Abstrakt
The entirely granitic catchment of the Plešné Lake is an exception from other lake catchments in the Bohemian Forest (Šumava Mts, Böhmerwald), dominated by metamorphic bedrock. No remarkable enrichment of the local granite (the Plechý/ Plöckenstein type) with any trace elements has been noted.
However, the literature data including monazite and zircon chemistry (Breiter 2016) imply that at least 60 % of the uranium (U) budget in the Plechý granite is bound in minerals relatively less resistant against weathering like apatite, possibly uraninite and secondary minerals (or even in non-crystalline compounds), and can be relatively easily mobilized. Therefore, the granite weathering may be a significant source of U in the catchment.
We evaluated U concentrations in a core brought out by Kullenberg piston corer comprising complete sequence of the Plešné Lake sediments, representing a period of the last ~ 15 kyr. The 5.49 m long core was split into 3-cm pieces, the portions of which were used for a non-destructive determination of U and some other trace elements by XRF method.
Uranium concentrations are closely related to those of yttrium (Y) (Fig. 1). In the lower part of the core (549-309 cm), which is formed by sediment poor in organic matter, the U concentrations range from 16 to 51 ppm.
The peak value (1127 ppm U) is at 267-270 cm depth, in organic-rich sediment. This sample represents the Early Holocene, being only 3-6 cm above its base at 273 cm.
From 249 to 0 cm, the U concentrations exhibit a nearly monotonous exponential decrease to the recent value of 41 ppm. Qualitative gamma-ray spectral measurement indicates that U concentration in the sample from 264-267 cm is very high compared to common rock- or soil samples (for spectrum see Fig. 2).
Quantitative determination of radionuclides is a subject of an ongoing research. Similarly to enrichment with Y and REE (Norton et al. 2016), the Early Holocene maxima of U may reflect a period of exceptionally intense chemical weathering (which had been suppressed in the preceding cold climate) and high input of U into the lake in soluble species.
Immobilization of U in the lake may be related to photolytic destruction of U-bearing complexes and adsorption of U by Al- and Fe-hydroxides. Reduction of dissolved uranyl compounds in anoxic environment at the lake bottom may be another important factor. (C) 2020 Czech Geological Survey.
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