RADIOCARBON AND URANIUM-SERIES DATING OF THE PLITVICE LAKES TRAVERTINES

Radiocarbon and uranium-series ages of the calcareous deposits of the Plitvice Lakes show that travertines were deposited during three warm, humid, interglacial oxygen isotope stages. According to our measureme nts, only calcite crystals or crystal aggregates represent reliable ma terial for both Th-230/U-234 and U-234/U-238 dating. Compact old trave rtine in the form of sandstone is less reliable; it can be dated by bo th methods provided that its detrital contamination is not significant , demonstrated by very low C-14 activity (< 1.5-2.0 pMC) and a high Th -230/Th-232 ratio. Old porous travertine contaminated with recent carb onates and Th-bearing clay (pMC > 5, Th-230/Th-232 < 5) gives erroneou s results by both methods. Stage 1 (Holocene) deposition is shown prim arily by C-14 dating corroborated by sedimentological and palynologic studies as well as by both Th-230/U-234 and U-234/U-238 disequilibrium methods. The intensive growth of travertine barriers coincided with s ignificant climate warming in the Holocene. Stage 5 deposition is conf irmed by the Th-230/U-234 dating of crystalline calcite aggregates emb edded in the travertine matrix and by concordant Th-230/U-234 and U-23 4/U-238 ages, assuming that the U-234/U-238 activity ratio of 1.88 obs erved in modern streams and in Holocene deposits can be extended to pa st epochs. The travertine deposition period was very short, peaking ca . 120 +/- 10 ka BP. Stage 11 deposition is indicated by U-234/U-238 da ting only, the period being within the U-234 decay range, but not that of Th-230. Stage 11 travertine was deposited ca. 420 +/- 50 ka BP. We did not find travertine samples with U-series ages indicating a growt h period during relatively warm Stages 7 and 9; due to the scarcity of old travertine outcrops, these and possibly other stages cannot be ex cluded on the basis of presented data. All of these isotopic dating re sults concur with the field relation of the travertine complex of the Plitvice Lakes.