Geochemical signature in off-shore sediments from the Gulf of Cádiz inner shelf Sources and spatial variability of major and trace elements

摘要

The geochemical characterization of 15 surficial sediments from the Gulf of Cádiz (GoC) inner shelf was assessed by examining their granulometric, mineralogic and major–minor elemental composition, in order to evaluate the sources of contaminants and describe environmental conditions. The study involved the comparison of the geochemical signature of the shelf sediments with that of surficial sediments from the main fluvial systems of the region: Guadiana, Piedras, Tinto and Odiel. Although the major supply of sedimentary material in this area is discharged by the Guadiana River, the high absolute concentration of elements associated to sulphide deposits (P, As, Cu, Hg, Pb and Zn) observed in some sediments indicated the important influence of the historically polluted Tinto–Odiel fluvial system in certain locations. The significant levels (>99%) of positive correlation observed between Al, Fe, K, Ni, V and, in a lesser extent, Na and Ba, suggested the association of these elements in the form of aluminous clay minerals. No correlation was observed between Al and Si (0.20), in contrast to the strong positive correlation found for the latter and Ti (0.91). On the other hand, an inverse relationship occurred between terrigenous–detrital sources and carbonate fractions, presumably related to biogenic origins, according to the significant negative correlations (>99%) observed for Si, respect to Ca (0.82), Mg (0.96) and total inorganic carbon (0.81). The general enrichment of As, Cu, Hg, Pb and Zn detected in the shelf sediments, together with strong and moderate positive correlations observed among these metals and P, suggested a common origin for all these elements, potentially related to pollutant-anthropogenic activities developed in the industrial area of Huelva city. Those sediments situated closer to the Tinto–Odiel mouth displayed enrichment factors (EF) equal or higher than 2 for As, Cu, Hg, Pb and Zn, particularly evident in one sample (S7), which showed EF of 2.7, 18.7, 8.7, 4.0 and 5.7, respectively. Cluster and Principal Components Analyses allowed us to differentiate four groups of shelf sediments governed by similar geochemical patterns: (i) a first group including the sediments under the Guadiana river influence, formed by coarser siliclastic sediments, with dominant terrigenous–detrital signatures and enrichment of Co and Mn; (ii) a second group of off-shore muddy sediments, Ni-associated, showing moderate enrichment of Cu, Hg, Pb and Zn, as well as important contribution of carbonate forms, which represent the transition towards a dominant marine source; (iii) sample S7, formed by muddy materials and enriched in sulphide associated-heavy metals and phosphorous, according to its proximity to the Tinto–Odiel fluvial system; and (iv) the coarsest sample S13, with high concentration of carbonates and moderate enrichment of heavy metals, which reflects a mixed signature between the detritic influence from Guadiana and Piedras rivers and marine-biogenic sources.