A LATE ORDOVICIAN, CONODONT-BASED δ18O CURVE

The Late Ordovician mass extinction is the second most severe Phanerozoic event in terms of percent extinction and is associated with evidence implicating a short lived glaciation as a potential cause. However, studies have also suggested that cooling may have begun in the Middle Ordovician and/or that there was an interval of Late Ordovician warming prior to the glaciation. Discriminating among these hypotheses is difficult due to the lack of precision of climate estimates based on faunal and sedimentological data and because of diagenetic concerns regarding δ¹⁸O analyses of Ordovician carbonates.

To address this problem, we analyzed the δ¹⁸O values of the conodonts Drepanoistodus suberectus and Phragmodus undatus from four sections that together span most of the Cincinnatian Series. Specimens are well preserved and both taxa yielded values between 17‰ and 18.3‰. These vales suggest paleotemperatures between 28° and 34°C assuming an “ice-free” seawater δ¹⁸O value of -1‰_V-SMOW. D. suberectus results exhibited considerable variation through the section which is attributed to the fact that as few as 6 elements were analyzed in some samples introducing possibly large sampling biases. The curve for P. undatus, though, is based on tens of elements per sample and seems to show considerable stratigraphic coherence. In previous work we found Mohawkian values for P. undatus of at least 18‰, but Edenian and early Maysville values were consistently between 17 and 17.5‰_V-SMOW. These low values are followed by a sharp, ~1‰ increase in the middle Maysville at ~1150 composite standard units (csu). Values remain near 18‰ into the Richmondian followed by a sharp ~1‰ decrease at ~1220 csu at a level near the base of the Hirnantian (1232 csu). If these results reflect only temperature, they indicate a 4-5°C cooling step in the middle of the Maysville and, surprisingly, ~5°C of warming in the Richmondian. Facies changes and an increase in abundance of Plectodina tenuis above 1200 csu suggest apparent Richmondian warming could result from a local seawater δ¹⁸O effect related to Late Ordovician regression. Regardless, the conodont δ¹⁸O data support neither a simple Hirnantian cooling step nor progressive Late Ordovician cooling; they do indicate conodont isotopic analyses have great potential to advance paleoclimatic studies of the early Paleozoic.