EARLY TO MIDDLE ORDOVICIAN HIGH-FREQUENCY (104-105 YR) CLIMATE CHANGES DETECTED USING OXYGEN ISOTOPES OF MARINE APATITE FROM CYCLIC MARINE CARBONATES

The Ordovician greenhouse to icehouse climatic transition occurs in three general phases: 1) warm, but unidirectional cooling during the Early Ordovician, 2) sustained moderate temperatures in the Middle through mid Late Ordovician, followed by 3) abrupt cooling in the latest Ordovician. The onset of Gondwanan glaciation began by the Middle Ordovician recognized, in part, by clear records of stratigraphic cyclicity driven by glacio-eustasy.

This study focuses on the origin of meter-scale, high-frequency (10⁴-10⁵ yr) stratigraphic cyclicity developed during the warm and moderate climatic intervals of the Early (Floian) and Middle Ordovician (Darriwillian). Three different cyclic carbonate successions in western Utah were sampled for oxygen isotope analysis of conodont apatite to evaluate the origins of the cyclic facies changes.

Measured oxygen isotope values range from ~15‰ to 18.5‰ SMOW and overlap, in part, with previously reported conodont apatite isotopic values from other global locations. Of the 13 cycles analyzed, 70% record lower isotopic values in the deeper water facies (cycle base or mid cycle) and higher values in the shallower water facies (transgressive cycle base or cycle cap) consistent with water-depth changes being controlled by glacio- or thermo-eustasy. The magnitude of intracycle isotopic change in these cycles ranges from ca. 0.5‰ to 1.2‰. The remaining 30% of cycles record the opposite trends with higher isotopic values at the base/mid cycle and lower values at the cycle top. Results from the dominant intracycle isotopic trends suggest that even during the warm and moderate climatic phases, high-frequency climate changes driving glacio/thermo-eustasy were influencing subtropical marine depositional systems.