Recent Sediments of Makirina Cove (Northern Dalmatia, Croatia): Their Origin Viewed Through a Multidisciplinary Approach

Abstract

Makirina Cove was formed by the Holocene sea-level rise whichcaused a marine ingression into the depression formed within Albian–Cenomanian dolomites at approximately 4.5 ka B.P. At present, MakirinaCove represents an restricted, stressed, shallow-marine (<2 m)ecosystem characterized by varying seawater temperatures (0–35°C)as well as fluctuating salinities (up to 41‰) affected by seasonallyenhanced evaporation, continuous freshwater supply through on-shoreand submarine springs associated with the coastal karst area and surface run-off episodes. These environmental conditions have beenconducive to high primary production of organic matter resulting inthe formation of organic-rich deposits which contain up to 5 wt.% oforganic carbon. Up to the present times, 3.5 m of sediments have beendeposited indicating a relatively high sedimentation rate estimated at0.75 m/1.0 ka in the northern central part of the Cove.The sediments are being deposited mostly as poorly sortedclayey–sandy silts. The distribution and concentration of most of thechemical elements is dependant on the mineralogical composition andgranulometric features of the Makirina sediments, which show valuesmore or less similar to those from the Central Adriatic. Accordingly,there is a positive correlation with Al and K concentrations increasingoff-shore and with the depth being associated with increasing concentrations of clay minerals within the clay fraction. The same holdstrue for concentrations of some trace elements, especially Mo and Sewhich is consistent with the distribution pattern of sulphides.Selenium is preferentially enriched in authigenic pyrite and itis probably the major source of Se in the Makirina Cove sediments.The concentrations of Ca, Mg and Sr decrease off-shore and they arelinked to the composition of the surrounding carbonate rocks. Thesaturation indices show that the water is supersaturated with respectto carbonates enabling the precipitation of authigenic amorphous orcrystalline carbonate phases from the pore water in the upper segmentof the sediment column. According to the oxygen isotopic (δ18O) composition,molluscs precipitated their carbonate shells mostly duringwarmer periods (May to November) at or near isotopic equilibriumwith their ambient waters. The carbon isotopic δ13C composition ofmollusc carbonate shells is environmentally affected due to oxidationand decomposition of organic matter as well as influxes of fresh waterinto the Cove, indicating their formation out of the predicted isotopicequilibrium with atmospheric CO2. Palynological and organic carbonisotopic (δ13C) composition shows that the sedimentary organic matter(SOM) is 70–90% lipid- and hydrogen-rich and on average 2/3 marinederived (mainly phytoplankton, bacteria and marine macrophytes) and1/3 terrestrially derived (mainly woody tissue). The variations in composition of SOM have been noted as a function of the distance fromthe shore. The type and the preservation state of SOM and pyrite aswell as the measurements of Eh, pH, total alkalinity, dissolved inorganiccarbon (DIC) and the enrichment of redox-sensitive trace elements,indicate oxygen-depleted depositional conditions and that thesediment is highly reductive even in the uppermost segment at thesediment/water interface. According to the results obtained from theapplied methods, the features of Makirina sediments strongly reflectthe given depositional conditions within this restricted, stressed, shallow-marine environment where these organic-rich sediments originate,and may therefore serve as a calibration standard in further investigations