228Ra, 228Th and 234Th in the upper 300m of the water column were measured to quantify new production and export flux of POC at the intercalibration station in the South China Sea during November 1997. Surprisingly high 228Ra concentrations of about 210dpmm-3 in the surface mixed layer were observed. The concentration decreased considerably, from >200dpmm-3 at 25m to ～43dpmm-3 at 200m, then increased to 87dpmm-3 at 300m close to the bottom (460m), coincident with the typical distribution pattern of 228Ra in open oceans. Concentrations of dissolved and particulate 228Th ranged from ～45 to ～27dpmm-3 and from ～40 to ～10dpmm-3, respectively, and showed a general decrease with increasing depth. A 234Th deficit relative to 238U was also observed in the upper 150m, as quantified by b-counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled 228Ra-nitrate approach and further converted into a new production of 4.4mmol Cm-2 d-1 based on a Redfield ratio of 6.6 for C: N. The 234Th-238U disequilibrium and the measured ratio of POC to particulate 234Th yielded a POC export flux of 5.7mmol Cm-2 d-1, which is in general agreement with the new production derived from nutrient budgets. POC export based on the 228Th-228Ra disequilibrium was estimated to be 1.7mmol Cm-2 d-1, significantly lower than the derived new production. DOC transport or accumulation, data uncertainties and the different time scales to which the various methods were applied could be reasons for the discrepancy. The vertical fluxes of nitrate and phosphate to the euphotic zone bear a N/P molar ratio of 35, which is significantly higher than the Redfield ratio and suggests P-limitation in the southern South China Sea.

Marine colloids could be an important source of nitrogen for bacteria and photoplankton. But elevated concentration of colloids may stimulate algal growth and lead to red tides in coastal waters. The effects of colloidal organic carbon (COC) concentration on the growth of photosysthetic bacteria (PSB) were investigated under different colloidal treatments in the laboratory. The PSB growth was inversely proportional to COC concentration and was restricted by high-molecular-weight (HMW) colloids (>10KDa) in treatments with non-nutrient or just inorganic nutrient with low COC concentration (≤5μMC). However, the PBS growth was enhanced in the presence of HMW colloids in the treatment with inorganic nutrient and high COC (127 and 255μMC) or with both inorganic nutrient and low-molecular-weight organic matter. Both bacteria number and bacteria growth ratio increased significantly when the concentration of COC was ≥5μMC. Our results suggest that marine colloids can be utilized by bacteria and might regulate primary productivity in coastal waters.

Seawater samples were collected in the water column from the Canada Basin aboard RV Xuelong in August 1999. Concentrations of d D, d 18O, nutrients (NO3-, PO43-, SiO32-) and dissolved oxygen were measured, along with hydrographic parameters (salinity and temperature). Our results showed that the upper layer of the water column was characterized by the occurrence of the upper halocline water (UHW) and the lower halocline water (LHW). The UHW was associated with a salinity of 33.1 (～150m depth) and maximums of nutrients, NO and PO*, whereas minimums of NO and PO* (PO*=PO3- + O2/175-1.95mmol/dm3) occurred at the depth of LHW (～300m depth). Two tracer systems, S-d 18O-PO* and S-d D-SiO3 2-, were used to estimate the fractions of the Atlantic water, Pacific water, river runoff and sea ice meltwater in water samples. Combined with the nutrient ratio NO/PO, it was suggested that the UHW was derived from the inflow of the Pacific water through the Bering Strait. These waters were modified to obtain the high salinity and nutrients in the Chukchi shelf or/and the east Siberian shelf. The LHW was maintained by inflow of the Atlantic water through Barents Sea and subsequent mixing with freshwater in the shelf region to produce the signals of NO and PO* minimums. In study basin, the river runoff signals were confined to water depths less than 300m and the fractions of river runoff decreased with the increasing depth. Water column inventories of river runoff and sea ice meltwater were calculated between the surface and 300m. The river runoff inventories in the Canada Basin were higher than those in other sea areas, suggesting that the Canada basin is a major storage region for Arctic river water. The sea ice meltwater signals suggested that the Canada Basin is a region of net sea ice formation and the inventories of net sea ice in the upper water column increasing from the south to the north.

Primary production, bacterial production, particulate organic carbon fluxes and organic carbon burial rates were quantified during the summer period of 1999 in the Arctic Ocean via 14C uptake, 3H uptake, 234Th/238U disequilibrium and 210Pbex dating, respectively. The integrated primary production in the water column was as high as 197mmolC/(m2

Thorium isotopes (228Th, 230Th, 232Th and 234Th) are useful tracers for studying particle dynamics and trace element scavenging in marine environments. In this study, surface waters were collected along a salinity gradient from the Jiulong River estuary, China, for determination of activity concentrations of 228Th, 230Th and 232Th in different size fractions, namely, the >53μm, 10-53μm, 2-10μm, 0.4-2μm, 10kDa-0.4mm and the <10kDa fractions. Our results indicated that the activity concentrations of 228Th, 230Th and 232Th in the Jiulong River estuarine waters were significantly higher than most of the previously reported values in coastal and oceanic seawaters, suggesting a higher lithogenic U and Th contribution from the Jiulong River Basin. When normalized to the particulate mass concentration, the activity concentrations of the three thorium isotopes decreased with increasing particle size, demonstrating the important role of surface areas of particles in controlling the scavenging of thorium from the water column. The partitioning of three thorium isotopes showed a common characteristic, i.e., the >53μm fraction had the least share (0-1%), while the 10-53μm fraction had the largest share of Th isotopes. The average value of the 230Th/232Th activity ratio (230Th/232Th) A.R. increased from 0.8 in the >53μm fraction to 3.7 in the 10kDa-0.4μm fraction, indicating that the radiogenic Th isotopes are preferentially scavenged by the small size particles. (230Th/232Th)A.R. in the <10kDa and 10kDa -0.4μm fractions were similar, however, suggesting a similar chemical composition and/or equilibrium partitioning between the low molecular weight and colloidal Th. It was very interesting to note that the geochemical behaviors of the three Th isotopes were different from each other. Dissolved 228Th had the highest concentration in the mid-salinity region, showing a non-conservative behavior with additional input. In contrast, dissolved 232Th showed a concave profile, indicating a net removal of 232Th during the mixing of fresh water with seawater. The behavior of Th isotopes in the 10kDa fraction followed those in the dissolved phases. The difference in geochemical behaviors among three Th isotopes was ascribed to their different sources in the estuary.