| The efficiency of the biological pump over the continental shelf is largely modulated by the transport of biogenic particulate organic carbon (POC) across the shelf and represents the capacity of net carbon sequestration of atmospheric CO2in the shelf area.234Th/238U disequilibrium was used to quantify234Th export flux from the water column. In combination with the depositional flux of234Th on the seafloor, we established a1-D mass balance model to estimate the flux of234Th (and hence particulate organic carbon export flux) across the shelf. We used the small-volume manganese dioxide coprecipitation method to measure total234Th activity in seawater, and a modified protocol to measure the excess of234Th activity in sediment. Samples were collected from the northern South China Sea shelf in August2012.234Th/238U disequilibrium was used to quantify the magnitude of the transport flux of POC and to study the mechanisms that control the transport of POC across the shelf. This study will provide a new reference and is helpful for understanidng carbon cycling in the South China Sea.Over the continental shelf, we found that234Th removal phenomenon was evident in all of the water column, with the234Th/238U ratio varying between0.06to1.07. The234Th/238U ratio in the nearshore area and in the bottom was relatively small. With distance offshore, the removal of234Th on particles decreased. At all stations, the deficit of234Th appeared to be more pronounced in the bottom than in the surface water. This low234Th signal corresponded to high TSM values in the bottom water and indicates that resuspension of particles may ocurr in the bottom of the shelf. The chlorophyll a distribution suggests that the high particle concentrations were more likely to be caused by resuspension than by in-situ biological production. Notably, high TSM value may not necessarily correspond to high POC concentration. Overall, the depth profiles of POC、TSM and234Th/238U suggest that particle resuspension may lead to the transport of POC across the shelf. The export flux of234Th over the continental shelf was characterized by large spatial variability. It varied between435±17-2984±80dpm/m2/d, with a mean value of1849±75dpm/m2/d.234Th excess was found in the surface of0-4cm sediment of the South China Sea shelf. The penetration of234Th into sediment is controlled mainly by bioturbation. In the study area, hydrodynamic and the nature of sediment affect the inventory of excess234Th in sediment. The inventory of234Th excess in sediment ranges from67±30dpm/m2/d to1832±167dpm/m2/d, with a mean value of800±99dpm/m2/d.Based on the234Th export flux from the water column and the depositional flux in sediment, we calculated the234Th export flux across the continental shelf in the northern South China Sea. The distrbition of234Th flux along isobath was relatively uniform. Overall, lateral transport of234Th occurred mainly in the region with a water depth of50-100meter. Through the analysis of the balance of234Th in the shelf, a one-dimensional (1-D) steady state of234Th box model was established. It was inferred that there was a mechanism of234Th transport across the continental shelf in the northern South China Sea.Finally, through an analysis of focusing factor (FFTh) and exporting factor (EFTh), we found that about60%of FFTh values fell in the range of1:2-1:4, suggesting that only25-50%of the total export of234Th deposited in sediment. About50-75%of234Th was thus transported in a form of particulate matter from the shelf to the slope and/or further offshore to the deep basin. With234Th export flux and POC/234Th ratio, we estimated the cross-shelf transport flux of POC to be6.8±0.99×107mmol C/m/d. This offshore transport process makes POC more conducive to be sequestrated in the deep sea or even in sediment, and is thus important for the regulation of CO2concentration in the study area. |
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