Geochemical signature as paleoenvironmental markers in Holocene sediments of the Tinto River estuary (Southwestern Spain) J. Borrego ) , N. Lo´pez-Gonza´lez, B. Carro Coastal Geology Group Department of Geology, Facultad de Ciencias Experimentales, University of Huelva, Campus de El Carmen, 21071 Huelva, Spain Received 2 April 2004; accepted 25 July 2004 Abstract High absolute concentrations of metals associated to sulfide deposits located at the river source (Fe, Cu, Zn, Pb, Cd, and As) have been determined from the study of major and trace element concentrations in sediment samples obtained in two vibracores in the Tinto River estuary. In most cases, the enrichment factor (EF) for these metals is over 2. Cu, Zn and As have much higher EF, over 10 in surficial samples. Fe oxy-hydroxide precipitation in high-chlorinity zones and its scavenging effect on metals is the main control process on the chemical composition of sediment. Vertical evolution of the sedimentary and geochemical characteristics of the cores show that the estuary has undergone a process of progressive shallowness and a relative increase in acidic river water volume with respect to seawater. This effect has originated a downstream displacement of acid mixing processes affecting the estuary, which is reflected in higher metal concentrations in sediments of the outer zones of the system. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: acidic estuary; geochemistry; enrichment factor; Holocene sediments; Tinto estuary; Spain 1. Introduction Metal behavior in systems with fluvial–marine in- teraction (rias, estuaries and deltas) has aroused a growing interest among the scientific community. This is due to alterations of anthropogenic origin which the biogeochemical cycles of trace metals have under- gone in these sedimentary environments (Salomons and Fo¨rstner, 1984; Siegel, 2002). A major portion of metals of natural or anthropogenic origin, introduced into estuarine environments, usually arrives in dissolved phases. They are immediately incorporated into organo- metallic compounds or other mineral phases of the suspended particulate matter at the beginning of the saline (salt-induced) mixing processes. Once they become part of the suspended particulate matter, they are transported in the water column and finally incorporated into the sediments (Stecko and Bendell-Young, 2000). In an estuarine system undergoing exclusively saline (salt- induced) mixing processes, the effects of metal and other trace elements adsorption onto Fe oxy-hydroxide particles take place in low-chlorinity zones (Sholkovitz, 1992; Millward, 1995; Sholkovitz and Szymczack, 2000). While in water systems affected by acid mine drainage or acid rock drainage, salt-induced adsorption/desorption processes are altered by wide pH variations (Achterberg et al., 2003), because low pH values ( !4) inhibit ad- sorption processes, and metals are held in dissolved phases until they reach high-chlorinity zones. Acidic contributions are characterized by low pH and high concentrations of Fe, sulphate and a wide variety ) Corresponding author. E-mail address: borrego@uhu.es (J. Borrego). 0272-7714/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2004.07.004 Estuarine, Coastal and Shelf Science 61 (2004) 631–641 www.elsevier.com/locate/ECSS