Investigating the Timing of Late Ordovician Cooling and Glaciation Using Oxygen Isotopes of Conodont Apatite

The Late Ordovician glacial episode is thought to have occurred during a time of elevated atmospheric CO₂ levels and it marked the end of a prolonged (>100 My) greenhouse climate mode. Interpretations on when Late Ordovician cooling/glaciation began and how long it lasted range from an abrupt initiation in the Hirnantian stage and lasting perhaps <1 My, to a protracted glacial episode with cooling/glaciation occurring up to ~14 My before the Hirnantian. We are testing the hypothesis that the glacial episode began at least several millions of years before the Hirnantian by analyzing oxygen isotopes from conodonts apatite within Richmondian carbonates. Similar to oxygen isotope analyses of Cenozoic calcareous foraminifera, conodont apatite can provide a high-resolution record of climatically driven changes in seawater temperature and ice-volume.

Upper Ordovician (Richmondian) carbonates of central Nevada (Hanson Creek Fm) and southern New Mexico (lower Montoya Group) are characterized by at least two ~30-100 m-thick depositional sequences (~1-3 My). The Hanson Creek sequences accumulated within predominantly deep ramp environments, while the Montoya sequences represent deep ramp through shallow ramp environments. Conodonts were collected within transgressive, maximum flooding, highstand, and lowstand systems tracts to evaluate intra-sequence isotopic trends as well as longer-term trends in oxygen isotope values leading into the Hirnantian glaciation.