Orbital-scale glacio-eustasy in the Middle Devonian detected using oxygen isotopes of conodont apatite: Implications for long-term greenhouse–icehouse climatic transitions

Abstract

Results from previous paleoclimate studies indicate Gondwanan glaciation, marking the onset of middle Paleozoic icehouse climate conditions, began by the Latest Devonian (Famennian). More recently, δ18O trends from conodont apatite suggest cooling Middle Devonian subtropical seasurface temperatures >20My prior to Famennian glaciation and high-latitude temperatures cool enough to permit the growth of continental glaciers which controlled longer term (My-scale) glacio-eustasy. This study evaluates the details and patterns of Middle Devonian high-frequency (<100ky) climate variability that are superimposed upon these longer term cooling trends and evaluates the origins of common and globally widespread carbonate cycles (or parasequences).Thirteen complete Middle Devonian (Givetian) subtidal cycles (0.5–3m thick) in the Little Cedar Formation of southeastern Iowa are characterized by oxygenated offshore facies (transgression), poorly oxygenated offshore facies (maximum flooding), a return to oxygenated facies, and are capped by bioturbated lower shoreface facies (regression/lowstand) which lack evidence of subaerial exposure. Across-shelf cyclostratigraphic facies relationships suggest relatively subdued water-depth changes were responsible for cycle development. Measured δ18O values from conodont apatite range from 15.9‰ to 18.4‰ and repeated, systematic intracycle trends are dominated by lower isotopic values in offshore facies and higher isotopic values in cycle-capping lower shoreface facies, which is consistent with glacio-eustasy driving the water-depth changes. The average ~0.6‰ magnitude of intracycle isotopic shift (0.2‰ to 1.0‰=total range) suggests ~15–23m glacio-eustatic oscillations were responsible for observed water-depth changes. The higher range of intracycle isotopic magnitudes may be the result of increased subtropical evaporation rates during glacial stages which locally enriched surface seawater δ18O values.Orbital-scale (<100ky) glacio-eustasy superimposed on Middle Devonian cooling trends ~20My prior to Late Devonian glaciation highlights the difficulties in attempting to pinpoint the specific timing of the middle Paleozoic greenhouse-to-icehouse climate transition and calls in to question the traditional delineations of greenhouse versus icehouse conditions.