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