Elemental and isotopic records in recent fluvio-lacustrine sediments in karstic river Krka, Croatia

摘要

A history of metal input and changes in the environmental conditions of the karst ecosystem of Krka River during the last several decades was investigated by statistical evaluation of the combination of several proxies including sediment content of minor (Pb, Zn, Cu, As, Cr, Ni, Co, V, Ga, Y, Rb, Sr) and major (Ca, K, Fe, Mn, Ti, Al) elements, concentrations of sedimentary organic carbon (C_(org)) and nitrogen (N), their stable isotope compositions (δ~(13)C_(org) and δ~(15)N), as well as stable isotope composition of sedimentary carbonate (δ~(13)C_(carb) and δ~(18)O_(carb)). The results show that measured trace and major elements are predominately of terrigenous origin, reflecting the geological background of the area, whereas the sedimentary organic matter is predominantly authigenic Although Zn amounts in sediments are results of anthropogenic pressures, its distribution in investigated sediment columns is influenced by natural processes, i.e. occasional re-sedimentation and self-purification of the water column caused by the particulate Zn deposition in lakes formed by the tufa barrages. Levels of Pb in sediment columns are low except in the K2 which is close to the crossroads, especially in the period of lead fuel use. Trace element concentrations in sediment columns are compared to consensus-based sediment quality guidelines (SQG) for freshwater ecosystems. Some of the As, Zn and particularly Ni sediment concentrations were found to be above threshold effect concentration (TEC) range. The stable isotopes of C_(org) and C_(org) /N ratios suggest that the sedimentary organic matter is predominately of algal and bacterial origin, while the carbonate is a mixture of terrigenic detritus and in-situ precipitates. The average grain size distribution and carbonate content of sediment indicate decreasing input of terrestrial non-carbonate fraction, as a consequence of afforestation and land use changes in studied area.