Radiometric dating of sediment records in shallow coastal lagoons (Pialassa Baiona and Piomboni, Italy)

Sediment cores from two shallow coastal lagoons (Ravenna, Italy) were collected as a part of a study on impact of natural and/or anthropogenic environmental changes and their temporal evolution. The two areas represent a vulnerable aquatic ecosystem covering a surface of about 1,800 ha that belongs to the list of Wetlands of International Importance under the Ramsar Convention, and lies parallel to the Northern Adriatic Sea shoreline. Natural and man-made changes over the time, led to existing physical features resulting in a number of different wetlands of less than 1 m in depth, broad and irregular in shape, separated by levees and crossed by a network of artificial channels. From the fifties, the delicate balance of life in Pialassa Baiona lagoon has been threatened by the presence of a petrochemical industrial complex, fuel oil power generation, inland intensive agriculture, as well as close contact with the densely populated town and coast, and human negligence. The goals of this study were: (1) to estimate recent sedimentation rate using a 210Pb dating technique; (2) to detect the surface and in-depth distribution of heavy metals of environmental concern such as Ni and Zn in different areas and sediment strata; (3) to highlight patterns between metal concentrations, and organic carbon in sediments from different areas of the lagoon with reference to their accumulation time. The method for dating sediment cores was on 210Pb, and its unsupported levels were determined by removing the supported fraction as measured via parent 226Ra activity. Measurements were accomplished by using a HPGe gamma-spectrometer connected to a multichannel analyser, whereas metals concentrations were determined using inductively coupled plasma spectrometry (ICP-OES). Although contaminant inputs from industrial and municipal discharges, as well as agricultural runoff, the major sources of contamination to Ravenna wetlands, have drastically declined during the last three decades, bottom sediments from the lagoons showed considerable surface and in-depth concentrations of Ni and Zn, and organic carbon depending on the location area, sedimentation rate, and time. Organic carbon and toxic metals show a similar vertical profile in sediment cores, whereas spatial distribution shows a marked gradient of pollution southward. These results show that sediments from the area may have a considerable influence on the environmental status of the lagoon areas in having the potential to act as a temporary sink or source for heavy metals. High organic C accumulation rates can be observed in the sediments due to a high organic matter supply from land and a high productivity of the water bodies as a result of eutrophication.