NEW STABLE OXYGEN ISOTOPE ANALYSIS ON SINGLE-TAXON CONODONT SAMPLES FROM A SECTION IN DICKEYVILLE, WI PROVIDE NO SUPPORT FOR VOLCANICALLY FORCED CLIMATIC SHIFT IN THE LATE ORDOVICIAN

Oxygen isotopic measurements of conodonts have been used in two recent studies to examine potential climatic effects of large volcanic eruptions during the Late Ordovician. This question is important, first, for testing whether there are geologically resolvable climatic effects of mega-eruptions such as those that resulted in the deposition of the Deicke and Millbrig K-bentonites and, second, for documenting possible relationships between volcanism and long-term climate evolution during the last 10 my of the Ordovician. Most earlier estimates of climate evolution for this interval have relied on qualitative and/or indirect methods of inferring climatic change (e.g. δ¹³C, faunal and facies patterns) or δ¹⁸O analyses of carbonate with largely unconstrained diagenetic overprints. Compared to calcareous material, conodont apatite is considered to have a high preservation potential of the primary oxygen isotope composition because of its more dense microcrystalline structure, and conodont measurements should provide a more robust record of temperature changes than other data types.

We measured δ¹⁸O values on single-taxon samples of two different conodont taxa in eight samples in the Dickeyville road-cut section (Dickeyville, WI) that spans from below the Deicke K- bentonite to above the Elkport K-bentonite, i.e., across the Turinian-Chatfieldian stage boundary. The studied interval at the road-cut section exposes the uppermost part of the Grand Detour Formation, the Quimbys Mill Formation, the Spechts Ferry Formation and the lowermost part of the Guttenburg Formation. Similar to results from Minnesota and Iowa, we found conodonts have δ¹⁸O values of 19.2 to 20.5‰ and, more importantly, see no trends either through the interval or associated with the K-bentonites. That is, the Dickeyville sections samples provide no support for a resolvable climatic effect of the Late Ordovician mega-eruptions at the scale of sampling.