International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Uranium isotopic ratios and their implication for uranium–uranium dating and groundwater circulation studies: A case study from speleothems of the Demänová caves, Nízke Tatry Mts., Slovakia

Published: 01 Feb 2020

Pages: 61 - 72

DOI: https://doi.org/10.31577/GeolCarp.71.1.5

Authors: HELENA HERCMAN, MARCIN BŁASZCZYK, ANNA MULCZYK, PAVEL BELLA

Abstract: Detailed analyses of speleothems collected in caves of the Demänová Valley, Nízke Tatry, Slovakia, have found significant variability in uranium concentrations and initial 234U/238U activity ratios, plus correlation between these variables. They are related to the ages of samples and their differing locations in the caves. Strong variability of initial 234U/238U activity ratios is the reason for serious limitations in applying the 234U/238U Regional Uranium Best Estimate method as an age estimator for speleothems older than the range of the 230Th/234U method (500 ka). The strong variability of uranium contents in speleothems, rocks and waters and their isotopic compositions allowed us to distinguish distinct periods with changes in water circulation paths or distribution/weathering degrees of deposits in surface alimentation areas or in the epikarst zone, that result from the more ready leaching of the 234U isotope from sediments. Periods characterised with higher initial 234U/238U activity ratios are most likely associated with the deposition of “fresh” colluvial deposits and/or soil development (potential uranium source) in the alimentation area. Gradual decrease of initial 234U/238U activity values indicate prolonged stable weathering and uranium leaching conditions. The Demänová results indicate periods of intensive erosion and “fresh” sediment deposition in the alimentation areas at ~120 ka, ~80–70 ka, and during the Holocene.

Keywords: Uranium isotopes, U-series methods, Demänová Cave System, Western Carpathians

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Volume 71 no. 1 / February 2020