Tropical cyclothems suggest obliquity control on early glaciation of the Late Paleozoic Ice Age (late Mississippian, Upper Silesia)

Abstract

The Late Paleozoic Ice Age (LPIA; ∼355–255 Ma) represents an internally complex interval of icehouse conditions, comprising a series of discrete, Myr-scale episodes of cryosphere expansion and contraction. A short-term variability on astronomical time scales has been identified in many studies, but the detailed structure of these changes and their interaction with Myr-scale cryosphere fluctuations is not fully understood. Here, we examine tropical cyclothems, which formed in the eastern part of the Variscan foreland (Poruba Member of the Upper Silesian Basin) and may represent a far-field record of glacioeustasy during an early phase of Gondwanan glaciation. A combination of new and published radioisotopic U-Pb zircon ages constrains the timing of Poruba cyclothems to 325.2–323.4 Ma, between Myr-scale phases of glacial expansion identified as C1 and C2 in eastern Australia. Spectral estimates of lithofacies profiles suggest that sea-level variations were paced predominantly by the 173-kyr cycle of obliquity modulation, with smaller scale changes in the ∼35-kyr obliquity band. Short-eccentricity signature is localized to the vicinity of source area, possibly reflecting upstream changes in sediment input. In spite of nonglacial conditions in eastern Australia, the results suggest that dynamic ice sheets reacting to integrated summer insolation likely formed in the polar region. The Upper Silesian cyclothems further suggest that a Myr-scale deglaciation in peripheral glacial centers (termination of C1 in eastern Australia) was associated with an increase in the frequency of ice-sheet variability. This change can be considered inversely analogous to the mid-Pleistocene transition, but its frequency structure differs, pointing to distinct insolation control and feedback mechanisms during LPIA.