Biological Production and POC Export Fluxes in the Canada Basin and the Bering Sea

Primary production (PP), bacterial production (BP) and particulate organic carbon (POC) fluxes in the Canada Basin and the Bering Sea were measured during the summer period of 1999 aboard R/V Xue-Long. Primary production and bacterial production were determined through in-situ C-14 and H-3 incubation experiments. Activity concentrations of dissolved and particulate Th-234 were measured using beta counting techniques to quantify the scavenging and residence time of Th-234 as well as POC export fluxes. Our results showed that significant Th-234 scavenging was observed in the upper 100 m water column in the study areas, with up to 40% of Th-234 deficit found at the Bering Sea stations and about 15% of Th-234 deficit at the Canada Basin station. Measured PP decreased from about 12.5 mmol-C/m3/h in surface water to near zero at about 100 m depth, with an integrated PP of 3.83 mmol-C/m2/d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol-C/m2/d, which is up to 52% of the measured PP. Particulate organic carbon export fluxes derived from Th-234/238U disequilibrium were about 1 mmol-C/m2/d in the Canada Basin and about 10 mmol-C/m2/d in the Bering Sea, with fluxes in the later area being 5-10 time higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of Th-234-derived POC export to primary production) of 0.26 and 0.7 for the Canada Basin and the Bering Sea, respectively. The higher ThE ratios suggest a decoupling between production and particulate export in the high-latitude ocean. Profile of particulate Th-234 in the Canada Basin showed a unique characteristic, with particulate Th-234 activities increased with increasing depth, and thus, ratios of POC to particulate Th-234 (umol-C/dpm) decreased with increasing depth. The enrichment of Th-234 with depth likely resulted from a continuous Th-234 scavenging and particle coagulation coupled with preferential remineralization of organic carbon during settling in the water column.