Orbital-scale climate changes detected in Lower and Middle Ordovician cyclic limestones using oxygen isotopes of conodont apatite

Abstract

Recent studies using δ18O trends from conodont apatite suggest that the Ordovician greenhouse-to-icehouse climate transition was characterized by repeated Myr to multi-Myr-scale sea surface temperature (SST) oscillations throughout the Ordovician making it difficult to pinpoint the temporal onset of cooling and glaciation. This study utilizes high-resolution δ18Oapatite trends from Lower and Middle Ordovician meter-scale carbonate cycles to evaluate orbital-scale (~104–105 yr) climate changes superimposed on the Myr-scale oscillations. Detailed facies analysis and δ18Oapatite variations from 14 cycles deposited along the western Laurentian margin (Nevada and Utah) show trends indicating glacio- and/or thermo-eustatic sea-level changes along with associated wetter versus drier climate shifts controlled meter-scale cycle formation and δ18O signals. Using the range of magnitudes of intracycle isotopic shifts (0.45–1.4‰) and reasonable isotopic proportionalities, ~25–45 m glacio-eustatic changes with ~2–6 °C SST variations are estimated. However, observed cyclostratigraphic facies changes do not support the higher end of estimated eustatic changes suggesting that the apparent overestimation is due to local δ18O shifts related to variations in paleotropical surface ocean evaporation and/or dilution by freshwater runoff during orbital-scale wetter versus drier climatic changes. These new δ18Oapatite results indicate that orbital-scale climatic oscillations, including glaciation, were superimposed upon Myr-scale and ~107 yr-scale Ordovician SST variations as early as the late Early Ordovician and that the greenhouse-to-icehouse climatic transition was more complex and protracted than previously thought.