INCREASED REMOVAL OR DECREASED DELIVERY OF 12C: INTERPRETING THE LOWER SHEINWOODIAN (SILURIAN) POSITIVE CARBON ISOTOPE EXCURSION FROM THE MIDCONTINENT

The Lower Sheinwoodian positive carbon isotope excursion began at or near the top of the Pterospathodus amorphognathoides conodont biozone (herein referred to as the Ireviken Excursion). This excursion is recorded in three previously unpublished sections from Tennessee, Ohio and Iowa. Carbon isotope values begin at +2.5 per mil at the base of the Maddox Member of the Wayne Formation in central Tennessee and increase to +4.9 and decrease back to a baseline of +1 halfway through the Maddox Member. In a core from Eastern Iowa, carbon isotope values begin at +1 per mil at the base of the Scotch Grove Formation. The Ireviken Excursion is recorded in a rapid shift to +4.5 per mil with heavy values continuing into the Gower Formation. Two quarries from Western Ohio provide a composite section with values beginning at +2.3 but only reaching a high of +3.42 in what may be erroneously labeled as the Dayton Formation. In each of the three localities, a pronounced unconformity marks the base of the excursion, however detailed sampling in Iowa and Tennessee has shown the entirety of the excursion was most likely recorded but is stratigraphically very thin. The beginning of the excursion is marked by a clastic or mud rich interval which grades into more carbonaceous lithologies with increasing carbonate grain size as the excursion progresses. This lithologic characteristic has been described at or near the top of the P. amorphognathoides zone in many localities in North America. The Ireviken Excursion occurred during a time of oligotrophic conditions, relatively high sea level, and decreased black shale deposition on continental shelves and epicontinental seas.