PB-210, MANGANESE AND CARBON - INDICATORS OF FOCUSING PROCESSES ON THE NORTHWESTERN MEDITERRANEAN CONTINENTAL-MARGIN

Organic carbon contents, (210)pb data, and organic and oxidised forms of manganese in sediments are used for reconstructing the shelf and de ep-sea transport of particulate matter and for evaluating early diagen esis conditions. The latter are used as indicators of focusing process es on the northwestern Mediterranean continental margin. Twelve differ ent physiographical sites were sampled between 400 and 2000 m in the P yrenean and Golfe du Lion canyon axes and on their open-slopes. In the canyon axes, which indent the Pyrenean slope, (210)pb activities show a regular exponential decrease with sediment depth: computed sediment ation rates range from 170 to 198 cm 10(-3) yr. For an identical regul ar exponential decrease in the Golfe du Lion canyon, the sedimentation rate is lower (119 cm 10(-3) yr). On the lower open slopes and deep-s ea fans, rapid deposition (turbidites and surficial sliding) occurs on to sediments that were deposited regularly at a rate of about 175 cm 1 0(-3) yr. A deposit enriched in organic carbon is identified at mid-sl ope between 500 and 1300 m. In this area, surface sediments of the can yon axes are richer in organic carbon than at the adjacent open slope sites (0.9% against 0.6% dry weight). Canyon axes are more active than open-slopes as channels for organic matter transfer. In the axial cha nnel of the Pyrenean canyon, organic carbon decreases exponentially in the uppermost 12 cm. In the Golfe du Lion canyons, only the upper 2 c m of sediment are enriched in organic carbon. This is related to lower sedimentation rates in the Golfe du Lion area as compared to the Pyre nean area. On the upper and lower open slopes, the organic carbon cont ents decrease rapidly within 1-2 cm of sediment. There is no exponenti al evolution profile and the residence times at the sediment-water int erface are longer than in the canyons. The highest surface Mn concentr ations are also found in the middle of the continental slope. The aver age is 64 mu g g(-1) for Mn associated with the organic phase (Mn(org) ) and 1400 mu g g(-1) for (Mn(Ox)). As for organic carbon contents, ca nyons are areas of high concentrations. The lowest values (4 mu g g(-1 ) for Mn(org) and 495 mu g g(-1) for Mn(Ox)) are found for the lower o pen-slope (1200-1700 m) where there is active surgical remobilisation and gravity mass movements. Moreover, the vertical distribution of Mn has allowed us to identify sites located at depths of about 1000 m whe re the redox discontinuity is located in the first few centimetres and where the (Mn(Ox)) and the (Mn(org)) profiles display a positive grad ient from the deepest layers to the sediment-water interface. The dist ribution of the various parameters was caused by three different mecha nisms: (1) bottom advection of particulate matter transferred from the continental shelf deposits to the slope along the canyon axes; (2) sc avenging of the dissolved phases of metals and (3) sedimentary focusin g of the particles in a ''favoured'' area bf the continental slope-a m id-slope depocenter. These data contribute to our knowledge of the rol e of continental margins in the global recycling of carbon and other m aterials.