Recent sediments (peloids) from Morinje Bay (Middle Dalmatia, Croatia): Their origin and anthropogenic pollution viewed through a multidisciplinary approach

The Recent sediments of the Morinje Bay have been multidisciplinary investigated in order to define their origin and to estimate the degree of anthropogenic pollution. Due to the fact that the peloids of Morinje represent an important source of healing mud with a great potential in medical applications, the investigation has been focused in the northern coastal part of the Bay where the city dump of Sibenik is placed. As a consequence of sea-level rise during the Holocene, the sea ingression through the narrow Jadrtovac channel towards the Morinje area started aproximally 8000 years B.P. The depression of Morinje was sunk by the sea ingression at approximately 4000 years B.P. During that period up to 4m of sediments were deposited in the central part of the Bay. Morinje Bay extents on 3 square km and represents a depositional environment of a restricted, marine shallow-water (0.3-1m) depression under seasonal fresh-water influence. The Bay communicates with open sea by a narrow (100-200m) channel. According to mineralogical composition and distribution of major elements, the sediments of Morinje Bay are classified as clayey siltites that originated from the erosion of the surrounding Palaeogene flysch-deposits. In a lesser amount biogenic quarz and calcite are also present. The palynofacies of the clayey siltites is characterized by a high portion of lipid-rich, mainly amorphous organic matter, but always accompanied with palynomorphs of phyto- and zooplanktonic origin. These results are in good consistency with the biological content of the sea-water column. The peloids are characterized by an overall high content of organic matter thus indicating at least seasonal eutrophication of this eco-system. The analysis of the stable isotopic composition of organic carbon (δ 13C‰) show that the seaweed vegetation has a great influence on the sedimentary organic carbon isotope composition. The gas chromatograms show that the qualitative composition, i.e. organic compounds, of the extractable low molecular organic matter is in all analysed samples the same, but differs in quantitative distribution. High sulphur content, that was directly incorporoted into the amorphous organic matter, indicates anoxic conditions. The amorphous organic matter was formed within anoxic conditions and is found to be in the humin phase of diagenesis. The organic matter is well preserved and protected against secondary oxidation in which seaweed roots play an important role by fixing the loose sediments. The sedimentary organic matter of the Morinje Bay originated in a marine, shallow-water environment where the continous transgression and seasonal run-off enabled high biological production of phyto-, zooplankton and seaweeds. Contemporaneously, the restricted environment and high biological production, together with the terrigenous detritus input, initiated within the sediments fast oxygen consumtion and anaerobic degradation of the biomass that was accumulated mostly as amorphous, lipid-reach organic matter and preserved in conditions of a high sedimentation rate (1m/1000y). The results of isotopic composition of organic δ 13C‰ and δ 15N‰ shows no impact of anthropogenic organic pollutants. This is in good consistency with the biological content of the sea-water column measured during June. This period is characterized by a relatively low production of phyto- and zooplankton which indicates low anthropogenic influence. Otherwise, it would have resulted in a constant increase of plankton activity as examplefied in some similar eco-systems along the Croatian coast. The use of aluminium to normalize heavy metals show that surficial sediment is relatively unpolluted. The exception represent following elements: Pb, Zn, Cu, Sn, Mn, P, Mo and Ba. They most likely originate from city dump Sibenik and agriculture activity.