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Geochronological and paleoclimatological investigations at the Plitvice Lakes

The group of scientists from Radiocarbon and Tritium Laboratory of the Ruđer Bošković Institute in Zagreb started investigation at the Plitvice Lakes National Park in 1976, when first radiocarbon dating of tufa was performed. Since that time the scientists continued systematic and comprehensive investigation encompassing both conditions required for calcite precipitation and age determination of tufa and lake sediments. Tufa is created from carbon partly of biogenic origin, and therefore its age can be determined by radiocarbon (14C) dating method. However, there are several factors that affect the accuracy and reliability of 14C dating of carbonates: (i) since tufa represents a very porous material, there exists a great possibility for contamination with allochthonous calcareous deposits ; (ii) there is a possibility of carbon isotope exchange during the aging of the deposits. (iii) It is also necessary to know the initial 14C activity at the time of carbonate deposition (so called reservoir effect). It was determined by measurements of 14C activity in dissolved inorganic carbon (DIC) and showed a continuous increase along the Plitvice watercourse from 65% to 85% as the consequence of influence of atmospheric CO2 and respiration of water plants. More than 500 samples of tufa were collected from the whole area of the National Park, both from active tufa barriers located along the present-day water flows and from the remnants of old tufa deposits outside them. Results of 14C dating showed that tufa at active barriers was formed within last 7000 years, i.e. during Holocene. The age of old barriers was determined by 230Th/234U method, since it exceeds the maximum age that could be measured by 14C method. The results showed that majority of these barriers were formed in interglacial periods: from 90 000 to 130 000 years (Riss-Würm), as well as in the period between 250 000 and 300 000 years (Mindel-Riss). These results verify also that the tufa precipitation process was associated with warmer climatic periods. Retrieving of sediments from two bigger lakes (Kozjak and Prošće) with total depth up to 12 m, enabled numerous analyses which contributed to the knowledge of geochronology of Plitvice Lakes. 14C measurements showed the beginning of sedimentation in Lake Prošće about 7000 years ago and in Lake Kozjak about 6500 years before present. These measurements indicated that the sediment, like the tufa barriers, was deposited also in warmer geologic periods. Comparative dating of sediment layers by 14C method and distribution of characteristic non-arboreal pollen indication of density of settlement in the wider area of National Park in the past was obtained. Age determination of peat found in the same area showed that peat-bogs were formed in Holocene period, i.e. during last 7000 years.

Geochronology; Paleoclimatology; Plitvice Lakes; Holocene; Interglacials; Radiocarbon dating; 230Th/234U dating

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