Orbital-scale climate change and glacioeustasy during the early Late Ordovician (pre-Hirnantian) determined from delta O-18 values in marine apatite

This study focuses on the similar to 10 m.y. before the latest Ordovician (Hirnantian) glaciation; we test whether orbital-scale climatic fluctuations controlled the growth and melting of continental glaciers, resulting in glacioeustatic sea-level changes and the development of widespread marine sedimentary cycles. delta O-18 values of conodont apatite from 14 Late Ordovician (Katian) cycles range from similar to 17% to 21%. Isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow-water facies, supporting the hypothesis that glacioeustasy was the dominant control on water-depth changes. Measured intracycle delta O-18 changes of 0.7%-2.5% were controlled by changes in ice volume (<60 m sea-level changes), sea-surface temperatures (<5 degrees C), and potentially local increases in seawater evaporation during drier and/or windier glacial stages. These interpreted orbital-scale climate changes and resultant large glacial ice-volume changes support recent interpretations of a dynamic and prolonged Ordovician greenhouse to icehouse transition.