Conodont apatite δ18O values from a platform margin setting, Oklahoma, USA: Implications for initiation of Late Ordovician icehouse conditions

The uppermost Sandbian–lower Katian stratigraphic record documents a critical time period in Earth history. Lithological evidence and geochemical proxy records suggest that the uppermost Sandbian–lower Katian interval could represent a transitional phase from a long Cambrian–Ordovician greenhouse world to the Late Ordovician icehouse world. To examine this interval of Earthʹs history, oxygen isotopes of conodont apatite from the Bromide and Viola Springs Formations in southern Oklahoma were analyzed in order to provide sea-surface temperature estimates at a location that had little influence by terrigenous sedimentation. Our results are inconsistent with 1) previous suggestions that global cooling occurred across a prominent sequence boundary close to the Sandbian–Katian boundary across North America and 2) the hypothesis that there is a direct connection between volcanic mega eruption(s) and a significant long term impact on climate during the Sandbian–Katian of Laurentia. Furthermore, δ18O values spanning the Guttenberg Carbon Isotope Excursion (GICE), an interval that has been interpreted as evidence for an early onset of the Late Ordovician glaciation, are variable. A + 1.5‰ δ18O shift is observed in the lower GICE that is followed by a return to δ18O values that fluctuate around a mean of 19.0‰ in the upper GICE. Together, these observations suggest a short-lived cooling event within the lower GICE. This cooling event may represent enhanced carbon burial and drawdown of atmospheric pCO2 followed by a return to more stable low-latitude temperatures in the upper GICE.