Palaeosol structures and isotopic records of sulfates and carbonate horizons in Neogene Poznań clays of the Polish Lowlands, western Poland

Abstract

This study examines two profiles of Neogene fossil soils within the Poznań clays formation in the Polish Lowlands, where well-developed carbonate and sulfate nodules are present. The clay fraction in these profiles indicates a mineral composition consistent with Vertisols, shaped by fluctuations in temperature and water availability that promoted the formation of carbonate nodules and gypsum crystallisation. Chemical analysis further elucidates the dominant hydrologic regime during palaeosol formation. Stable isotope data from gypsum horizons and carbonate concretions were crucial for characterising the sedimentary environments.

The δ³⁴S and δ¹⁸O values in gypsum ranged from + 1.1 to + 6.1 ‰ and − 6.0 to − 3.4 ‰, respectively, suggesting formation through sulfide oxidation, likely from pyrite. These sulfur and oxygen isotopic signatures indicate continental conditions with an arid to semi-arid climate during palaeosol development, along with gypsum recrystallisation and intercalations of clay minerals and organic matter.

Carbon isotopic composition (δ¹³C) ranges from − 37.7 to − 16.6 ‰, while oxygen isotopic composition (δ¹⁸O) ranges from − 6.1 to − 3.3 ‰, reflecting a combination of pedogenic and post-sedimentary processes. The δ¹³C values of pedogenic carbonates (PCs) (− 27.3 to − 23.3 ‰) suggest that the primary carbon source is likely the biomineralisation of C3 plants remains, with C3 photosynthesis contributing to carbonate precipitation in the topsoil. The minimal δ¹⁸O variability (approximately 1 ‰) in these carbonates may correspond to short-term environmental fluctuations. Additionally, some carbonates exhibit isotopic patterns indicative of modification by vegetation and contemporary weathering processes.