POC fluxes estimated from 234Th in late spring–early summer in the western subarctic North Pacific

Abstract

Measurements of 234Th, particulate organic carbon (POC), nutrients and net primary production were made at station K2 (47°N, 160°E) in the western subarctic North Pacific Ocean six times from 3 June to 17 July 2006. Net primary productions varied from 263 to 732 mgC m−2 d−1. 234Th was used as a tracer to estimate POC fluxes from the surface layer to 100 m depth. POC fluxes varied from 14 to 284 mgC m−2 d−1. The maximum POC flux and net primary production in this study were much higher than those observed previously at station K2 (179 mgC m−2 d−1 in July 2003 for POC flux, 552 mgC m−2 d−1 in May 2004 for net primary production). Because maximum chlorophyll a in this study (1.3 µg L−1) was also much higher than previous values at station K2 (1.0 µg L−1 in August 2004), we assumed that high-frequency observations had allowed measurement of a near annual peak of biological production and biological pump activity. The net primary productions and POC fluxes changed greatly during the study period. Biological production and export in this region varied greatly during this season. The export ratio (POC flux/net primary production) in this season (23 ± 1 %) was less than the annual export ratio in this region (29 ± 1 % in 2002–2005), although this ratio was higher than those in the other oceans (5–10 %). POC export (5.0 ± 0.1 gC m−2) in late spring–early summer was about half that of net community production (10.2 ± 0.6 gC m−2) estimated from the drawdown of phosphate during the study period. We suggest that the pathways of carbon export, other than the sinking of particles, were transport by zooplankton and so on.