Mineralogical, organic and isotopic composition as palaeoenvironmental records in the lake sediments of two lakes, the Plitvice Lakes, Croatia

摘要

Six sediment cores, the top 30-40 cm, from two lakes of different sizes, Lake Prosce and Lake Kalu-Cerovac, situated in the karst region of the Plitvice Lakes system, Croatia, were studied by mineralogical and structural, C/N and total organic carbon (TOC) and isotopic analyses (a(14)C and delta C-13 of carbonate and organic fractions). The study of the two karst lakes where sediment composition is mainly calcite, showed that organic matter (OM) also plays an important role, and that both fractions analysed simultaneously give a good overview of the processes in the lakes. The morphological and structural properties of the calcite particles at micron and submicron levels indicate that the calcite mineral phase is formed by active authigenic precipitation in the studied lake systems. In the small Lake KaluCerovac, the measured distributions of carbonate and OM fractions, C/N, TOC, and a(14)C and delta C-13 values of both fractions along the sediment depth showed similar values for all three cores, with small variations in each core indicating very homogenous precipitation of autochthonous calcite inside the lake, without allochthonous fraction. In the bigger Lake Prosce, the large difference in sediment composition of the three cores indicated different conditions of sediment precipitation inside the lake. Significant fractions of land-derived carbonate and OM were recognized by mineralogical, C/N, a(14)C and delta C-13 values in the shallow, coastal area indicating that the sediment was a mixture of allochthonous and autochthonous fractions. In contrast, the composition of the deep-water sediment in the same lake indicated in-situ calcite precipitation and aquatic OM produced in the lake. Sedimentation rates of 0.7 cm/yr and from 0.3 to 0.7 cm/yr for Lake Prosce and Lake KaluCerovac, respectively, were determined based on the a(14)C of macrofossils. Extreme hydrological events in 1981 and 2010 were identified by disturbances in carbon isotopes distributions in sediment profiles. No evidence of anthropogenic influence in either lake on the local level was found, but the response to the global C-14 contamination was observed by a14C peaks in both, carbonate and organic fractions. Increased bioproductivity in the recent decades was observed in Lake Prosce which can be correlated with a slight increase in lake water temperature in the last three decades. This process can be associated with enhanced eutrophication in Lake Prosce. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.