Radiocarbon and Uranium-Series Dating of the Plitvice Lakes Travertines

Abstract

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 measurements, only calcite crystals or crystal aggregates represent reliable material for both 230 Th/ 234 U and 234 U/ 238 /U dating. Compact old travertine in the form of sandstone is less reliable; it can be dated by both methods provided that its detrital contamination is not significant, demonstrated by very low 14 C activity (< 1.5–2.0 pMC) and a high 230 Th/ 232 Th ratio. Old porous travertine contaminated with recent carbonates and Th-bearing clay (pMC > 5, 230 Th/ 232 Th < 5) gives erroneous results by both methods. Stage 1 (Holocene) deposition is shown primarily by 14 C dating corroborated by sedimentological and palynologic studies as well as by both 230 Th/ 234 U and 234 U/ 238 U disequilibrium methods. The intensive growth of travertine barriers coincided with significant climate warming in the Holocene. Stage 5 deposition is confirmed by the 230 Th/ 234 U dating of crystalline calcite aggregates embedded in the travertine matrix and by concordant 230 Th/ 234 U and 234 U/ 238 U ages, assuming that the 234 U/ 238 U activity ratio of 1.88 observed in modern streams and in Holocene deposits can be extended to past epochs. The travertine deposition period was very short, peaking ca. 120 ± 10 ka bp. Stage 11 deposition is indicated by 234 U/ 238 U dating only, the period being within the 234 U decay range, but not that of 230 Th. Stage 11 travertine was deposited ca. 420 ± 50 ka bp. We did not find travertine samples with U-series ages indicating a growth period during relatively warm Stages 7 and 9; due to the scarcity of old travertine outcrops, these and possibly other stages cannot be excluded on the basis of presented data. All of these isotopic dating results concur with the field relation of the travertine complex of the Plitvice Lakes.