On The Export Fluxes, Seasonality And Controls Of Particulate Organic Carbon In The Northern South China Sea

Continental margins play a disproportionally important role in the global ocean carbon cycling and the flux of biologically produced organic carbon from the upper ocean implies the net sequestration of carbon dioxide(CO₂) and the recycling of organic matter.However our understanding of the spatial and temporal variability of particulate organic carbon(POC) flux in the extremely dynamic marginal seas remains limited.In this study,we attempt to elucidate the spatial and seasonal variation of POC export based on the high resolution ²³⁴Th sampling in the northern South China Sea(SCS).²³⁴Th were measured with a modified small-volume MnO₂ eoprecipitation method.Samples were collected in four cruises conducted in the winter(Feb.2004),the summer(Jul.2004),the fall(Sept.2004) and the spring(May. 2005).In this study we examined the effect of different sampling methods on POC and particulate ²³⁴Th(PTh) data,which were collected by small-volume samplers(1-2 L) and large-volume(100-600 L) pumping methods.POC and PTh derived from small-volume samplers were frequently observable to be higher than from large-volume pumping.The absorption of dissolved organic carbon(DOC) and dissolved ²³⁴Th onto the filters appeared to be a primary factor causing the discrepancy.We also examined POC/²³⁴Th and POC/²²⁸Th ratios in three particle size classes (1-10μm,10-53μm,＞53μm).The various trends in the POC/²³⁴Th ratio versus particle size were observed,which may be explained by a combination of ²³⁴Th decay and the preferential remineralization of POC relative to Th during particle aggregation.During the survey,the deficit of ²³⁴Th generally decreased with the distance offshore in the northern SCS.Depth profiles of ²³⁴Th may be categorized into three types with seasonal variations.In the winter,the deficit of ²³⁴Th appeared to be substantial in the surface and decreased with depth.In other seasons,²³⁴Th basically showed a stratified structure,i.e.,the subsurface maxima of ²³⁴Th deficit.Such a stratified structure is characteristic of oligotrophic ocean.At some locations on the shelf,²³⁴Th activities were lower than ²³⁸U throughout the water column and the enhanced deficit existed at the base of the eupotic zone,which were mainly caused by the resuspension of the bottom sediments.Besides the distribution of ²³⁴Th,POC distributions on the shelf and the slope were also investigated from 2000 to 2004.POC concentration decreased with the distance offshore,which indicates the influence of the Pearl River.POC concentration in the summer was higher than in the spring and the autumn and was lowest in the winter,which is consistent with the seasonal variation of the discharge of the Pearl River.POC concentration generally showed a decrease with depth.At some locations on the slope,POC peaked at 200 m and 400-500 m,which suggests the horizontal particle-transport or the particle transport along the slope in the northern SCS.Using a three-dimensional steady state model,²³⁴Th flux was estimated.It is shown that horizontal and vertical transport account for less than 10%of the overall ²³⁴Th balance for most of stations.However,the contribution of physical transport to the ²³⁴Th flux was up to 30-40%at physically dynamic regimes such as at the salinity fronts.This strongly suggests that high resolution ²³⁴Th sampling is necessary to correctly resolve the variability of POC fluxes in the marginal sea. Combined with the measurements of POC/²³⁴Th ratio on suspended particles,²³⁴Th fluxes were converted to POC export rates and regarded as the upper limit of POC flux in the northern SCS.In the northern SCS,POC export on the shelf varied from 8.0 to 11.3 mmol C m^-2 d^-1 with no apparent seasonal variations.In contrast,the average POC export in the regime of the slope and the basin changed from a low of 3.6±2.2 mmol C m^-2 d^-1 in the spring to a high of 9.0±6.4 mmol C m^-2 d^-1 in the winter,with a medium value in the summer(4.7 mmol C m^-2 d^-1) and the autumn(4.8±4.8 mmol C m^-2 d^-1).The seasonal pattern of POC export matched the pattern of primary production and the community structure of the phytoplankton,the enhancement of which in winter may be related to the nutrient supplies from the wind-induced mixing in the winter.ThE ratio(²³⁴Th-derived export production/Primary production ratio) in the northern SCS varied in the range of 4-68%,with an average of 24%,which was higher than the tyical value in the oligotrophic ocean(＜10%).Using a simple box model,we estimated that the diffusive flux of dissolved organic carbon(DOC) was 0.91 mmolC m^-2d^-1 and equaled to～30%of POC export in the northern SCS in the spring,the efficiency of biological pump in the northern SCS would thus be underestimated without the consideration of the downward flux of DOC.Our updated synthesis of ThE in some marginal seas indicated the seasonal variation of ThE,which showed were two or three times of difference between seasons. Assuming that this seasonal signature is of characteristics of the marginal sea,it is estimated that POC export of the marginal sea might contribute up to 30%of the global export flux.