HIGH RESOLUTION CHEMOSTRATIGRAPHY OF THE IREVIKEN CARBON ISOTOPE EXCURSION IN THE ARBUCKLE MOUNTAINS, OKLAHOMA

The middle and late Silurian is characterized by globally recorded positive excursions in marine carbonate carbon isotopes (δ¹³C), known as the Ireviken, Mulde, Lau, and Silurian-Devonian boundary excursions. The cause of these excursions is not fully understood, and is hypothesized to record changes from humid to arid conditions, whereby the resultant changes in ocean water circulation account for the increase in δ¹³C. Our 10 cm sampling of the Late Ordovician Keel Limestone through the Ludlow Henryhouse Formation in the Arbuckle Mountains of south central Oklahoma provides higher resolution data for carbonate δ¹³C and δ¹⁸O than has been previously published. Our data demonstrates that significant, yet smaller fluctuations in δ¹³C and δ¹⁸O are recognized within the Ireviken Excursion. The upper Cochrane records an average δ¹³C value of -0.5 ‰, which we use as a baseline for seawater isotopic composition. The uppermost 20 cm of the Cochrane Formation demonstrates an abrupt δ¹³C shift of +0.5 ‰, which may represent the earliest onset of the Ireviken Excursion. The maximum δ¹³C of the Ireviken Excursion is recorded in the Fitzhugh Member of the Clarita Formation at +4.5 ‰, yet the maximum excursion demonstrates three fluctuations in δ¹³C of ~0.5 ‰ magnitude. An overall trend of decreasing δ¹³C after the Ireviken maximum excursion illustrate multiple minor excursions of 0.5 ‰ – 1.4 ‰ magnitude. Similar trends are observed in δ¹⁸O data over the sampled interval, but are less pronounced than δ¹³C, obtaining a maximum δ¹⁸O of -2.3 ‰ (VPDB). Applying the hypothesized models of paleoceanographic change, the observed small-scale isotopic fluctuations (10’s of cm) suggest a rapid onset of arid climate and outer platform anoxia, followed by a more gradual change to a humid climate and inner platform anoxia that is punctuated by short periods of environmental instability.