Mid-Holocene horizons of strongly decomposed peat and problems of dating paleohydrological changes in mires in the Racibórz basin, Southern Poland

During litho-stratigraphic studies of deposits in small mires in the upper basin of the Oder River, mid-Holocene hiatuses identified by the presence of horizons of strongly decomposed peat (HSDP), were identified at four sites. Radiocarbon dating results indicate long (10³ years) breaks in peat accumulation and the dates obtained from the underlying layers of deposit are inconsistent with the pollen record. In order to explain these discrepancies, an attempt was made to recreate the conditions of HSDP formation under the influence of endo- and exogenous factors determining peatland productivity. In the light of macrofossil analysis, the development of HSDP during the Atlantic and Subboreal periods, when the humidity of habitats decreased, occurred at an advanced stage of development of swamps with Alnetea glutinosae forest communities. The gradual rise in water levels in the mires that started in the Subboreal period and intensified during the Subatlantic period was recorded as a slow shift in vegetation towards non-forest, sedge-moss communities belonging to the Magnocaricion alliance or the Scheuchzerio-Caricetea nigrae class. One consequence of the mechanism of HSDP formation is the existence of limitations on the dating of the duration of the dry phase owing to the presence of organic matter of different ages in the degraded peat layer. The determination of the age of wet phases offers better prospects. In the light of radiocarbon dating, which is essentially in line with the pollen analysis results, the transition towards positive peatland productivity occurred between ca. 9^th century BC and 7^th century CE.