Late Holocene natural and man induced environmental changes in the Western Iberian coast: acessing forcing factors

Abstract

The Western coast of the Iberian Peninsula stands as an interface between both the Atlantic and Mediterranean climatic influences and marine / fluvial conditions. The paper aims to assess the environmental changes in the last ca 6000 years (both natural and anthropogenic induced) using multiproxy analysis (geomorphological and sedimentological data, elemental and stable isotope content, microfossil assemblages, radiocarbon dating, and historical records) applied to an embayed coast. The results showed the prevalence of marine environment until 6272-6000 cal BP, but with fluvial infilling of the inner embayment, even before present sea level was reached (ca 4500-4000 cal BP). The influx of sediments was probably the result of the four recorded wet episodes (at modelled age cal BP (2σ): E1 – 6067-4770, E2 – 5806-4409, E3 – 5383-4088, and E4 – 4086-3905). These great sediment influxes along the previous 3000 years continues until 2110-1962 cal BP triggering the downstream migration of the river mouth, the development of a sand barrier coast and of a sheltered lagoon inside the palaeo-embayment. Afterwards the sedimentation rate (SR) reached high values (0.19-0.48 cmyr-1), as the result of Roman intervention in the drainage basin where pastures and local fires are recorded, together with an increasing dryness. A major disturbance is recorded in the Pollen Assemblages Zones (between PAZ II and PAZ III) and in the sediments around 1863-1706 cal BP (2σ), in the transition from the Roman Age to the Muslim invasion period, probably reflecting a hiatus in the sedimentary record. Onwards, SR reached 0.21-0.57 cmyr-1. Two major hydro marine episodes may have contributed to this fact: the latter is the Lisbon tsunami (1755 AD) while the former may reflect the 16th Century tsunamis or a great marine storm episode. An aeolian environment prevailed since then and the embayment was transformed into a dune field interrupted only by the narrow channel of the river whose mouth is often closed.