Stratigraphic and oxygen isotope evidence for My-scale glaciation driving eustasy in the Early-Middle Devonian greenhouse world

Million year-scale (3rd-order) depositional sequences (few 10s to similar to 100 m-thick) are common in the Devonian marine record and are correlated between sedimentary basins and across widely separated continents implying a eustatic origin. Tectonically driven changes in mid-ocean ridge spreading rates/lengths are too slow to account for the calculated 3rd-order rise/fall rates and typical orbital frequencies (similar to 20-400 ky) are too fast. We analyzed oxygen isotopes from conodont apatite to evaluate if Early-Middle Devonian 3rd-order sea-level changes were controlled by paleoclimatically driven ice volume (glacio-eustasy) and seawater temperature change (thermo-eustasy). Two successive depositional sequences in the western U.S. (Nevada) and Czech Republic (Prague Basin) were sampled. The delta O-18 values range from 17.8 parts per thousand to 20.2 parts per thousand (SMOW) and show similar trends across the Emsian-Eifelian boundary with increasing and peak values in the lower costatus, decreasing values in the mid costatus, and increasing values in the upper costatus to australis zones. The similarities between these widely separated localities indicate that the isotopic trends are global rather than controlled by local variations in seawater temperature, evaporation/freshwater influx, or diagenesis. In the Nevada section, where the sea-level history is the most complete and best understood, the isotopic values increase and peak in late highstand and lowstand system tracts and decrease to the lowest values in the transgressive and maximum flooding intervals suggesting the My-scale sea-level changes were paleoclimatically controlled. The magnitude of isotopic shift from lowstand to maximum flooding intervals ranges from 0.8 parts per thousand to 1.5 parts per thousand and occurred over time spans of <1.7 My The only known mechanism to explain such large and rapid isotopic shifts is glacio-eustasy. If a reasonable range of My-scale subtropical sea surface temperature changes are assumed (similar to 1-3 degrees C), then this requires glacio-eustatic sea-level changes of >35 m to explain the remaining isotopic shifts. These amplitudes imply that significant continental ice sheets grew and melted on the <1.7 My time scales during the Early-Middle Devonian and, like recent reports for the mid-Cretaceous and early Eocene, challenge previous assumptions of ice-free greenhouse climates. A plausible climate driver for these My-scale paleoclimate changes is longperiod eccentricity (similar to 2.4 My) and obliquity (similar to 1.2 My) variations. (C) 2009 Elsevier B.V. All rights reserved.