Impact of the Mediterranean Outflow Water on particle dynamics in intermediate waters of the Northeast Atlantic, as revealed by 234Th and 228Th

Over the last decade Th-234 has become increasingly used to study particle transport in the ocean on a timescale of weeks. The application of Th-234 is mainly focused on the determination of particle and associated carbon fluxes from oceanic surface water. However, Th-234 is also suitable for investigating particle dynamic from the upper ocean down to interface sediments, as illustrated by the present work which reports unexpected behavior of Th-234 in intermediate waters associated with the Mediterranean Outflow Water (MOW). Concentration profiles of dissolved U-238 and Ra-228, and dissolved and particulate Th-234 and Th-228 were measured in the Mediterranean Outflow Water (MOW) near the Gibraltar Straits and at two sites (36 degrees 30'N-15 degrees 35'W, Nicole; 36 degrees 27'N-10 degrees 35'W, Yseult) which had hydrographic characteristics of Meddies, i.e. MOW that propagates as eddies in the Northeastern Atlantic at intermediate depths. There are marked differences in the distribution of thorium between MOW and the surrounding Atlantic waters. At the youngest Meddy Nicole salinity maximum at 1000 m depth, Th-234(total):U-238 and Th-228(total):Ra-228 activity ratios are significantly lower than radioactive equilibrium, indicating an unusual deficit of short half-life thorium nuclides. This implies an export of thorium, presumably on particles, from intermediate Meddy Nicole waters. This process is supported by an increase of particulate thorium fluxes measured in sediment traps deployed for two weeks above and within Meddy Nicole. In contrast, offshore Meddy Yseult has more typical profiles of both thorium nuclides that are nearly in equilibrium with their parents. These results indicate that at intermediate depths, the presence of MOW affects the exchange of reactive elements between particles and dissolved forms and enhances the downward flux of particles from intermediate waters in the Northeast Atlantic. (c) 2005 Elsevier B.V. All rights reserved.