SEDIMENT GRAIN SIZE AND ORGANIC CARBON DISTRIBUTION IN THE CABRAS LAGOON (SARDINIA, WESTERN MEDITERRANEAN)

The aim of this work was to investigate the spatial variability of sediment bulk properties in the surface sediments as well as down-core trends, and to assess the relationship between sediment distribution and levels of total organic carbon (TOC), in the Cabras lagoon, Sardinia. Grain size distributions and TOC contents were measured in the surface sediments (0-2 cm horizon) in a grid of 31 stations. Vertical profiles along the 0-24 cm depth horizon were also measured in three stations. In the superficial horizon, TOC content was very high, up to 43 mg g(-1), with a mean of 33 mg g(-1). Core profiles showed a marked reduction of TOC content with depth to similar to 20-25mg g(-1) and a concomitant shift in particles size towards a sandier composition (mud content decreasing from 95-100% to 70-85%) at 3-7 cm core depth. Total organic carbon contents in the uppermost layers were associated with the grain size fraction <8 mu m. The results suggest that a major change in the sedimentary regime of the lagoon, associated with internal trapping and re-distribution of organic C-bounding fine particles, has been occurring in the last few decades. The cause would appear to be the construction of a dam at the lagoon mouth rising up to the high tide level in order to maintain a constant lagoonal water level.