engleski

Geochemical and isotopic analyses of palaeoenvironmental records in the lake sediments of the Plitvice Lakes, Croatia

The presented comprehensive multi-proxy study of lake sediments from two karst lakes of different sizes, Lake Prošće (0.68 km2) and Lake Kaluđerovac (0.02 km2), both belonging to the karst area of the Plitvice Lakes system, involved mineralogical, chemical (C/N and TOC) and carbon isotope analyses (a14C and δ13C of carbonate and organic fractions) of 6 sediment cores. The main aim of the study was to find out the following: 1) How does the lake sediment respond to the environmental conditions and what is the difference between two lakes of different sizes? 2) Which fraction, organic or carbonate, of the lake sediments better describes paleoenvironmental conditions? 3) What can we conclude about the changes in the environment of the Plitvice Lakes during the last ≈150 years by studying ≈40-cm-long sediment cores? Sediment cores were taken in each lake at three positions having different water depths and characterized by different environmental conditions. Some sites were close to the shore (P4, P5, Ka2), some were influenced by the stream of tributaries (P4, P5), and the process of eutrophication was observed at some of them (P4, P5, Ka1). The length of sediment cores varied from 22 cm to 46 cm. The correlation of measured values (organic/carbonate ratio w, C/N, and isotope analyses a14C and δ13C for organic and carbonate fractions) represented by their mean, minimal and maximal values, in 6 sediment cores from the two lakes, with water depth is presented in Figure 1. Lake Prošće is almost 30 times bigger than Lake Kaluđerovac, significantly deeper and at a higher altitude. The water composition of both lakes showed slight differences and in both lakes water is supersaturated with regard to CaCO3, leading to expressed precipitation of lake sediment in the form of calcite. Both lakes are oligotrophic. In the small Lake Kaluđerovac all the cores were sampled in relatively shallow water, from 1.2 m to 14 m in depth. The measured parameters of carbonate and OM fractions (C/N, a14C and δ13C) showed similar values for all three cores (Ka1, Ka2, Ka3) with small variations with depth in each core. Different water depths above these three cores do not particularly influence either of the measured parameters indicating mostly uniform conditions of sediment precipitation in Lake Kaluđerovac, regardless of the position of the sediment core inside the lake. All analyses indicated autochthonous calcite precipitation. Some differences in the sedimentation rate were observed, and the sedimentation rate decreases downstream in the lake (Ka1 > Ka2 > Ka3). In the big Lake Prošće significant differences among the three cores are evident. In the deepest point of the Lake (P1, 40 m) all parameters show small variations and all data indicate the autochthonous origin of the calcite and mostly aquatic OM with increased primary productivity in the surface sediment layers. For shallow points P4 and P5, which are close to the coastline and under the influence of the Sušanj stream and the Matica River, respectively, the variations of each parameter are larger, particularly for the P5 core. The main reason for this is the strong influence of the land-derived material forming the sediment which becomes the mixture of allochthonous and autochthonous material. As a consequence, the sedimentation rate of the P4 core is higher than that of the P1 core and some extreme hydrological events were recorded. In the big Lake Prošće significant differences among the three cores are evident. In the deepest point of the Lake (P1, 40 m) all parameters show small variations and all data indicate the autochthonous origin of the calcite and mostly aquatic OM with increased primary productivity in the surface sediment layers. For shallow points P4 and P5, which are close to the coastline and under the influence of the Sušanj stream and the Matica River, respectively, the variations of each parameter are larger, particularly for the P5 core. The main reason for this is the strong influence of the land-derived material forming the sediment which becomes the mixture of allochthonous and autochthonous material. As a consequence, the sedimentation rate of the P4 core is higher than that of the P1 core and some extreme hydrological events were recorded. According to all the analyses of the Prošće and Kaluđerovac lake sediments we can conclude the following: 1) In the bigger Lake Prošće, the uppermost of a series of 16 lakes, settled mainly in dolomite and surrounded with mostly deciduous forest, the large differences in sediment composition from different cores indicated different conditions of sediment precipitation inside the lake and varied sources of sedimentary material. Significant fractions of land-derived both carbonate and organic components were indicated by mineralogical, C/N, a14C and δ13C values in the shallow, coastal area indicating that the sediment was a mixture of allochthonous and autochthonous fractions. In contrast, the composition of deep-water sediment in the same lake indicated in-situ calcite precipitation and aquatic OM produced in the lake. The same analyses of the sediments in the small Lake Kaluđerovac, settled in the limestone canyon, indicated very homogenous precipitation of autochthonous calcite inside the lake, without an allochthonous fraction. Finally, the response of the lake sediment to the environmental conditions does not depend particularly on the size of the lake, but the surrounding environmental conditions could have great influence on the sediment composition. If one wants to study regional and long-term paleoclimatic records in lake sediments, the chosen sampling location should be far from local-scale influences where mostly authigenic calcite precipitates from DIC in water and organic matter is of limnic origin. Sediment locations closer to the shore and influenced by local water inputs (feeding streams) that may occasionally bring terrigenic material to the lake are more appropriate for the determination of local short-term paleoenvironmental events. 2) The study of two karst lakes where the sediment is composed mainly of calcite showed that the OM fraction also has an important role and that analyses of both fractions and the relation between them give good overview of the processes in the lake. The a14C and δ13C values of the carbonate fraction showed that all sediment from Lake Kaluđerovac and deep-water sediment (P1) from Lake Prošće are authigenic, i.e., formed by in situ precipitation from DIC in water, while two sediment cores in Lake Prošće are the mixture of the authigenic and allogenic carbonate. The authigenic/allogenic fraction ratio was determined based on δ13Ccarb values of sediments. The sedimentation rate was determined based on the radiocarbon dating of macrofossils in Lake Kaluđerovac sediments (0.3 to 0.7 cm/yr) and the known occurrence of extreme hydrological events in Lake Prošće sediments (0.7 cm/yr). These values are in good agreement with the previously determined sedimentation rates by 210Pb dating. Isotope analyses of the organic fraction and correlation with C/N and TOC values of OM give additional information on carbon cycling and evidence of the biota that have lived in both lakes. 3) Using the lake sediment profiles of 30 – 40 cm in length from the two lakes which cover the time period of the last ≈150 years we can follow the influence of anthropogenic contamination and changes of environmental conditions in the area of the Plitvice Lakes. Extreme hydrological events in 1981 and 2010 were identified by disturbances in carbon isotopes distributions. Local anthropogenic influence was not evident, but the response to the global 14C contaminations was observed by the occurrence of the a14C peak in both carbonate and organic fractions. Increased bioproductivity in the recent decades was found in Lake Prošće, which can be correlated with a slight increase of the lake water temperature in last three decades (Sironić et al, 2017). This process can be associated with the enhanced eutrophication in Lake Prošće. The results of this work, particulary the isotope analyses in the lake sediments will be compared with previous study of the Plitvice Lakes sediments formed in the last ≈150 yr (Horvatinčić et al., 2008, 2014).

project REQUENCRIM ; workshop ; quaternary ; isotope methods ; Plitvice lakes ; lake sediment

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