SEA LEVEL FORCING OF LATE ORDOVICIAN CARBON ISOTOPE EXCURSIONS

Despite observations that carbon isotope excursions in Paleozoic marine carbonates are often coincident with facies transitions, the influence of sea level change on the carbon isotope budget of epeiric seas is not well understood. Most explanations for Paleozoic carbon isotope excursions have invoked changes in ocean carbon-cycling driven by changes in ocean circulation. In these studies, it is assumed that the carbon isotope value of the epeiric sea is inherited from the neighboring ocean, and that the epeiric sea does not cycle carbon independently of the ocean.

We present inorganic and organic carbon isotope, and epsilon Nd profiles for Late Ordovician Galena Group carbonates of the central Midcontinent region, Iowa, which show four covarying isotope excursions that correlate with sea level. The epsilon Nd variations record the submergence history of the craton during the studied interval, providing an additional proxy for sea level. We report one positive carbon isotope excursion in the Decorah Formation, which has been observed by others, and four additional positive carbon isotope excursions, three in the Dunleith Formation, and one in the Wise Lake Formation, that are reported for the first time. Since it has been shown that the Midcontinent contained several water masses with distinct carbon isotope signatures, we suggest that sea level change is not just correlated with the carbon isotope excursions, but may be forcing changes in carbon isotope values as a result of increased seawater exchange between water masses of the central Midcontinent in Iowa, and neighboring water masses that were relatively enriched in ¹³C.