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Comparing spatial and temporal changes in metal trends (Cr, Ni, Pb and Zn) on the Portuguese shelf since the 1970s

Journal article
Authors Mario Mil-homens
Rodney Stevens
Ingemar Cato
Fátima Abrantes
Published in Environmental Monitoring & Assessment
Volume 186
Issue 10
Pages 6327-6340
ISSN 0167-6369
Publication year 2014
Published at Department of Earth Sciences
Pages 6327-6340
Language en
Keywords Portuguese shelf, marine sediments, metal contents, spatial and temportal variations, gradient method
Subject categories Earth and Related Environmental Sciences, Environmental Sciences, Sedimentology


Comparison between an archive dataset, collected during the 1970s (1974–1977), and samples taken during the PALEO1 cruise (2002) enabled evaluation of large-scale and long-term spatiotemporal changes in major (Al) and metals (Cr, Ni, Pb, Zn) in bulk surface sediments (<2 mm fractions) in four areas of the Portuguese continental shelf, using the so-called Gradient Method and normalising to Al, a proxy for terrigenous clay content. From north to south, these were situated in the north-western (offshore the Ave and Douro rivers), central (offshore the Lis River) and south-western (offshore the Mira River) shelf sectors, all sampled in both the 1970s and in 2002, and the southern (offshore Algarve) sector, sampled only in the 1970s. One-way Anova ANOVA and Kruskal-Wallis tests were used to identify differences among study areas and periods of sample collection. Despite a A comparison of metal contents in 2002 samples with the Portuguese classification scheme used to regulate dredged materials (CSURDredMat) deposition in coastal areas (CSURDredMat) sediment environmental, indicatesing good environmental quality in terms of Cr, Ni, Pb and Zn levels., Nevertheless, the GM results suggest an increase in metal loadings relative to Al in the 2002 samples (except for Pb) since the 1970s. The Algarve area on the southern shelf is characterized by having high number of sites with metals level corresponded to class 2, explained justified by the occurrence of a substantial natural metal enrichment resulting supply from drainage by the Guadiana, Tinto and Odiel rivers (Spanish rivers) in the drainage areas of the Iberian Pyrite Belt formations. This supply has, probably been magnified by present and past mining activities. For quality evaluation of marine sediments, especially when using total metal contents, this study shows the importance of considering all the sources of metal loading. and tThe natural grain-size and composition variability since these features are also crucially influential on the metal contents and the combined evaluation allows a more holistic perspective.

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