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.

Pb-210 and Po-210, a parent-daughter pair, are particle-reactive radionuclides. Pb-210 tends to be associated with inorganic particles but Po-210 prefers organic particles. In the context of these characteristics the purpose of this study is to determine Po-210 and Pb-210 in the surface water plankton of the northern South China Sea (SCS) and the Luzon Strait (LS) areas in order to understand their temporal and spatial distributions and the extent of their radioactive disequilibrium. As the LS has provided a pathway for the exchange between the Kuroshio and the SCS waters, the study area has the characteristics of an open ocean and a marginal sea. The plankton Po-210 activities in the study area are about 10~400 dpm/g, but may reach 1200dpm/g in an El Nino year, the effect of which on Po-210 is not clear at present. The Po-210 in the plankton is mainly affected by the surface water Po-210 , biomass concentration, and the Po-210 in the surface water as well as plankton transported from other area(s). If the biomass concentration (as indicated by Chlorophyll-a) increases, the Po-210 in the plankton decreases, i.e. they are inversely correlated. Based on the planktonic Po-210 distribution, the Kuroshio water which has both high Po-210 and high Po-210-bearing plankton has evidently intruded into the northern SCS. The Pb-210 activities of in the plankton vary from 5 to 25dpm/g; the variation trend is similar to that of Po-210 but with much lower activity, resulting in a Po-210/Pb-210 ratio much greater than unity. Compared to the suspended particles, the plankton is highly enriched in Po-210 but it strongly repels Pb-210. Based on earlier studies and this one, the extent of Po-210 enrichment (as indicated by the Po-210/Pb-210 activity ratio) in various organisms increases sequentially from plankton to mussel, large swimmer and then to marine fish, i.e. the higher the level of organism in the food chain, the higher the ratio becomes. However, the Po-210 and Pb-210 activities per unit mass of these organisms are generally lower than those of plankton. Organisms of higher food-chain level may accumulate Po-210 in the digestive system, but part of it may be excreted. Po-210 accumulation does not occur in other parts of these organisms. In terms of adsorption, the plankton with greater surface area to volume ratio, can adsorb more Po-210 resulting in a higher specific activity, whereas large organisms with smaller surface area to volume ratio can adsorb less Po-210 yielding a lower specific activity. The enrichment of Po-210 in various organisms must have been achieved by absorption and adsorption with unknown proportion. Based on a simple box model calculation for the northern SCS water within the upper 100m layer, the excess Po-210 in the planktonic biomass can account for about 70% of the total deficit in this layer. But in the LS area, the excess and the deficit are balanced. This suggests that the Po-210 deficit in this surface water is due to absorption and adsorption by organisms as evidenced by their large Po-210 enrichment.

Abstract In this study, we have analyzed systematically the oxygen isotopic compositions of South China Sea (SCS) and Luzon Strait (LS) seawater so that a comprehensive interpretation of their temporary and spatial variability can be delineated. The oxygen isotopic compositions of the samples collected in the two areas were determined using the Epstein-Mayeda technique, and the overall precision of the d18OSMOW measurements is +0.1 ?. Our data suggest that LS seawater is a mixture of SCS and Kuroshio waters. The precipitation (1.5 mm) on SCS in April 1998 (during ORI517 cruise) is much less than that (169.5 mm) in April 1999 (during ORI546 cruise). As LS and SCS waters are concerned, the profiles of d18OSMOW generally share the same trend as those of salinity. The d18OSMOW values of the SCS surface water decrease toward the south, while those of the LS surface water decrease toward the west. These suggest that the effect of the Kuroshio water decreases are it moves from northeast toward southwest after its intrusion through LS. The salinity of Kuroshio water reaches the maximum at 34.92 with the corresponding d18OSMOW value of 0.42 ?, whereas the salinity of the representative SCS surface water is 33.34 and the corresponding d18OSMOW is -0.25 ?. As these two data were selected as end members, the estimated proportion of the intruding Kuroshio water in the composition of LS water could be high up to 80 % at 121.5o E. The plot of d18OSMOW versus salinity for cruises OR517 and 546 shows a difference between the slopes of the two regression lines, indicating the effect of variability in precipitation and the depth of the mixed layer. The average d18OSMOW value for surface water decreases toward the west as the water moves from Western Philippine Sea (WPS) to LS and SCS. The average d18OSMOW values for the surface, subsurface, intermediate, and deep waters are listed in the corresponding order in parentheses preceded by the studied area as follows: WPS (0.29 ?, 0.25 ?, -0.06 ?, -0.10 ?); LS (0.15 ?, 0.17 ?, -0.02 ?, -0.08 ?); SCS (-0.03 ?, 0.18 ?, -0.05 ?, -0.08 ?).

Abstract This research investigated the distribution of Trichodesmium in the South China Sea and the ecological factors that affect it. The survey periods were from August 1999 to October 2000. A total of 5 cruises were conducted, mostly in summer and autumn, including cruise 559 (August 18~25, 1999), cruise 629 (May 22~27, 2000), cruise 588 (July 3~14, 2000), cruise 597 (October 4-13, 2000), and cruise 657 (October 16-21, 2000). The sampling stations were located on the continental shelves, the slope zones, and the basin in the South China Sea between 13~22∘N and 114~120∘E. The density of Trichodesmium in the South China Sea varied greatly among the continental shelves, the slope zones, and the basin zones. Trichodesmium density in the continental shelf and the slope was high, with an average density of 150¡Ó297¡Ñ103 trichomes/m3 (4~1008¡Ñ103 trichomes/m3) in the surface water and 151¡Ó322¡Ñ103 trichomes/m3 (13~1114¡Ñ103 trichomes/m3) in 2-60m water depth. This could be caused by the influence of the mixing of the Kuroshio water and the substances of terrestrial origin. In the basin, Trichodesmium density was low, with an average density of 5¡Ó7¡Ñ103 trichomes/m3 (0~22¡Ñ103 trichomes/m3) in the surface water and an average density of 6¡Ó7¡Ñ103 trichomes/m3(0~22¡Ñ103 trichomes/m3) in 2~60m water depth. The Trichodesmium communities are mainly composed of T. thiebautii and T. erythraeum, with T. thiebautii being more abundant than T. erythraeum. Their relative importance at the continental shelf and the slope zones was 58% and 36% respectively, and 64% and 31% in the basin respectively. Trichodesmium appeared commonly as a single trichome, rarely in colony. The vertical distribution of Trichodesmium concentrated at 0-100m. The highest density was near the surface, varying between 4~1008¡Ñ103 trichomes/m3. In summer, the Trichodesmium in the continental shelf and the slope zone of the South China Sea was slightly higher than that in the East China Sea. The influences of temperature, nutrient abundance and wind speed on the distribution of Trichodesmium are not obvious. Trichodesmium in shallow water became abundant after the passage of a typhoon. The maximum density of Trichodesmium was observed in shallow water and average number of trichome per colony decreased. The influences of the typhoon on Trichodesmium species composition and colony density were not obvious. High density of Trichodesmium occurred at shallow waters and the water column with low stratification index (SI) which repressed the properties of waters in the continental shelf and the slope zones. The regions in which high-density Trichodesmium appeared, were the continental shelves and slope which showed water properties similar to those of the Kuroshio water, indicating possible intrusion of the Kuroshio water in summer into the continental shelves and slope of the northern South China Sea.

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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.

Abstract The South China is the largest marginal sea in the western Pacific. It¡¦s unique geographic settings and high sedimentation rates preserve the paleo- climatic signals with larger amplitude than those from the open ocean. In this study, grain size and elemental compositions of the fine fractions (<63 mm) from the sediments collected by the ODP Leg 184 Sites 1143 and 1146 were analyzed to reconstruct the depositional settings for the last 8 My. Particle size and elements analyses, in conjunction with the carbonate contents and sedimentation rates from core sediments, reflect the possible increase in precipitation, which was caused by the strengthened summer monsoon between 5 and 3 Ma. In addition to the grain sizes change from silt-domain to clay-domain, Ti/Al ratio of sediments increase while the Si/Al, Zr/Al, and K/Al ratios decrease, which could be related to the enhanced sediments input through rivers. These environmental changes could be attributed to the uplift of Tibet plateau and the formation of Western Pacific Warm Pool. On the contrary, the impact of climate changes is not evident in the loess plateau in the northwestern China. It is likely that the climate in the South China Sea became warm and humid from 5 to 3 Ma were regional changes. Key words: Grain size, element analyses, South China Sea, summer monsoon

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.

The dynamics of marginal seas is complex in terms of carbon dioxide absorption and release. This thesis analyzes data collected in the southern Taiwan Strait and in the South China Sea. In order to deduct the influence of temperature on the fCO2, fCO2 is normalized to the average water temperature (fCO2 mean). In the spring of 2008, in the Taiwan Strait, when salinity was smaller than approximately 33.8, measured fCO2 mean and salinity had a negative correlation; but when the salinity was higher than approximately 33.8, the correlation was positive. When salinity was smaller than apprx. 33.8, fCO2 cal. mean correlated slightly negatively to chlorophyll. This indicates that the low fCO2 cal. was not only caused by the increase of the CO2 solubility at lower temperatures, but also by the biotic photosynthesis. On the contrary, when the salinity was higher than apprx. 33.8, fCO2 cal. mean and the chlorophyll held positive correlation. It indicates that the influence of photosynthesis was reduced. In this case, the primary factor of fCO2 cal. change was due to the mixing of the high normalized dissolved inorganic carbon (NDIC=35¡ÑDIC/S) China Coastal Current with low NDIC seawater. With a raise of seawater temperature, then a decrease of the CO2 solubility, seawater became a source of carbon dioxide. In the summer of 2008, the northern South China Sea was influenced by Pearl River plume, resulting in lower fCO2 and salinity. The fCO2 of the China coast was influenced not only by the Pearl River plume, but also by the Jiulong River plume and upwelling. The Taiwan Strait water mass mainly contains the South China Sea water, a Kuroshio branch and the China Coast Current. During an El Niño year, the monsoon weakens, so that the volume of Kuroshio entering the South China Sea increases. However, for La Niña years, the monsoon strengthens, therefore the volume of the Kuroshio entering the South China Sea decreases. As a result, the Taiwan Strait water changes interannually due to different mixture of seawater of the Kuroshio and the South China Sea. The southern Taiwan Strait could be divided into the Penghu Channel and the western strait. During an El Niño summer, the Penghu Channel is occupied by waters with high temperature, salinity and pH, but low NDIC and nutrients. This is because more Kuroshio waters enter the South China Sea, then move northward to the southern Taiwan Strait. The hydrology in the Penghu Channel in normal years shows different result from season to season. In the summer, the Penghu Channel contains low temperature, salinity and pH water. In winter, waters with high salinity and pH, but low AOU, NDIC and nutrients prevail. This indicates that less Kuroshio waters enter the South China Sea in summer than in winter. The hydrology of the Penghu Channel changes decidedly from season to season in a normal year but spring, summer and fall have no clear change in the El Niño period, because more Kuroshio waters enter the South China Sea in summer. The wind effect during the El Niño period becomes weakened, have the hydrology during summer monsoon is similar to the hydrology in spring and summer. The waters of the Penghu Channel reach the highest pH, but the lowest AOU, NDIC and nutrients in winter. Older waters from upwelling move to the north in the western Strait during spring and fall in a normal year. However, during the El Niño period, possibly due to the weaker monsoon, such upwelling signal is reduced. Waters of the western strait in winter have higher temperature, salinity and pH, but lower NDIC during the El Niño period compared to a normal year. This indicates that the El Niño influences not only the Penghu Channel but also the entire southern Taiwan Strait in winter.