Palaeoenvironmental reconstruction of the Konin Basin (central Poland) during lignite accumulation linked to the mid-Miocene climate optimum

Abstract

We reconstruct palaeoenvironmental conditions during the formation of the first Mid-Polish Lignite Seam (MPLS-1) of the Konin Basin of central Poland, which is of mid-Miocene age. In terms of macropetrography, the MPLS-1 comprises different lithotype associations, predominantly xylodetritic lignite, and represents the peat deposits of mires most likely inhabited by Myricaceae-Cyrillaceae shrubs. Palynological assemblages obtained from the MPLS-1 at the Adamów Lignite Opencast Mine indicate the existence of wetland/palustrine and mesophytic vegetation during the accumulation of the peat. Applying the Coexistence Approach to the palynological assemblages, climate is inferred to have been warm temperate and humid with a mean annual temperature (MAT) of 15.7–18.0 °C with plant communities similar to modern pocosins developed on coastal plains in southeastern USA. The palynological assemblages from the MPLS-1 at Jóźwin IIB Lignite Opencast Mine show palaeofloristic cyclicality. In the studied section, five cycles were identified, dominated by swamp forests (Taxodium/Glyptostrobus–Nyssa) similar to modern forests in southeastern USA and China. The climate at that time was warm temperate and humid (MAT = 15.7–20.5 °C) based on CA application. This is consistent with the significant contribution of warm temperate plants, typical of mixed mesophilous forests, which grew in the vicinity of the swamp forests. The results of geochemical analyses are generally in agreement with palaeobotanical studies, indicating angiosperm dominated the peat-forming vegetation. However, the geochemically-derived average temperature ranges of ca. 24.2–25.0 °C, based on the carbon preference index (CPI), is significantly higher. These differences between palynologically- and geochemically-derived MATs can be explained by the influence of changes in vegetation on the CPI values. The periodic contribution of algae and microorganisms to the biomass while the water table was raised is indicated by elevated abundances of short-chain n-FAs. In contrast, under acidic conditions, bacterial activity was limited, as indicated by ββ/(ββ + αβ) hopane ratios and low sulfur content. Based on sedimentological data, the MPLS-1 was mainly formed either proximal (Jóźwin IIB, Tomisławice) or more distal (Adamów) to the river channels as evidenced by the presence or absence of siliciclastic partings within the MPLS-1. In summary, the MPLS-1 accumulated mostly as low-lying mires under freshwater conditions, corresponding to the last peak of the Mid-Miocene Climatic Optimum and to the period thereafter when the climate showed a cooling trend. Our findings are placed in the context of palaeotectonics and palaeoclimates at regional and global scales.