GLACIO-EUSTATIC CONTROLS ON PALEOZOIC STRATIGRAPHIC CYCLICITY EVALUATED USING OXYGEN ISOTOPES OF MARINE APATITE

High-frequency (10⁴-10⁵ yr) sedimentary cycles and My-scale depositional sequences have been recognized in the Paleozoic marine record for over a century and are most commonly attributed to sea-level changes. Early studies focused on repeated shallowing and deepening facies changes, subaerial exposure features, and widespread lateral extent to argue for eustasy. Subsequent studies utilized computer modeling, statistical and time series analysis to also argue for eustatic drivers and for orbital-scale periodicities. Continued refinement of the numeric time scale using newly discovered ash beds indicate that the durations of many of the Paleozoic cycles and sequences lie within Milankovitch-band frequencies.

Recently the origins of common Paleozoic cycles and sequences have been evaluated using oxygen isotopes from marine conodont apatite to test for glacio-eustatic origins. Oxygen isotopic trends from cyclic successions accumulating during greenhouse (Silurian, Devonian), icehouse (Pennsylvanian), and transitional (early Late Ordovician, Early Mississippian) climate modes support the hypothesis that targeted cycles and sequences were generated by glacio-eustasy. Decreasing and low oxygen isotope values occur within deepening and deepest water facies, whereas increasing and high isotopic values occur within shallowing and shallowest water facies. The magnitudes of isotopic change and by implication, the magnitude of climatic change observed across cycles and sequences developed in greenhouse intervals are similar to those recorded in icehouse and transitional climates and lie within the range of isotopic shift reported for Neogene glacial-interglacial stages. We suggest these large magnitudes of Paleozoic isotopic shift, particularly for the high-frequency cycles include the combined effects of changes in ice volume, seawater temperature, and potentially increased evaporation rates during drier/windier glacial stages. These oxygen isotope results combined with earlier stratigraphic, modeling, and statistical studies strongly support short (<400 ky) and long-period (~1-2.5 My) Milankovitch-forced glacio-eustasy controlled cycle and sequence development throughout the Paleozoic.