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A large freshwater lake formed (lake area > 5000 km<sup>2</sup>) during the Stephanian B (˜304 My) period of the Late Pennsylvanian in the central-east equatorial Pangaea and covered a substantial part of the continental Late Palaeozoic basins in the central Bohemian Massif (the Czech Republic). Lacustrine mudstones of the Mšec Member were acquired from two shallow boreholes 80 km apart and were analysed for lamination structure and periodicity, mineral composition and dispersed organic matter, in order to obtain more detailed information on the palaeoenvironmental change with a near annual resolution. Two hydrological states of the lake were identified, including high lake-level periods with dysoxic conditions on the lake floor, under a permanently thermally stratified hypolimnion favouring organic matter production and storage during periods of condensed sedimentation producing an irregular lamination pattern. A series of continuous regular couplet lamination reflect conditions of partially, possibly seasonally stratified hypolimnion with advanced lake-water mixing and organic matter oxidation. The couplet lamination results from an alternation of organic-clay laminae, consisting of variable proportions of autochthonous algal and microbial organic remnants, with detrital silt laminae accompanied by authigenic siderite and humified or burnt terrestrial plant debris. Similarly as in some modern tropical lakes, such regular high-frequency variations in sediment supply suggest seasonal changes in rainfall to be responsible for couplet formation. Couplet thickness distribution shows statistically significant periodicities, resembling multiannual to multidecadal scales observed in recent sediment archives, and so indirectly supports seasonality in precipitation in Pangaean low-latitudes, at least during part of the Stephanian B

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Lacustrine couplet-lamination: evidence for Late Pennsylvanian seasonality in central equatorial Pangaea (Stephanian B, Mšec Member, Central and Western Bohemian basins)