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Circulation, biogeochemistry and carbon cycling in ocean margins
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Vertical structure of 234Th deficit in the euphotic zone of the oligotrophic South China Sea
P-M1-13-S
Yifan Ma* , State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Weifang Chen, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Kuanbo Zhou, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Minhan Dai, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
Presenter Email: yivanma@stu.xmu.edu.cn |
| We conducted high-resolution profiling of 234Th in the euphotic zone to examine the fine structure of export productivity based on a cruise in the summer of 2017 to both the northern (Station SEATS at 18N, 116E) and southern basin (Station SS1 at 14N, 116E) of the South China Sea (SCS). Samples for total and particulate 234Th and particulate organic carbon (POC) were collected with a depth interval of 10 m in the euphotic layer. Using our unique data set, we are able to examine the partitioning in export fluxes between the nutrient-depleted layer (NDL) above the nutricline and the nutrient-replete layer (NRL) below the nutricline. The activity of total 234Th in the euphotic zone ranged from 1.98 to 2.60 dpm/L for these two stations. Small but visible deficit in 234Th relative to 238U was observed in the NDL at station SEATS. As expected, the lowest activity of total 234Th (2.00 dpm/L) appeared at 65 m around the deep chlorophyll maximum (DCM) at station SEATS. While a similar 234Th pattern held at station SS1 in the southern SCS where the lowest activity of total 234Th (1.98 dpm/L) also appeared at the DCM (75 m) in the NRL, 234Th was nearly in equilibrium with 238U in the NDL in the southern SCS. At station SEATS, the total 234Th flux was 583.9 dpm/m2/day, among which 299.0 dpm/m2/day was from the NDL, close to that from the NRL (284.9 dpm/m2/day). In contrast, at station SS1, though the total 234Th export flux was similar, amounting to 529.1 dpm/m2/day, the flux from the NDL was much lower (143.4 dpm/m2/day) as compared to that at SEATS while the fluxes in NRL was 385.7 dpm/m2/day. Such difference in 234Th export partitioning between the NDL and NRL at these two stations might be related to the differing nutrient sources in the NDL between southern and northern SCS. |
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