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Early Middle Pleistocene palaeolacustrine sediments of the Ferdynandów site (E Poland), serving as a stratotype for the Ferdynandovian interglacial, were subjected to a new drilling in 2011. The obtained profile, covering the late Sanian 1 glaciation, two interglacial successions (Ferdynandovian 1 and 2), the cold Ferdynandovian 1/2 interval, and the early Sanian 2 glaciation, permitted a high-resolution plant macroremains analysis, correlated with the results of a palynological examination. In detailed studies of plant macroremains from the profile, new taxa were discovered for the Polish Pleistocene flora: specifically, species that are extinct or not found nowadays in Poland. On the basis of the taxonomic diversity and occurrence of bioindicators, the division into warm and cold units of the Ferdynandovian interglacial as well as units associated with the Sanian 1 and 2 glaciations, already applied in pollen studies, were confirmed, and the periods were described in terms of plant macroremains.

Lacustrine sediments at the Łuków site bear a record of the Ferdynandovian interglacial, correlated with Marine Isotope Stage (MIS) 13-15, including two warm periods of interglacial rank (climatostratigraphic units Ferdynandovian 1 and 2) separated by cooling/glaciation (Ferdynandovian 1/2). On the basis of plant macroremains analysis, the type of local vegetation in the lake and its surroundings as well as changes in climate, trophic conditions and water level were reconstructed in detail. Ferdynandovian 1 was a time of development of tall sedge swamps. The presence of Najas marina and N. minor also suggests high levels of eutrophication, particularly in the younger part of the climatic optimum. The occurrence of Zannichellia palustris indicates habitats of variable water level and high salt content. In the terminocratic phase of Ferdynandovian 1, the communities showed the reoccurrence of Betula nana, B. humilis and Larix sp., the disappearance of thermophilous trees, and the intensification of succession processes linked to climate cooling. In the cool Ferdynandovian 1/2, Betula nana and Cenococcum geophilum increased their frequencies, most likely due to enhanced supply of mineral matter to the basin. During Ferdynandovian 2, the next climate warming of interglacial rank, communities of aquatic vegetation with the highest share of thermophilous taxa included the extinct Aldrowanda borysthenica, Brasenia borysthenica, and Scirpus atroviroides, as well as Cyperus glomeratus, a species not presently found in the flora of Poland. Another cooling in the Sanian 2 (Elsterian 2) glaciation is indicated by the development of peat communities, with numerous Carex sp., Menyanthes trifoliata, Eriophorum vaginatum, and Andromeda polifolia, accompanied by the extinct Carex paucifloroides, Caulinia macrosperma, and Potamogeton praemaackianus.

New palynological data from the Ferdynandów site are presented and interpreted against the background of the earlier division of this stratotype pollen sequence by Janczyk-Kopikowa (1975), with special attention to a comparison with the nearest profile of a complete Ferdynandovian succession in Łuków (Łuków Plain). The proposed division of the new pollen diagram from Ferdynandów - into two warm periods of interglacial rank separated by a succession typical of glacial periods - is based on the new division of the Ferdynandovian pollen sequence s.l., applied for the first time by Mamakowa (1996) to the Podgórze B1 pollen profile close to Nowe Miasto on the Pilica river. The two warm units and the cold one between them in the new pollen diagram from the Ferdynandów site correspond to the climatostratigraphic units named Ferdynandovian 1 and 2. Together with the cooling/glaciation (Ferdynandovian 1/2) separating them, the whole Ferdynandovian sequence s.l. can be related to the early Middle Pleistocene Cromerian Complex (Cromerian II Westerhoven and Cromerian III Rosmalen) and Marine Isotope Stages (MIS) 13-15.