| Abstract: | The Cenomanian-Turonian Boundary Oceanic Anoxic Event (OAE2) led to deposition of organic-rich sediments on a global scale. However, the origin of this event is still unclear. Recently, the OAE has been linked to methane hydrate dissociation based on large and abrupt negative carbon isotope excursions (-5 to -8 per mil) in the  record of calcium carbonate from the Bass River borehole in the New Jersey coastal plain (ODP 174AX). The dissociation of large volumes of methane hydrate in continental margin sediments could have depleted oxygen in the deep ocean and caused the OAE.
Three sections were sampled at high resolution (1-3 cm) to test the methane hydrate dissociation hypothesis. Two localities are in the Western Interior Basin (Cuba, Kansas and Pueblo, Colorado) and the other locality is the Bass River borehole. Carbon isotopes were analyzed on the organic carbon and carbonate fractions of sediments. Methane hydrate dissociation should result in an abrupt negative carbon isotope excursion in both the organic carbon and calcium carbonate fractions.
Almost no change in the values of the organic fraction coincide with those in carbonate suggesting that methane hydrate dissociation was not responsible for initiating the Cenomanian-Turonian Boundary OAE. Diagenesis is likely responsible for the large and abrupt negative excursion observed in calcium carbonate. More specifically, contemporaneous recrystallization of calcium carbonate and oxidation of organic matter appears to have caused the extremely low carbonate carbon isotope values. |