CHALLENGES TO EARLY-MIDDLE DEVONIAN GR EENHOUSE CLIMATE INTERPRETATIONS: OXYGEN ISOTOPIC EVIDENCE FROM CONODONT APATITE

The Early-Middle Devonian is traditionally characterized by globally warm climates based mainly on latitudinally expanded carbonate and evaporite sedimentation belts, globally high long-term sea levels, and lack of evidence of glacial ice. Given these apparent greenhouse climatic conditions, it is of particular interest to understand the origins of eustatic 3^rd-order (1-5 My) sea-level changes which generated globally correlative transgressive-regressive sequences.

We collected samples from two stacked late Lower Devonian to early Middle Devonian (Emsian-Eifelian) 3^rd-order sequences (30-180 m thick) in Nevada, southern France, and the central Czech Republic for O-isotope analysis of conodont apatite. High-resolution conodont biostratigraphy provides the time control to correlate among these separate localities. Results from the Nevada sequences indicate that decreasing d¹⁸O values correspond to transgressions and increasing and peak d¹⁸O values correlate to regressions and lowstands, respectively; these results clearly support a glacio-eustatic origin for the 3^rd-order sea-level changes. We are presently processing the French and Czech samples to substantiate these glacial interpretations. The isotopic shift between 3^rd-order lowstand and maximum flooding in both depositional sequences is between 1 and 1.5 per mil. Using the Quaternary as a guide, this magnitude of Devonian isotopic shift corresponds to ~100⁺ m sea-level changes and subtropical seawater temperature changes of 1-2°C over time spans of 1-2 My. If our interpretations of these significant glacio-eustatic sea-level and seawater temperature changes are correct, this challenges the concept of an Early-Middle Devonian greenhouse and suggests that considerable continental ice existed in Gondwana and fluctuated in volume through time.