The paleoenvironmental reconstruction of Aveiro Lagoon is being carried out based on the sedimentological, paleoecological, geochemical and mineralogical characterization of sediment samples collected systematically in the lagoon. This paper presents results concerning the changes in the chemical and mineralogical composition of the fine fraction (<63 µm) and the clay fraction (<2 µm) in sediment samples from Aveiro Lagoon, mainly collected from the lower intertidal plain and subtidal deposits located along the Mira, ÿlhavo and Ovar channels. A zonography of the lagoon was established based on the parameters analyzed. The results show that the surface and near-surface sediments of Aveiro Lagoon trap relatively large quantities of trace metals, such as Cu, Pb, Co, Ni, Cd, Zn, Fe, Mn and Cr, considered to be partially adsorbed, particularly on silty and clayey particles, and coprecipitated on Fe and Mn oxyhydroxide forms precipitated in the topmost layers of sediments. Series of statistically significant relationships were established between trace metal concentrations and clay content and semi-quantitatively determined clay mineral species in the sediments.
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
A chemical analysis of Al, Ca, Fe, Mg, Cu, Pb, Zn, Cr, Co, Ni and Cd was carried out in 22 surface sediment samples of the continental shelf and upper slope between Espinho and Aveiro (Portugal). To reduce the disturbing effects of grain size, all samples were size-normalized by wet sieving (<63 µm). The fine fraction was digested with concentrated acids and analyzed by flame atomic absorption spectrometry. The Al and heavy metal concentrations displayed a similar distribution pattern, decreasing seaward. Exceptions to this general pattern are Ca, Mg and Pb distributions. The Ca contents are related to carbonated biogenic particles that are very abundant in the outer shelf. Multivariate statistical analysis was applied to all data to assess the behaviour and sources of each element analyzed. The results from factor analysis showed that terrigenous supply (from Aveiro Lagoon flow, coastal erosion and littoral drift), grain size (clay) sorting and anthropogenic influences from Aveiro Lagoon are the major factors controlling spatial variations of chemical elements in the continental shelf between Espinho and Aveiro. Unusually high contents of Pb were found in the middle shelf off Furadouro, but we could not determine the possible source of this contamination.
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