Separating eustatic from local environmental effects: a late-Holocene record of coastal change in Albufeira Lagoon, Portugal

Diatom and sedimentological analyses of a 7.6-m core of the Albufeira coast al lagoon, western Portugal, provide information about the forcing factors and environmental history of the evolution of this lagoon during the last 2 500 years. At this timescale, eustasy is considered to be a minor environme ntal control on the dynamics of the barrier-lagoonal system. Instead, these dynamics are forced by local factors, such as changes in the sand-barrier permeability. Lithological and diatom facies are compared with present-day environments and show that the lagoon evolved essentially as a slightly bra ckish water body with a salinity of around la,. This pattern was disturbed by three major episodes of sedimentation associated with threshold response s in the permeability of the barrier. The first (c. 2370 BP) did not produc e a specific lithological signal and is characterized by a peak in freshwat er euplanktonic diatoms, implying an almost permanent isolation of the basi n and freshwater flooding. A second episode was identified at c. 1600 BP, w hen a dramatic opening of the tidal inlet provoked a significant change in the sedimentation regime, which became dominated by inorganic, minerogenic sediments and allowed marine/brackish epiphytic diatoms to flourish togethe r with allochthonous marine planktonic species. Finally, a third threshold occurred at c. 1225 BP corresponding to the establishment of the present-da y muddy or sandy-muddy sedimentation pattern. Both the sedimentological fea tures and the diatom assemblages indicate the establishment of a more tidal ly influenced environment. This reflects regular artificial breaching of th e barrier associated with increasing anthropogenic intervention in the syst em.