Particulate Organic Carbon Export Flux In The Kuroshio Extension

The magnitude of POC flux from the euphotic zone reflects the capacity of sequestration of atmospheric CO2.As an important sink of atmospheric CO2,the northwestern North Pacific has attracted the attention of numerous scientists.However,most of the previous investigations were conducted in the Kuroshio-Oyashio mixed regions and the Oyashio current.In comparison,few studies has been carried out in the Kuroshio Extension where there are a great number of mesoscale eddies in all year round.The mesoscale eddies,both cyclonic eddy and anticyconic eddy,can change the POC flux inside.Accordingly,the Kuroshio Extension probably has a great capacity of CO2 fixation.In this study,we measured the POC flux from the euphotic zone within the Kuroshio Extension using 234Th/238U disequilibrium and discussed its controls.This study helps to understand the role of the northwestern North Pacific in the sequestration of atmospheric CO2.Our investigation was carried out in April 2015.We collected samples of total 234Th,particulate 234Th and POC from 30°N to 40°N and from 140°E to 160°E in the Kuroshio Extension.We divided the study region into an anticyconic eddy region and a common ocean region.The deficit of 234Th was evident in the euphtic zone at all the stations.The largest deficit always occurred in the surface.234Th reached secular equilibrium with 238U at a depth of?100 m.234Th/238U ratios varied between 0.57±0.02 to 1.15 ± 0.04.In general,234Th deficit in the anticyconic eddy region was larger than that in the common ocean region.The Chl-a and POC data also suggest that the 234Th deficit in both regions was caused by scavenging of biological particles.The distribution of 234Th flux is consistent with that of 234Th deficit in the Kuroshio Extension.The average 234Th flux in the anticyconic eddy region was 1470± 120 dpm m-2 d-1,which was 3 times the flux in the common ocean region(467 ±380 dpm nm-2 d-1).We converted the 234Th flux estimates into a POC flux using POC/234Th ratio in the suspended particles.The average POC flux in the anticyconic eddy region was 8.4 ± 3.5 mmolC m-2 d-1,while that in the common ocean region is 4.5 ± 0.7 mmolC m-2 d-1.Regardless,POC flux in the common ocean region varied greatly due to different mixing effect.At the stations characterized by relatively weak mixing,the average POC flux was 2 times the flux at the stations with strong vertical mixing.Compared to POC fluxes in other oceanic regions,the POC flux in the Kuroshio Extension is relatively high.Finally,we attempted to examine the relationship between POC flux and POC inventory using a linear least square analysis method.Though the data is limited,we found a significant correlation between POC flux and POC inventory.