| Silicon (Si) is the second most abundant element on the earth and is widely distributed in all the earth spheres. The chemical weathering of silicates on the continents and photosynthesis of diatoms in the ocean play important impact on the atmosphere CO2 at various time scales. The main mechanism for the marine carbon sinks is "biological pump". Diatoms are the most contributor to the global ocean primary production, which comsume Si during the growth to form the siliceous cells, therefore, the biological pump of the ocean is driven by the biological silicon pump to a large extent. Given these reasons, investigating the transition of Si from the lithosphere to the dissolved silica (DSi) pool, the transportation of DSi from the terrestrial to the marine system as well as the biogeochemical behaviors of DSi in the coastal environment, are of great importance to understand the Si cycle and its impact on the carbon cycle.The East China Sea (ECS) is characterized with high primary production. Previous studies have demonstrated that the DSi in the ECS shelf mainly originates from the incursion of Kuroshio waters, the input from the Changjiang and the northward waters from the Taiwan Strait. However, the contribution of each water mass to the euphotic stock of DSi and the spring diatoms blooms in different region of ECS still calls for further detailed investigation. As the dominant source of fresh water and important contributor of DSi to the ECS, there have been a few studies about the Si isotopic composition of the Changjiang river water. Nevertheless, more questions like how will the DSi concentration and isotopic composition discharged by the Changjiang response to the biological and geochemical processes, as well as the human activities within the drainage basin, still requires further investigations. In this study, for the first time we considered the Changjiang drainage basin, the Changjiang Estuary and the ECS as a whole system, and investigated the behaviors of DSi in these areas on the basis of both DSi concentration and Si isotopes techniques.(1) Analytical methods for silicon isotopes. A long-term precision of 0.08‰～0.10‰ (2sd) could be obtained for 829Si and 830Si under optimized instrumental settings. By using ammonia as the co-precipitator and optimizing the co-precipitation condition, DSi in various waters could be completely recovered within single magnesium induced co-precipitation and purified by following cation exchange resin separation technique, without inducing detectable isotopic fractionation. We participated the first inter-comparison study of the stable silicon isotope composition of seawater in international GEOTRACES program, and the measured 830Si for ALOHA300 and ALOHA1000 were+1.69±0.24%o (n=8,2sd) and+1.27±0.04%o (n=8,2sd), which were well agreed with the means values reported by 11 laboratories from 7 countries.(2) Silicon isotopic composition of DSi in the Changjiang Drainage and the influencing factors. The DSi concentration varied from 91.3 μmol/L～124.7μmol/L in the mainstream and from 58.4 μmol/L～194.0μmo/L in the tributaries, respectively. δ30Si ranged from+1.00‰～1.86‰ in the mainstream and from +0.83‰～+2.27‰ in the tributaries, respectively. Generally, an increasing trend in 830Si was observed from the upper to the lower reaches, and this increase was more obvious in the non-flooding season than the flooding season. The mass balance calculation showed that, during the flooding season, the exported silicon isotopic signature of the Changjiang was mainly controlled by the mixing process between the upstream waters and the tributaries and lakes in the middle and lower reaches. However, in the non-flooding season, the biological consumption of DSi by fresh diatoms might induced further fractionation of silicon isotopes. The supplement of ground water to the Changjiang mainstream might insert only limited impact on the silicon isotopic composition in the mainstream.On the basis of both silicon isotope ratio and the major ions, we proposed that the weathering process might not be the controlling factors on the silicon isotopic composition in the Changjiang basin. Besides, it was found that the 830Si of the tributaries showed significant positive correlation with the coverage broadleaf within the secondary drainage basin (r2=0.78,p<0.05), suggesting the influence of biological fractionation of fresh diatoms and higher plants. Based on the observations conducted in the Three Gorge Reservoir (TGR) and the output water of the TGR at Yichang station, we proposed that there was no convincing evidence of the retention effect of the TGR on the DSi has at this stage. Compare with the previous published studies, the evolution of silicon isotopic composition from upper to lower reaches displayed different trends in the Changjiang basin among different observation periods. Particularly, significant difference in δ30Si mostly occurred at the locations with tributaries input, inferring that this difference were mainly due to the heterogeneity caused by the mixing between the mainstream and tributaries.(3) Silicon isotopic chemistry in the Changjiang Estuary and coastal regions. The observed DSi varied from 11.2 μmol/L～116.5 μmol/L in summer and from 5.92 μmol/L～112.9 μmol/L in winter, respectively, and the δ30Si of DSi ranged from+1.48‰ ～+2.3‰ in summer and from+1.54‰～+1.95‰ in winter, respectively. The DSi concentration and δ30Si in the fresh water end-member were estimated to be 112.7±3.2 μmol/L and　+1.61±0.07‰. The DSi concentration was dominanated by the mixing process between the fresh and saline end-members. In summer, the δ30Si remained close to the river end-member up to salinity of 20, beyond this salinity it showed a significant increase due to stronger biological consumption of DSi.The isohaline of 20 might be taken as an important boundary of DSi transport in the Changjiang Estuary and the limiting factors for the growth of diatoms. Within this salinity, the light limitation and abundant DSi supplement from the riverine input lead to a limited utilization of DSi, thus the 830Si mainly inherited the signature of the Changjiang fresh end-member; beyond this salinity, the respelling of light limitation and reduced nutrients concentration resulted in an enhanced depletion of DSi by the diatoms, thus leading to the fractionation of silicon isotopes. On the basis of summer samples and Steady State Model, the Si isotope fractionation factor (30ε) of diatoms was estimated to be-0.95‰, in agreement with the previous laboratory cultured experiments for the Skeletonema costatum.(4) Contribution of different source of dissolved silica to the spring diatom bloom over the East China Sea shelf. During the observation period, the DSi ranged from 1.3 μmol/Lto 116.5 μmol/L, and 830Si ranged from+0.85‰～+2.53‰ with a total variation range up to ca.1.7‰. The DSi in the surface layer exhibited a decreasing trend on the off-shoreward, while the bottom waters showed an overall higher DSi concentration than the surface water. With respect to the δ30Si, the surface water was more enriched with heavier Si isotopes compared to the underlying waters, indicating a stronger biological fractionation process in the surface layers.Diatoms in the coastal waters of ECS were mainly controlled by the Changjiang Diluted Water and Zhe-Min coastal water; in the middle shelf, diatoms in the surface water were largely supported by the DSi from the incursion of Kuroshio Subsurface Water. The lateral advection of the Kuroshio Surface Water and mixing with other water masses was also identified, while its influence area was mainly confined in the outer shelf and slope areas. In the southern ECS, the surface water was mainly controlled by the Taiwan Strait Water, and its impacting range was gradually limited in the outer shelf during its northward movement. Based on the depth profiles of DSi concentration and δ30Si, the ECS shelf behaved a strong silicate pump effect. Moreover, the remineralization of biogenic silica become more intensive from the inner shelf to the slope area.Overall, based on the integrated studies of the Changjiang basin, Changjiang Estuary and the ECS shelf, the dissolved silica from the river to the marine systems was demonstrated be to influenced by multiple factors, including the physical mixing, biological consumption, remineralization processes, etc., and the application of silicon isotope index can help to better decipher the physical and biological impacts. |
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