Trace-element concentration and distribution in the sediments of Antigua, British West Indies
Approximately 120 samples collected around the island of Antigua during January of 1979 were analyzed for Mg, Cr Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb. The study was undertaken to investigate five aspects of trace-element geochemistry: One, to investigate the relationship between drainage basin lithology and the trace-element concentrations found in nearby sediment accumulations. Two, to investigate man's influence on the trace element concentrations found in various sediment samples. Three, to investigate the important carriers of trace-elements in the estuarine and marine environments. Four, to test the feasibility of employing trace-element ratios as environmental indicators, and five, to discuss any trends found in the distribution of trace-elements. The geology of the area offers a concentration gradient of trace-element source rocks ranging from the intrusive and mixed volcanic terrain of the mountainous southwestern third of the island to the volcaniclastic, terrigenous, and marine sedimentary rocks of the central third of the island, to the relatively pure carbonate terrain of the northeastern third of the island. Three bays and harbors and four reef transects were sampled. Results indicate that anthropogenic influences are dominant' in two of the bays. The concentrations of trace elements there are the highest found during the study. Concentration of trace elements is governed chiefly by the composition of the contributing source area, where anthropogenic inputs are not dominant. Under reducing conditions, the major factors controlling the distribution of trace elements are sulfides and organic material. Under oxidizing conditions, however, the main controls are clay minerals, where present, and Mn and Fe oxides. The basic trend visible in the data from all locations is decreasing trace-element concentrations going away from shore, which attests to the intimate association of trace elements and terrigenous material.