High-resolution environmental changes recorded in ostracod and mollusc fauna from the Holsteinian palaeolake at Ortel Królewski II, eastern Poland

To better understand the environmental variability during the Holsteinian interglacial, fauna of the palaeolake deposits at Ortel Królewski II, eastern Poland was investigated. The analysis covered the uppermost part of the profile, representing the so-called pre-optimal part of the Holsteinian interglacial (MIS 11c), namely the Taxus and Pinus-Larix pollen zones. Variability within the structure and composition of the faunal assemblage enabled a high-resolution palaeoecological reconstruction – lake level and temperature fluctuations, its trophy and energy of the environment. During the Taxus pollen zone the lake was relatively deep and characterized by medium trophy conditions, which is indicated by a low abundance of molluscs of temporary water bodies and low numbers of Metacypris cordata and Scottia tumida. The temperature was rather stable with only small fluctuations during the transition between the Taxus and Pinus-Larix zones. The occurrence of some cold-loving ostracod species within the beginning of the Pinus-Larix pollen zone indicates some deterioration of thermal conditions, which was additionally confirmed by using the mutual ostracod temperature range (MOTR) method. Lake level drop and expansion of reed zones within the Pinus-Larix pollen zone are inferred from the increased abundance of shallow water ostracod and mollusc species as well as from an increased Bithynia-index (BIN). The short cooling event documented early in the Pinus-Larix zone in the studied profile might be an equivalent to the mid-MIS 11c cooling, the so-called Older Holsteinian Oscillation. Thus, the improved knowledge about the climate dynamics recorded within the sediments from this site contributes to a better understanding of the spatial extent and pattern of the climatic changes within the Holsteinian interglacial across Europe.