Lateglacial and Holocene environmental changes in Portuguese coastal lagoons 1: the sedimentological and geochemical records of the Santo André coastal area

Abstract

The sedimentological and geochemical study of a core corresponding to sedimentation in the Santo André lowland since 14 000 BP allowed the reconstruction of environmental changes of this area and discussion of the relative importance of local versus global forcing factors responsible for its evolution. The pre-Holocene section of the core represents terrestrial sedimentation in a fluviatile environment, contemporaneous of a low sea level and a distal shoreline. Sediment inputs were terrestrial and resulting from intensive weathering and high erosion rate in the adjacent watershed. During the Early Holocene ( c. 10 020 to 5380 BP) the sea invaded this lowland and defined an open marine shallow environment corresponding to a wide gulf which has been disturbed by multiple terrestrial inputs that might have been induced by massive episodes of fluvial discharge. The first symptoms of confinement were noticed close to 5400 BP, and c. 5380 BP a major environmental threshold occurred (possibly reflecting the marked deceleration of the sea-level rise rate) corresponding to the establishment of an efficient detrital barrier that completely isolated the bay and changed this lowland into a closed coastal lagoon. After 5380 BP, the lagoonal environment evolved as a function of local forcing factors, among which the frequency and efficiency of exchanges with the ocean predominate: until c. 3570 BP the lagoon was restricted, fed by fresh water and terrestrial sediment inputs; a second episode of lagoonal sedimentation lasts until c. 1620 BP, reflecting brackish conditions and increase in marine influence due to a reduced efficiency of the barrier. The upper section of the core represents the final stages of filling of the lagoonal margin by fluvial activity. Human settling and intervention in the barrier are possible causes of a short-lived return to brackish conditions recorded in the uppermost sediment unit of the core and of the dramatic increase of the sedimentation rates