SEA LEVEL FORCING OF NEODYMIUM AND CARBON ISOTOPE EXCURSIONS IN EPEIRIC SEAS

Despite observations that δ¹³C 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. Explanations for Paleozoic δ¹³C excursions recorded in epeiric sea carbonates have invoked changes in ocean carbon-cycling driven by changes in ocean circulation. It is assumed that the δ¹³C value of the epeiric sea is inherited from the neighboring ocean, and that the epeiric sea does not cycle carbon independently of the ocean. Nd isotopes in epeiric sea sediments have also been used to monitor the ε_Nd composition of adjacent oceans. However, correlation between ε_Nd excursions and sea level change in Midcontinent Ordovician carbonates demonstrated that the Nd isotope balance in epeiric seawater changes due to local changes in the weathering flux of dissolved Nd from source terranes as sea level fluctuates. As a result, ε_Nd profiles of epeiric sea carbonates may be used as geochemical sea level curves.

Similar trends between sea level fluctuations, determined by lithofacies and ε_Nd profiles, and six positive δ¹³C excursions from carbonates of the Ordovician Galena Group of central Iowa demonstrates that sea level fluctuations may also drive local changes in the δ¹³C composition of an epeiric sea. Repeated episodes of sea level rise and flooding of the Galena carbonate platform by cool, nutrient-rich, eutrophic waters from the Sebree Trough, which was connected to the oxygen minimum zone of the adjacent Iapetus ocean, generated locally increased rates of primary productivity and enhanced organic carbon burial. Supporting this interpretation is the fact that the positive δ¹³C excursions are associated with increased %TOC and hardground omission surfaces.

Interpreting δ¹³C excursions as sea level driven changes in local carbon cycling within an epeiric sea challenges the more conventional view that δ¹³C excursions reflect perturbations of the ocean carbon cycle. The sensitivity of epeiric sea C-cycles to sea level forced perturbations suggests that even globally correlated δ¹³C excursions found in the deposits of different paleo-continents may reflect eustatic sea level forcing of local changes within the C-cycles of multiple epeiric seas.