Transient Climate Events at the Cenomanian/Turonian Boundary (OAE2)

The recent discoveries of large, brief negative carbon isotope excursions associated with the long-term Mesozoic carbon burial events requires a more detailed examination of the nature and cause(s) of these events. Negative carbon isotope excursions immediately preceding OAEs have been reported for the Toarcian (Early Jurassic) and OAE1a (Aptian) events. It is hypothesized that catastrophic releases of carbon (in the form of methane) may have led to the anoxic conditions. Stable isotope records from the Cenomanian-Turonian (Late Cretaceous) boundary Ocean Anoxic Event (OAE2) from New Jersey show two short-lived (~10 k.y. each), large (>5 per mil), negative carbon isotope excursions. The duration of the transient events and of OAE2 is constrained by cyclostratigraphy developed using downhole gamma logs and confirmed by high-resolution stable isotope records. The transient events in the carbon isotope records occurred after an initial phase of carbon burial. Methane release(s) from gas hydrates is the only known mechanism that can explain the magnitude and rapidity of the carbon isotope excursions. No precursor carbon isotope decrease is associated with OAE2, unlike the Toarcian (Early Jurassic) and early Aptian (Early Cretaceous) OAEs. Therefore, "methane burps" did not trigger OAE2; rather, anoxia preconditioned the ocean for methane accumulation and release. The New Jersey section also shows that the transient release occurred after a 4 to 5^oC cooling, in contrast to hypotheses that invoke deep water warming as the causal mechanism for thermal dissociation of methane hydrate. Abrupt 3.5% oxygen isotope decreases coincide with the carbon excursions at New Jersey and record transient greenhouse warming events, indicating that methane escaping into the atmosphere resulted in greenhouse warming.