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

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 Al, Cr, Ni, Pb and Zn contents in bulk surface sediments in four areas of the Portuguese shelf. The so-called gradient method (GM) with normalisation to Al, a proxy for terrigenous clay content, was applied. The studied areas are distributed from north to south along the western margin, with the northernmost located off shore the Ave and Douro rivers, a central one located off shore the Lis River and a southern one located off shore the Mira River. One-way ANOVA and Kruskal-Wallis tests were used to identify differences among study areas and periods of sample collection. A comparison of metal contents in 2002 samples with the Portuguese classification scheme regulation for deposition of dredged materials in coastal areas indicates 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 from the 1970s to the 2002 samples (except for Pb). The Algarve area on the southern shelf is characterised by having high number of sites with metal level on class 2. These high values have traditionally been explained by a substantial supply of natural metals from the Guadiana, Tinto and Odiel rivers that drain areas 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 possible sources of metal loading. The natural grain size and composition variability have also a crucial influence on the metal content, and the combined evaluation allows a more holistic perspective.