. These results implied that DOM must play an important role in modulating nutrient cycling and food web dynamics in the euphotic layer.

In this study, the sediment cores taken in the northern South China Sea (SCS) as well as the settling particulates collected from time-series sediment traps deployed in the same area have been analyzed for Pb-210 activities in order to estimate the sedimentation rate and mass flux from core data and to obtain temporal variations in mass flux and Pb-210 from the time-series sediment traps. The main purposes are to compare and to discuss the mass balance problem between the sediment trap and core results in terms of mass flux, Pb-210 activity and its fluxes. The time-averaged particulate fluxes measured from different depths at M3S and M1T sites in the northern SCS generally increase with depth, reflecting an increasing effect of the lateral transport. The upper and middle traps display a synchronous trend in mass flux variations. The mean particulate flux at S5 site near Luzon Strait is clearly higher than the two sites mentioned above probably because of the effect of topography and bottom current. Distributions of Pb-210 are influenced by particulate scavenging: the longer the settling particles stay in the water column the more the surrounding Pb-210 will be scavenged. The temporal variations of Pb-210 at M3S show a similar trend and an increase with depth. At M1T site, the temporal variations of Pb-210 show no clear trend due to insufficient samples. It has been commonly observed in the marginal sea that Pb-210 activity is inversely correlated with the associated mass flux, i.e. higher Pb-210 is associated with lower mass flux in terms of their temporal variations. This study is also in line with such observations. The mean water content of the core at I located near the shelf break in the northern SCS is about 28%, and its mean loss on ignition (L.O.I.) is 3.2%. These are similar to those observed previously in the northern SCS (water content: 25-37%; L.O.I.: 3%-8%). The sedimentation rate as determined from the excess Pb-210 profile at core I is 18cm/100yr which is at the lower end of the previous study (16-52 cm/100yr) (Wu, 2006). The sediment flux and sedimentation rate estimated from both the Pb-210 inventory (I) and the upper limit of sedimentation rate are identical, respectively, at 0.32 g/cm2/yr and 18cm/100yr. Thus the mixing effect could be neglected. The Pb-210 flux estimated from the deep sediment trap at 2163m (M3S, 77.4 dpm/m2/d) is much lower than that observed from the core sediment (F, 761.1 dpm/m2/d). Although the specific Pb-210 activity of the particles is much greater than that in the surface sediment, the particle flux is too small relative to the mass flux of the sediment, suggesting that additional sediment with Pb-210 has been transported laterally from elsewhere and deposited here. This results in a large imbalance between the sinking particulates and the underlying sediment in mass flux and Pb-210 flux.

The carbon and oxygen isotopes of planktonic foraminifera, and faunal assemblages of towing samples, £_13C of dissolved inorganic carbon, and £_18O of seawater collected from northern South China Sea (SCS) were analyzed in this study. Plankton tows were collected between December 2002 and December 2005 at water depth of 100m. In addition, fauna and sea water samples were sampled at different depths ranging between 50 and 300m at some stations. Generally, the faunal assemblages are dominated by Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides aequilateralis, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia menardii. G. aequilateralis and G. menardii are dominated in warm months while G. ruber, G. sacculifer, and P. obliquiloculata are dominated in cold months. The relative abundances of G. sacculifer and G. aequilateralis obtained from towing samples in this study are significantly higher than those shown in underlying sediment traps and surface sediments, suggesting that the tests of these two taxa are prone to dissolution. In addition, the abundance of P. obliquiloculata which serves as an index taxon for the Kuroshio Current in Paleoceanography, is more abundant in the northern part than in the southern part of the SCS basin. Nevertheless, the £_13C and £_18O of G. sacculifer and P. obliquiloculata are significantly lighter than that in sediment traps and surface sediments, indicating some potential effects caused by the differential dissolution. Possible dwelling depths of P. obliquiloculata is estimated between 50 and 90m, whereas N. dutertrei is estimated between 20 and 80m according to Kim and O¡¦Neil (1997). The latter changes to 40~160m when the equation of Bouvier-Soumagnac and Duplessy (1985) is applied. Oxygen isotope compositions of G. sacculifer and G. ruber display a significant correlation with temperature (T) in the surface mixed layer. The slopes of £_18O/ T for these two species are similar to each other. The differences of £_18O (£G£_18O) between G. sacculifer and N. dutertrei, G. sacculifer and P. obliquiloculata, G. ruber and P. obliquiloculata are proved to be a good proxy indicator for reconstructing the upper water column based on the statistic relationship between £G£_18O and £GT (difference between mixed layer and 100m) in this study.

The purposes of this study are to understand the variations of the particulate and dissolved Pb-210 and Po-210 in profiles in the South China Sea and to estimate the residence time of these nuclides through their extent of radioactive disequilibrium. The profiles of Pb-210 and Po-210 in particulate and dissolved phases were measured at four stations in the northern South China Sea (station B, C, D and F) and three in the Bashi Channel (stations W2, W3 and W5) from four Ocean Researcher I cruises (ORI-575, ORI-580, ORI-588 and ORI-606). At Station C two profiles were taken, one before and the other after a typhoon to see its effect, if any, during the ORI-588 cruise. As the associated Ra-226 profiles were not measured their relations to Pb-210 profiles are not available for discussion in this thesis. The mean suspended particulate matter (SPM) concentration for each station is about 0.2 mg/kg with small deviations. Activities of Pb-210 and Po-210 in the northern South China Sea were all higher than those in the sea off NE Taiwan easured during the KEEP program. The dissolved Pb-210 profiles generally display a maximum around 15 to 25 dpm/100kg in the surface layer due to atmospheric input, and a minimum of about 5 to 15 dpm/100kg from 300 to 1000 m. The activities below 1000 m vary only slightly with a deep maximum as high as 20 dpm/100kg around 2000 to 3000 m, as seen at Station C. The particulate Pb-210 is around 3 to 5 dpm/100kg with small variation for the entire water column. The dissolved Po-210 profiles are somewhat similar to the dissolved Pb-210 profiles, but lower by about 5 to 10 dpm/100kg in activity. The mean residence time is about 0.76 year for total Po-210 in the entire water column, and 0.35 to 2.13 years for total Pb-210 in the mixed layer. At each station, Po-210 is deficient relative to Pb-210 in either the particulate or dissolved phase. The mean total Po-210/Pb-210 activity ratio is about 0.58, indicating that Po-210 is more effectively scavenged. It is imperative to understand the fate and pathway of the missing Po-210. Judging from its biophilic nature, Po-210 may have been absorbed or consumed by biomass and enriched in various tropic levels in the food chain, in addition to being scavenged and removed by sinking particles.

This study reports the first sediment trap mooring results obtained under the SIBEX program (South China Sea Integrated Biogeochemical Experiments). Two strings of sediment traps were deployed respectively at M1 located to the southwest of Taiwan, and M2 in the northern basin of the South China Sea (SCS). Box cores were also taken at several sites to the south of M2. The main purposes are to measure settling particulate fluxes at various depths for the studies of temporal and spatial variations of the particulate flux and 210Pb activity. The box cores were used to determine the sedimentation rates. These are to enhance our understanding of the characteristics of the particulate distribution and the roles the particulate matter plays in the biogeochemical processes in the SCS. Particulate fluxes measured from different depths at M1 and M2 generally increase with depth. In temporal variation, M1 has higher amplitudes than M2, with highest amplitudes at 948 m where highest flux (2025 mg/m2/d) was observed. The particulate flux at 948 m has higher values than at 248 m, probably due to lateral transport. At M2, the particulate flux at 240 m has higher values with greater amplitudes than at greater depths, i.e. 1240 m, 2240 m and 3240 m, where their particulate fluxes show a synchronous trend with small amplitudes in temporal variation. The time-averaged particulate flux for each trap ranges from 199 to 554 mg/m2/d, consistently higher than previous observations (76~104 mg/m2/d). However, our values are comparable to the mean particulate flux (280 mg/m2/d) estimated from terrigenous inputs. The apparent changes in particulate flux in the SCS over the past ten years warrants further investigations. The temporal variations of Pb-210 show a synchronous trend and a rapid increase with depth as observed at M1 and M2. This rapid increase of Pb-210 with depth reflects effective scavenging by sinking particulates although particulate concentrations are low. The loss on ignition (L.O.I.) at M1 and M2 was inversely correlated with Pb-210, indicating that Pb-210 was expelled from biogenic particulates or organic matter. The particulate fluxes around 240 m at M1 and M2 were generally positively correlated with the L.O.I., suggesting that the high particulate fluxes are probably contributed by biogenic particulates or organic matter. The sedimentation rates as determined from excess Pb-210 profiles range from 9.01~23.13 cm/100yr. These rapid sedimentation rates reflect the effect of mixing and additional sediments accumulated through lateral transport. The surface layers of these cores were subject to perturbation and mixing, based on the penetration depths of the excess Pb-210. The Pb-210 flux estimated from the inventory of excess Pb-210 in the sediments is much greater than that observed from the traps, suggesting that additional Pb-210 has been accumulated via lateral transport and slumping of nearby surface sediments. Based on the mean particulate flux observed, one may roughly estimate the corresponding sedimentation rate of about 10 cm/ka, which is an order of magnitude lower than those determined by the excess Pb-210 method. Because the upper layers of the sediments have been strongly disturbed and mixed with the additional sediments accumulated through lateral transport, the sedimentation rates as determined by the excess Pb-210 method are probably over-estimated by a factor of 10 to 20.

The time-series sediment traps were deployed in the northern South China Sea (NSCS) to collect settling particles to study the spatial and temporal variations of particle fluxes, particle sources and particle compositions. Each recovered sample was analyzed to determine the particle flux, major compositions (OM, Carbonate, opal and lithogenic materials) and trace metals (Al, Fe, Mn, Cd, Zn). The data were employed to elucidate the biochemical processes in controlling particle and chemical fluxes. The results show that the particle fluxes vary significantly in time and space, and were apparently related to terrestrial inputs and monsoon-effected processes. Episodic events such as typhoon and earthquakes also effected the fluxes considerably. Vertical fluxes generally decreased with depth for biogenic components (POC, carbonate, opal) but increased with depth for the lithogenic component. Most lithogenic fluxes below the surface were likely derived from lateral transport and decreased with distance away from Taiwan Island. Biogenic fluxes in surface layers were generally higher in winter than in otherseasons, which may be resulted from the elevated primary production in winter. Typhoons also enhanced biogenic fluxes in surface layers but biogenic fluxes were diluted in deep layers by strong lithogenic fluxes. Carbonate dominated the biogenic fluxes in the upper layer and may play as the major ballast in the transportation of biogenic materials through the water column. The C/N ratios of organic matter increased generally with depth implying that nitrogen was preferentially decayed over carbon through the water column. Lithogenic elements such as Al, Fe, Mn were closely correlated and increased in fluxes with depth. However, microelements associated with biogenic materials (Cd, Zn) displayed a significant decrease in content with depth, showing a rapid recycling in water columns.

Seasonal dynamics of unicellular diazotrophs were investigated in the upstream Kuroshio and the northern South China Sea (SCS). Unicellular diazotrophs had been postulated as an important N2-fixing contributor for the phenomenon of N* in the SCS where abundances of filamentous Trichodesmium and Richelia were scarced. Samples were collected during four cruises between August 2008 and August 2009 in summer (CR1310 and CR910), winter (CR886), and late spring (CR899), respectively. Sampling stations located between 21¢XN-22¢XN and 116¢XE-122¢XE in the upstream Kuroshio off southeast Taiwan and covering the shelf and basin waters of the northern SCS. The abundance of the unicellular diazotrophs was determined using whole-cell immunocytochemical method in which antibody of nitrogenase was used as the probe. Cells containing nitrogenase can be visualized and counted after the antigen-antibody reaction under microscope. Unicellular diazotrophs were classified to four types according to their sizes and shapes. For diameters of those with 1-3 £gm and in coccoid shape are called 1-3 £gm C, diameters of 1-3 £gm and in rod shape are called 1-3 £gm R, diameters of >3-10 £gm and in coccoid shape are called >3 £gm C, and diameters of >3-10 £gm and in rod shape are called >3 £gm R. Surface abundance of the unicellular diazotrophs was highest in winter in both the Kuroshio and the SCS, followed by summer, and was least in late spring. Among four cell types, 1-3 £gm C usually was the most abundant group, followed by 1-3 £gm R and >3 £gm R, and was least for the group of >3 £gm C. The abundances between groups of 1-3 £gm C and 1-3 £gm R were positively correlated. Likewise, the abundances between >3 £gm C and >3 £gm R were positively correlated. However, the total abundance of small cells (1-3 £gm C+R) was not significantly related to the large cells (>3 £gm C+R). During summer and late spring, the abundance of unicellular diazotrophs in the SCS was 1.3-2 times of that in the Kuroshio. However, in winter the abundance in the Kuroshio was 1.2 times of that in the SCS. Surface water temperature was found negatively correlated to the abundance of 1-3 £gm C, >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R), respectively. Significant correlations among surface water temperature and surface chlorophyll a, [NO2+NO3], SRP and N:P ratio implicated that the dynamics of cell abundances could be attributed to the correlated ecological variables of surface water temperature. The dynamics for the abundances of >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R) were suggested to relate with the fluctuation of SRP concentration. Unicellular diazotrophs accounted for 60-90 % of total unicellular cells in terms of cell number. Vertical distributions of unicellular diazotrophs in the Kuroshio and the SCS were in similar trends, with maximum abundance in deep water during summer and late spring, and on surface water during winter.

Uranium and thorium radionuclides were measured on two gravity cores (T17G and T18G) and one box core (T19B) collected from the western South Okinawa Trough (SOT), one gravity core from off shore Southwest Taiwan (N3) and three box cores (C, D and E) from the northern South China Sea (SCS) in order to examine the variations of these radionuclides and their activity ratios in the sediments of the areas and to characterize the source function of the sediments and their geochemical implication based on these nuclides. For long half-life radionuclides such as 238U, 234U, 232Th and 230Th, the activities in the cores of the SOT and Southwest Taiwan areas show no significant vertical or areal variations, implying no significant variation in sediment supply or depositional environment within the past 100 years. The average activity of 238U is 1.65 dpm/g and 1.33 dpm/g in the SOT and Southwest Taiwan areas, respectively, and that of 232Th is 3.57 dpm/g and 3.34 dpm/g, respectively. The average activities of 238U and 232Th are, respectively, 1.37 dpm/g and 2.37 dpm/g in the SCS. The mean 232Th activity is lower in the SCS than in the SOT and Southwest Taiwan. The mean 232Th activity of the sediments in the SOT and Southwest Taiwan is quite comparable to that of the shale, slate and black schist in Taiwan, suggesting that these sediments are the terrigenous detrial materials from Taiwan. The 238U and 234U activities in the cores of these two areas show no significant vertical nor areal variations with activities ranging between 1.3 and 2 dpm/g, and their 234U /238U activity ratios being about 1.1, quite close to that of seawater (1.14). Since 238U and 234U are quite comparable among the three areas, the higher activity of 230Th in excess over 234U in the northern SCS may be due to greater water depth that allows more 234U produced 230Th to be scavenged from the water column. The uranium and thorium radionuclides and their activity ratios in the SOT and Southwest Taiwan sediments suggest that these sediments are the terrigenous detrial materials from Taiwan. The source function of the SCS sediments is more complex than that of the above-mentioned sediments.