THORIUM-234/URANIUM-238 DISEQUILIBRIUM AS AN INDICATOR OF SCAVENGING RATES AND PARTICULATE ORGANIC-CARBON FLUXES IN THE NORTHEAST WATER POLYNYA, GREENLAND

Analyses of particulate and dissolved distributions of the naturally occurring radionuclide Th-234 (half-life=24.1 days) in the water column of the Northeast Water Polynya (Greenland continental shelf) show deficiencies relative to equilibrium with U-238 that are related to uptake onto sinking particles. The deficiencies are generally enhanced in the upper 50 m of the water column (Polar Water), a zone of high suspended particle, particulate organic carbon, and pigment concentrations. In July-August 1992, residence times of dissolved Th relative to uptake onto particles ranged from 52 to 293 days and those of particulate Th relative to sinking ranged from 6 to 52 days in the upper 50 m. In contrast, removal was enhanced in July-August 1993, with dissolved and particulate Th-234 residence times ranging from 45 to 167 days and 4 to 57 days, respectively. The assumption of steady state used in these calculations is not strictly valid, in that both years showed progressive increases in Th-234 removal at stations reoccupied several weeks apart. Spatial variation in Th-234 removal from the water column also is evident and may be due, in part, to progressive Th-234 removal associated with the inferred anticyclonic circulation within the polynya. Inventories of particulate Th-234 in the upper 50 m correlate well with inventories of suspended particulate matter and particulate organic carbon (POC). Using the ratio of POC/Th-234 on suspended particles and the Th-234 fluxes obtained from disequilibrium with respect to U-238 fo calculate POC fluxes gives values of 12 to 45 (mean = 25 +/- 14) mmol C m(-2) d(-1) in 1992 and 17 to 45 (mean = 35 +/- 10) mmol m(-2) d(-1) in 1993. A direct comparison of the Th-derived POC fluxes from the 1992 data is possible with those obtained independently based on depletions of dissolved nutrients (29.2 +/- 23.5 mmol m(-2) d(-1) (Wallace et al., this issue)) and dissolved inorganic carbon (14 +/- 7.4 mmol m(-2) d(-1) (Yager et al., this issue)). The good agreement reflects the fact that during the summer, particle cycling and flux in the surface water of the polynya are apparently dominated by biogenic particles. Removal of Th-234 from th, water column is balanced, within the uncertainties, by inventories of excess Th-234 in bottom sediments, with both water column deficiencies and sediment inventories greater in 1993 than 1992. This indicates that on the timescale of several half-lives of Th-234 (similar to 100 days) the polynya is in balance with respect to Th-234 (and probably the particles with which it is associated).