engleski

Conditions of aragonite preservation in the recent sediment of a karstic marine lake

The Lakes of Mljet (Veliko and Malo jezero, Mljet Island, south Adriatic, Croatia) are small karstic coastal lakes. They are semi-enclosed, relatively deep depressions (47 and 29 m, respectively) connected with the open sea by a narrow shallow channel. The lakes were formed in Holocene when the seawater penetrated through the permeable karstified system 7.3 kyr B.P, whereas the marine ingression through the channel followed less than 4 kyr B.P. Since the Lakes are typical karstic features and represent an almost entirely closed water body with very limited water circulation, they are highly sensitive to anthropogenic stress due to the visitors of the proclaimed National park. Considerable amounts of aragonite were found in the sediment of Malo jezero, in contrast to Veliko jezero, where it is present only as a minor mineral phase. Most probably it was formed by inorganic precipitation in the water column during whiting events, a rare and unusual phenomenon which has recently been reported in the Malo jezero. The aim of the study was to explore the potential for preservation of aragonite precipitates during early diagenetic processes in the sediment columns. Geochemical analyses of the sediment and pore water were performed to estimate the saturation state of carbonate minerals (calcite, aragonite) in the sediment column, so as to evaluate the potential of the sediment to preserve aragonite phase during early diagenesis. Stable isotope analysis of C and O in carbonate and C in sedimentary organic matter showed lower δ 13C and δ 18O values in Malo jezero than in Veliko jezero. The isotope separation between SOM and carbonate was the same in both lakes, which indicates that a considerable fraction of the sedimentary carbonate is of authigenic origin and confirms the assumption that the whiting events contribute an important fraction of carbonate to the sediment. The saturation state of calcite and aragonite in both lakes varies seasonally ; the interstitial solution is supersaturated or close to the saturation with respect to calcite and aragonite in both lakes in autumn, so the aragonite can be retained, whereas in the spring the interstitial water is subsaturated with respect to both carbonate minerals. From the isotope mass balance of carbonate and dissolved inorganic carbon, the contribution of the decomposition of sedimentary organic matter to the total DIC concentration in the interstitial water was estimated to be between 80 and 90% in the uppermost sediment layer, whereas the rest derived from carbonate dissolution. Chemical analysis of bulk sediment showed increased concentrations of Sr in Malo jezero compared to Veliko jezero. The SEM-EDAX analysis of the needle-like aragonite crystals confirmed the presence of Sr in concentrations up to 0.97%. Long periods of saturation of interstitial water with respect to aragonite and stabilization of aragonite with Sr thus both contribute to the preservation of this mineral in the sediment.

aragonite; recent sediments; marine lake; Mljet Island

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