The widely held view that Pennsylvanian cyclothems formed in response to Milankovitch-controlled, glacio-eustatic, sea-level oscillations lacks unambiguous quantitative support and is challenged by models that are based on climate-controlled precipitation-driven changes in depositional style. This study shows that cyclothem successions do in fact contain a clear record of Milankovitch-controlled oscillating sea level, but that it is prerequisite that besides cyclothem thickness, cyclothem composition is taken into account. A simple subdivision of cyclothems into subaqueous and subaerial facies is sufficient to reveal the signal, provided that sufficiently long and complete successions are studied.
Two Duckmantian–Bolsovian (Westphalian B–C) successions were studied—one from a high-accommodation setting in the Netherlands and another from a medium-accommodation setting in Kentucky in the United States. The Dutch record comprises an exceptional, 1728-m-long, continuously cored interval, and it shows a distinct twofold cyclicity in the subaerial-facies ratio of subsequent cyclothems at wavelengths of ∼256 m and ∼59 m, which is confirmed by power-spectral analysis. The signal is not present in the Kentucky succession due to subsidence-controlled low preservation of only one out of three to four cyclothems, and that explains why many cyclothem studies have yielded inconclusive results.
Recent U/Pb ages indicate that the 256 m cycle represents ∼395 k.y., which matches with long eccentricity (413 k.y.). This then gives a 95 k.y. duration for the 59 m cycle (short eccentricity). Individual cyclothems in the high-accommodation Dutch succession are mostly between 5 and 35 m thick, which points to a sub-eccentricity duration (mean 21 k.y.). The highly variable thickness may be due to interference of precession-, obliquity-, and eccentricity-driven sea-level fluctuations or alternatively to autocyclic or climate-controlled variations in sediment supply. Integration of the results with U/Pb calibrated radiometric ages for “tonstein” ash layers from North America and Europe allowed refinements of the chronology of the main Westphalian (Moskovian–Bashkirian) coal interval; these refinements are consistent across Euramerica.
An analysis of cumulative coal-bed thickness further indicates that terrestrial-carbon (coal) storage patterns are comparable in the two remote areas: in the Netherlands ∼5 m coal per m.y. during the Langsettian (Westphalian A) and increasing abruptly to ∼20 m/m.y. at the start of the Duckmantian substage (Westphalian B). In Kentucky, storage rates were lower, but when standardized to Dutch subsidence, the pattern is identical. This suggests that burial of terrestrial carbon during the Late Paleozoic Ice Age was globally controlled and possibly very predictable.
- Received 13 February 2015.
- Revision received 7 April 2015.
- Accepted 20 May 2015.
- © 2015 Geological Society of America