Re-evaluating origins of Paleozoic orbital-scale and My-scale stratigraphic cyclicity using oxygen isotopes of marine apatite

High-frequency (10⁴-10⁵ yr) sedimentary cycles and My-scale depositional sequences in Paleozoic marine strata have been studied for over a century and though debated, their origins are most commonly attributed to sea-level changes. Early studies focused mainly on repetitive shallowing and deepening facies changes, subaerial exposure features, and widespread correlations to argue for eustatic drivers. Subsequent studies utilized 1D and 2D computer models and statistical and time series analysis to argue for eustasy and for Milankovitch-scale periodicities. With increasing high-resolution numeric age control provided by newly discovered ash beds, the durations of many Paleozoic cycles and sequences are found to lie within the Milankovitch-frequency band. Recently the origins of Paleozoic cycles and sequences have been evaluated using oxygen isotopes from marine apatite (conodonts) to specifically test for glacio-eustatic origins. Isotopic trends from well studied Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian marine successions support the hypothesis that the cycles and sequences were generated by glacio-eustasy with decreasing and low isotopic values occurring within deepening and deepest water facies and increasing and high values occurring in shallowing and shallowest water facies. Of particular interest is that the magnitudes of isotopic change and by inference, the magnitude of climatic change, observed across cycles and sequences developed in Paleozoic greenhouse time intervals (Silurian, Devonian) are as large as those observed in icehouse (Neogene, Pennsylvanian) and transitional (Late Ordovician, Early Mississippian) climatic intervals. These oxygen isotope results combined with earlier stratigraphic, modeling, and statistical studies suggest that short- and long-period Milankovitch-forced glacio-eustasy controlled cycle and sequence development throughout the Paleozoic.