| Summary: | Thorium-234 is used over the north-western Iberian Margin as a tracer of sinking particle fluxes on time scales of days to weeks. As a part of Ocean Margin Exchange (OMEX) project (II-II), samples were collected during pre-upwelling (June 1997) and winter (January 1998) periods. During the pre-upwelling situation, the distribution of total 234 Th is rather uniform, with a mean deficiency of 24%. In contrast, particulate 234 Th activities increase from the shelf to open ocean. As a result, particle residence times derived from 234 Th show a considerable range between 4 and 45 days, with a mean value of about 26 days. In the shelf waters, there is no clear trend in particle residence time, whereas the offshore stations present a more obvious signal with a gradual increase from about 10 days over the slope to nearly 30 days in the open ocean. In winter, total 234 Th activities are more variable, and both deficit and equilibrium situations are observed. The mean deficit of total 234 Th is low, about 10%, which may reflect reduced export fluxes at this time. Moreover, in the shelf waters during winter, particulate 234 Th presents higher levels when compared with the pre-upwelling situation, which may be due to more intense resuspension of shallow sediments. As a consequence, average near-surface particle residence times are significantly longer in winter, with a mean value of 113 days. Estimates of export flux of particulate organic carbon (POC) from the upper ocean, using the ratio of POC and 234 Th of particles, range between 1 and 56 mmol C m -2 day -1 in June 1997, and ca. 0 and 25 mmol C m -2 day -1 in January 1998. These variations reflect the large differences in hydrographic conditions and primary production levels between the two seasonal situations.
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