Sequestration of 12C in the deep ocean during the early Wenlock (Silurian) positive carbon isotope excursion

Marine carbonates from three biostratigraphically well-dated Silurian sections from the mid-continent of North America were analyzed for carbon isotopic (δ13Ccarb) study. The early Wenlock positive carbon isotope excursion (Ireviken Excursion) is recorded in three sections from Tennessee, Iowa, and Ohio, with values of >+4.5‰. This global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event (Ireviken Event) that spanned the Llandovery–Wenlock boundary. Although several oceanographic models have been developed to account for the coincident changes in lithology, biology, and carbon isotope stratigraphy during this interval of the Silurian, their proposed causal connections among glaciation, oceanography, primary productivity, and the global carbon cycle remain a matter of debate. This investigation provides carbon isotopic and lithologic evidence that the Ireviken Excursion occurred during a time of relatively oligotrophic conditions, a third order transgressive–regressive cycle, and increased carbonate production throughout the mid-continent of North America. The Ireviken Excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections. It is proposed that the Ireviken Excursion was the product of sequestration and burial of 12C in deep water as a result of altered deep ocean circulation.