Decadal decreasing trend in biological carbon pump estimated from 234Th in the western subarctic North Pacific

We measured thorium-234 (234Th), particulate organic carbon (POC), and chlorophyll a in the western subarctic North Pacific surface layer in 1997–2008. 234Th, POC, and chlorophyll a in the surface layer showed clear seasonal changes. As a result of approximation by using a trigonometric function, 234Th export flux estimated from the deficiency of 234Th relative to 238U and the ratio of POC to particulate 234Th (POC/234ThP) indicated constant and decreasing trends in the study area from 1997 to 2008, respectively. POC export flux estimated from 234Th decreased in the study area, showing that the strength of the biological carbon pump in this region declined in the studied decade. The ratio of POC/chlorophyll a decreased during the term. These trends were possibly caused by a decline in not the biomass but particle size of phytoplankton. Our results suggest that not only the biological carbon pump but also phytoplankton community structure change in the western subarctic Pacific Ocean in a decade.