234Th/238U disequilibrium as an indicator of the seasonal export flux of particulate organic carbon in the North Water

Abstract

Abstract Water-column deficits of 234Th (half-life=24.1 d) relative to its parent 238U were used to estimate fluxes of 234Th and particulate organic carbon (POC) through the euphotic zone of the North Water, a polynya in northern Baffin Bay. Samples collected in May 1998, early in the seasonal development of the polynya, showed small deficits of 234Th in the upper 100 m. Deficits were greater in July 1998 and August–September 1999. Sinking fluxes of POC were calculated from the water-column 234Th deficits by multiplying the latter by the POC/234Th ratio on sinking particles (assumed to be the >70-μm filterable fraction). POC fluxes were greatest in July 1998 (mean 27.4±13.4 mmol C m−2 d−1) and declined in August 1999 (mean 8.7±1.7 mmol C m−2 d−1), largely due to a decrease in the POC/Th ratio in the 1999 samples. Regionally, POC fluxes were greatest in the western and northern portions of the North Water in July 1998 and markedly lower in the west in August 1999. This pattern resembles spatial and temporal changes in primary production in the polynya determined independently. POC fluxes determined from water-column 234Th profiles agreed within factors of 2–5 with values determined from floating sediment traps deployed in the upper 100 m. Thorium-based indices of new production (ThE ratios) obtained within the polynya encompassed a wide range (0.1–0.9), with western areas showing substantially higher transfer efficiency of carbon fixed by phytoplankton at the surface. Thorium-derived POC fluxes at 100 m for both July 1998 and August 1999 correlated positively with biomass of the dominant diatom in the North Water, Chaetoceros socialis, suggesting that regional variations in carbon export can be further understood by factors controlling the distribution of this species.