A microfaunal and sedimentary record of environmental change within the late Holocene sediments of Boca do Rio (Algarve, Portugal)

The foraminifera and ostracods observed in a late Holocene sedimentary sequ ence within a fluvial valley at Boca do Rio in the coastal zone of the west ern Algarve, Portugal, reveal a general, though not smooth transition from marine to fluvial conditions. The relative influence of these two environme nts appears to depend on the degree of permeability of the barrier system a t the coast. Optically stimulated luminescence dates and palaeoecological i nformation obtained from the sequence suggest that barrier formation may be related to changes in climate and/or patterns of ocean current circulation . An unusual deposit rich in sand and gravel found within the otherwise mud -dominated sequence has been dated at AD 1801 +/- 76 years. This deposit co ntains foraminifera and ostracods which indicate marine conditions, and whi ch contrast markedly with the brackish-water, estuarine assemblages found i n the mud deposits. The rapid transition in the foraminifera and ostracod a ssemblages indicates a short-lived coastal flooding, which may represent th e tsunami associated with the Lisbon earthquake of AD 1755. The variations in the foraminifera and ostracod assemblages also suggest subsidence during the earthquake, with uplift having occurred in the period since then.