Enhanced short-term sediment deposition within the Nazare Canyon, North-East Atlantic

In January and May 1999, interface sediments were sampled within the Nazare Canyon, a large submarine canyon that deeply indents the western Iberian continental margin (NE Atlantic) at around 39 degrees 35N. Excess Th-234 profiles were established with high resolution to calculate inventories and bioturbation coefficients. A northern transect at 42 degreesN is briefly presented as a reference for the Western Iberian margin. Synthesis of these: parameters is used to examine short-term deposition within the Nazare Canyon. Th-234(xs) was always detected with significant activities on surface in most of the samples. Along the reference transect and in the adjacent area, surface Th-234 excesses are about 10-20 dpm g(-1) . In contrast, Th-234(xs) reaches very high levels (up to 170 dpm g(-1)) in the canyon. There is a large range of variability in inventories and bioturbation rates, in relation with depth and location. These results show a preferential deposition in the canyon and focus in the upper and middle part of the canyon. On a monthly time scale the Nazare Canyon appears as a significant locus of sedimentation for the Western Iberian margin. (C) 2001 Elsevier Science B.V. All rights reserved.