The in situ accumulation of large quantities of biogenic materials over millennia is very relevant and occurs only in a limited number of ecosystems. Peat bogs are probably the bestknown example among terrestrial ecosystems (e.g. Aaby & Berglund, 1986) and coral reefs among marine ecosystems (e.g. Clausen & Roth, 1975). In situ organic accumulations in the marine environment are rare, having been described for mangroves and seagrass species. The Mediterranean seagrass Posidonia oceanica accumulates large quantities of organic debris. Roots, rhizomes and leaf sheaths are progressively buried fonning a bioconstruction called 'matte'. The organic material remains with little morphological alteration for millennia (Mateo et aI. , 1997), constitnting a marine peat-like deposit with a great interest from a palaeoenvironmental point of view. The present paper evaluates the palaeoecological potential of the organic deposits generated by the marine phanerogame Posidonia oceanica in the Bay of Port Lligat, Girona, NE Spain. This stndy discusses the detailed evolution of the vegetation in the landscape of the area.

This research was funded by the projects PALEOMED (McyT-BOS2002-02247) and PIF-CSIC 20050F0231 'CARBOMED' >Magnitnd del sumidero de carbono asociado a la faner6gama marina Posidonia oceanica y potencial en paleoecologia>.

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