Distribution,Seasonal Variations And Isotopic Composition Of Silicon In The Oligotrophic South China Sea

Diatom,a single-cell phytoplankton,dominates the silicon cycles in the ocean.Featured by fast growth rates and higher demands of nutrients,diatoms utilize carbon dioxide for photosynthesis while absorbing silicates in seawater to form a hard siliceous shell with a faster rate of sedimentation but slower rate of degradation,which is crucial for the efficiency of the biological pump and carbon burial from the sea surface to the deep ocean.It is well known that picophytoplankton such as Prochlorococcus and Synechococcus is the dominate phytoplantonic composition in both autotrophic biomass and primary production in oligotrophic systems.It is also known that diatoms dominate the primary pruduction in eutrophic continental shelf and/or estuarine regimes.However,the role of diatoms in driving biological pump in oliogtrophic ocean,or"ocean desert" has only been recently noticed and the underlying mechanism remains to be explored.Limited studies have shown that the coupled carbon-silicon export process from the euphotic zone of the oligotrophic marine system would be much more complicated than what we previously thought.In stratified oligotrophic ocean,euphotic zone is often classically divided into two different areas,the upper Nutrient Depleted Layer(NDL)and the lower Nutrient Replete Layer(NRL)which are characterized by different nutrient input sources.The continuous development of multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)for the determination of silicon stable isotope provides an effective proxy for studying the internal processes of silicon in the euphotic zone.In this study,the South East Asian Time-series Study(SEATS)station in the northern sub-basin with another important contrasting station SS1 in the southern sub-basin,both known to be strongly and persistently stratified,are chosen to investigate the differences in silicon utilizzation,regenration and export in a framework of two-layer structure of the euphotic zone.With the data of the 234Th/238U disequilibrium and sediment traps,we estimated the layered export fluxes at SEATS and SS1.Data from 2017 year show different seasonal variations at SEATS and SS1 stations within the layered structure of euphotic zone characterized deeper mixed layer depth(MLD)and nutricline depth at SS1 station.At both stations,the depth of chlorophyll maximum layer(DCM)and the depth of silicon maximum(DBSiM)appear to rise upward in winter compared to summer.Similar chlorophyll concentration but distinct biogenic silica(BSi)distribution are found at SEATS and SS1.BSi concentration at SS1 station is much higher in winter than in summer,with winter maximum being 0.095?mol/L around 60 m.However,the underlying mechanism of abnormal and elevated BSi concentration(0.024?0.150 ?mol/L)at SEATS station in summer is still unclear:BSi maximum found in surface water of SEATS station is probably casued by the presence of stronger silicified diatoms due to strengthened nitrogen fixation in the surface,which need more proof from studies on diatom species.Overall,dissolved silicon isotope composition(?30Sisi(OH)4)show oppositing distribution to Si(OH)4 concentration at SEATS and SS1 with higher values in surface water and lower values in deep water.?30SiSi(OH)4 distribution show little difference in deep water but obvious and different seasonal features in surface water between SEATS and SS1 stations.The surface maxima of ?30SiSi(OH)4 at SEATS station(+3.09‰ in winter and+3.12‰ in summer)are higher than those at SS1 station(+2.63‰ in winter and+2.69‰ in summer),indicating stronger diatom production in the surface of SEATS station.Particulate silicon isotope composition(530SiBSi)show smaller variations in summer with barely changes at SS1 station of very low BSi concentration and diatom abundance.While in winter season,?30SiBSi in the NDL is higher than NRL due to lighter ?30Sisi(OH)4 consumption brought by strengthened water mixing and silicate released by BSi export.The apparent fractionation coefficient(A30Si)in the mixed layer of NDL region is generally low,mainly derived from the fractionation of silicate by diatoms,which becomes irregular in NRL due to the lighter Si(OH)4 input and the complexity of BSi composition.Combined analyses of Si(OH)4,BSi and?30SiSi(OH)4 distribution with two-end member mixing models show increased?30SiSi(OH)4 and nutrients near 50?60 m at SS1 station in winter and 85?95 m at SEATS station in summer can be related to extra nutrient input and enhanced diatom production in surrounding waters.The choice of BSi/234Th ratio is one of the controlling factors influencing the precison on the estimation of silicon fluxes.The export fluxes of BSi at 50 m and 100 m at SEATS station in summer in 2017 are estimated to be 1.45±0.10 mmol/m2/d and 1.00±0.07 mmol/m2/d by using the BSi/234Th ratios of particles on sediment traps,which is much higher than the results obtained by using the BSi/234Th ratios in the suspended particles.Export fluxes of 50 m,100 m and 200 m in summer at SEATS measured from sediment traps directly range from 0.191-0.424 mmol/m2/d in 2 017 and 0.034?0.083 mmol/m2/d in 2018 respectively,which decrease with depth with highest value at 50 m.BSi concentration and export fluxes in the summer of 2017 both increased greatly compared to 2018,which is similar to the frequent "summer bloom"happening in ALOHA station.And this summer outbreak of diatoms is thought to be triggered when nitrogen fixation enhances the production of some diazotrophic endosymbionts with diatoms in the mixed layer(Dore et al.,2008).Results from two methods both show that the NDL layer accounts for a larger proportion of the silicon export than the NRL layer.Given that DCM usually appears in the NRL layer,one can speculate strong mineralization processes in NRL layer.The processes,framework and mechanisms of silicon cycle in the euphotic zone in the oligotrophic basin of the South China Sea is preliminarily discussed in this paper.However,the controlling mechanisms of silicon export in the upper NDL zone still need to be futher explored.