210Pb, manganese and carbon: indicators of focusing processes on the northwestern Mediterranean continental margin

Abstract

Organic carbon contents, 210Pb data, and organic and oxidised forms of manganese in sediments are used for reconstructing the shelf and deep-sea transport of particulate matter and for evaluating early diagenesis conditions. The latter are used as indicators of focusing processes on the northwestern Mediterranean continental margin. Twelve different physiographical sites were sampled between 400 and 2000 m in the Pyrenean and Golfe du Lion canyon axes and on their open-slopes. In the canyon axes, which indent the Pyrenean slope, 210Pb activities show a regular exponential decrease with sediment depth: computed sedimentation rates range from 170 to 198 cm 10 −3 yr. For an identical regular 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-sea fans, rapid deposition (turbidites and surficial sliding) occurs onto sediments that were deposited regularly at a rate of about 175 cm 10 −3 yr. A deposit enriched in organic carbon is identified at mid-slope between 500 and 1300 m. In this area, surface sediments of the canyon 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 channel of the Pyrenean canyon, organic carbon decreases exponentially in the uppermost 12 cm. In the Golfe du Lion canyons, only the upper 2 cm of sediment are enriched in organic carbon. This is related to lower sedimentation rates in the Golfe du Lion area as compared to the Pyrenean area. On the upper and lower open slopes, the organic carbon contents decrease rapidly within 1–2 cm of sediment. There is no exponential evolution profile and the residence times at the sediment-water interface are longer than in the canyons. The highest surface Mn concentrations are also found in the middle of the continental slope. The average is 64 μg g −1 for Mn associated with the organic phase (Mn(org)) and 1400 μg g −1 for (Mn(Ox)). As for organic carbon contents, canyons are areas of high concentrations. The lowest values (4 μg g −1 for Mn(org) and 495 μg g −1 for Mn(Ox)) are found for the lower open-slope (1200–1700 m) where there is active surficial remobilisation and gravity mass movements. Moreover, the vertical distribution of Mn has allowed us to identify sites located at depths of about 1000 m where the redox discontinuity is located in the first few centimetres and where the (Mn(Ox)) and the (Mn(org)) profiles display a positive gradient from the deepest layers to the sediment-water interface. The distribution of the various parameters was caused by three different mechanisms: (1) bottom advection of particulate matter transferred from the continental shelf deposits to the slope along the canyon axes; (2) scavenging of the dissolved phases of metals and (3) sedimentary focusing of the particles in a “favoured” area of the continental slope—a mid-slope depocenter. These data contribute to our knowledge of the role of continental margins in the global recycling of carbon and other materials.