南海北部輸出生產力季節變化：釷-234/鈾-238不平衡估算及沈降顆粒收集器量測

Abstract

The time-series station, SEATS (18°N, 116°E) was visited four times during October 2006~October 2007 to carry out seawater sampling and floating trap deployments. This study is aimed to estimate and to directly measure export production using 234Th/238U disequilibrium and floating trap, respectively. 234Th fluxes in the euphotic layer estimated from the steady-state irreversible scavenging model showed small seasonal variation, which increased from 57 dpm/m2/d at 5 m to 1857 dpm/m2/d at 100 m in summer, 42 dpm/m2/d at 5 m to 1105 dpm/m2/d at 100 m in fall, and 161 dpm/m2/d at 5 m to 1584 dpm/m2/d at 100 m in winter. The floating traps were deployed at three depths, i.e., mixed depth (30 m), euphotic depth (100 m), subsurface (160 m). Large variations of total mass fluxes and 234Th fluxes were found at the mixed depth, ranging from 441 mg/m2/d in summer to 3548 mg/m2/d in winter and from 353 dpm/m2/d in summer to 3395 dpm/m2/d in winter, respectively. In contrast to the fluxes at the mixed depth, the total mass and 234Th fluxes at the euphotic depth (100 m) fell in a smaller range, from 597 to 1215 mg/m2/d and from 1201 to 2108 dpm/m2/d, respectively. Calculated from the ratio of measured 234Th fluxes and modeled 234Th fluxes, the trapping efficiencies of the floating traps at the euphotic depth (100 m) ranged from 0.8 to 1.3, giving a reliable POC fluxes measured by the floating trap. The POC flux at 100 m ranged from 9.8 mmol/m2/d in summer to 17.4 mmol/m2/d in fall. With trapping efficiency correction, POC fluxes ranged from 9.6 mmolC/m2/d in winter to 21.0 mmolC/m2/d in fall. The ratio in the particles is the determining factor in using the 234Th/238U disequilibria to estimate export flux. In order to investigate the relationship of ratio with different particle size, this study measured POC and particulate 234Th contents in size-fractionated particles (0.45-10 μm, 10-63 μm, 63-153 μm, >153 μm). The results showed that the ratio in 0.45-10 μm particles is close to that in the sinking particles, from which the POC flux was calculated. Using 234Th as a carbon proxy, the POC flux at the euphotic depth ranged from 10.6 mmolC/m2/d in summer to 25.4 mmolC/m2/d in fall.