ABSTRACT

The half life of Th-234 (t 1/2 = 24.1 d) is optimal for studying biogeochemical processes occuring in the euphotic zone on time scales of one to hundred days. Such time scales allow the assessment of temporal variability for primary productivity, zooplankton grazing activity and the resultant production of fecal pellets. As part of the DYFAMED programme, profiles of dissolved and particulate Th-234 between 0 and 200 m, as well as measurements of particulate Th-234 fluxes were made in 1986 and 1987 at two sites, one off the coast of northwestern Corsica and the other off the French Riviera, near Nice, in a 2000 m water column. Suspended matter and sediment trap samples were analyzed for a suite of radionuclides by nondestructive gamma ray spectrometry. Filtered 30 1 water samples were acidified on board ship and later analyzed for dissolved Th-234. Results indicate that such gamma counting techniques permit detection of vertical changes in dissolved and particulate Th-234 concentrations with an accuracy of ± 10%, as well as temporal changes in Th-234 particulate fluxes on a time scale of 6 days or less. The Th-234 specific activity in sediment trap samples ranged from 800 to 3000 dpm/g. Th-234 particulate fluxes at 200 m depth were closely correlated with total particulate mass fluxes. Our results are in agreement with studies in other oceanic areas which demonstrates the usefulness of Th-234 as a tracer of scavenging and particulate transport processes in marine surface waters.

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