Abstrakt
Palaeo-temperature reconstructions utilizing periglacial sorted patterned-ground features are challenging because these landforms can develop over a considerable temperature range. Consequently, most inferences made so far have been purely qualitative, and as such, they have been frequently considered as unreliable.
However, theoretical and experimental studies have demonstrated that patterned-ground geometry depends on temperature conditions at the time of its initiation. The size of sorted patterns is controlled by the frost depth in seasonally frozen regions and the thaw depth in permafrost areas, and their diameter-to-sorting depth ratio is constant, of c. 3.1 to 3.8 under subaerial conditions (e.g.
Ray et al., 1983; Hallet and Prestrud, 1986). Accordingly, temperature conditions at the time of patterned-ground formation can be inferred via the sorting depth, which closely approximates former frost or thaw depth.
In this contribution, we introduce a novel procedure designed to infer palaeo-temperature and palaeo-permafrost conditions prevailing at the time of the initiation of sorted patterned ground, which we demonstrate on the example of the Late Glacial or Early Holocene large-scale sorted nets located on the Luční pláň Plateau in the Krkonoše Mts., Czech Republic. The median diameter of the investigated sorted nets is 4.15 m and the estimated median sorting depth achieves 1.18 m, which is well consistent with our electrical resistivity tomography soundings and earlier observations in excavations at the study site.
The median modelled air thawing index is 285 °C.days with interquartile range of 164 °C.days to 478 °C.days. The median mean annual air temperature (MAAT) was calculated to be -8.3 °C with interquartile range of -10.0 °C to -6.5 °C.
The sorted patterns likely formed under continuous to discontinuous permafrost conditions because c. 55 % and c. 44 % of the probability distribution of MAAT is below -8.0 °C and -1.5 °C, respectively (sensu Gruber, 2012). Clearly, the presented method is able to provide palaeo-temperature and palaeo-permafrost estimates for sorted patterned ground, but we believe it also is applicable on other periglacial structures indicative of the base of palaeo-active layer.