Sediment and pollutant transport in the Northern Gulf of Cadiz: A multi-proxy approach

Abstract

This paper presents a multi-proxy study of sediment samples from the area of the northern Gulf of Cadiz Shelf (SW Iberia) influenced by the Guadiana River. 471 surface sediment samples from the northern gulf of Cadiz Shelf from January 1999 to November 2003 are analysed for grain size variations and composition, including a series of samples retrieved from the same key locations before, during, and after an important flood event in February 2001. Samples from the shelf and coast around the Guadiana estuarine mouth and the inside of the estuary are analysed for heavy mineral composition. Additionally, 26 samples recovered in November 2001 along three transects are analysed for mineralogy, elemental composition, heavy metals, OC, TN, and nannoplankton. The datasets cover mostly fair weather, low precipitation periods, with the exception of the February 2001 survey, which followed an unusually rainy winter and occurred during large-scale flooding in the Guadiana basin with water discharge levels of up to 3000 m 3/s. The results show, based on calculations from variations in sedimentary components that the bulk of bedload stemming from the Guadiana Estuary is deposited within the 15 m bathymetric line. An estimated 2–2.5 × 10 6 tons (or 7.5–9.5 × 10 5 m 3) of sand were exported during the winter 2000/2001 from the Guadiana Estuary onto the inner shelf. Simultaneously, fines were resuspended on the inner shelf, and re-deposited at the upper margin of the middle shelf. Sediments are predominantly transported eastwards by the littoral drift, although results also show a weaker westward component. This is true for the coastline, as well as the upper inner shelf, as shown by the distribution of heavy mineral associations, as the Guadiana has a very characteristic translucent heavy mineral signature, composed mainly by amphiboles and pyroxenes. Samples retrieved half a year after the February 2001 flood indicate a pollutant export from the Guadiana River basin onto the inner and the middle shelf. The signature of these pollutants diminishes in an offshore and eastwards direction on the middle shelf. Although other studies have shown that the Guadiana is less contaminated than the neighbouring Piedras and Tinto–Odiel Systems, our results show that the influence of contaminants from the Guadiana River on middle shelf sediments is much larger than from the heavier polluted Piedras and Tinto–Odiel systems. Pollutants from these two systems probably remain over longer periods of time within the inner shelf. Nannoplankton assemblages show considerable amounts fossil Cretaceous to Pliocene specimen probably originating from the shelf slope, documenting the influence of upwelling bottom currents on the lower shelf sedimentation.