Data-model comparison reveals key environmental changes leading to Cenomanian-Turonian Oceanic Anoxic Event 2

Abstract

The middle Cretaceous was a period characterized by elevated sea-floor spreading rates, enhanced volcanism, high atmospheric CO2 levels, warming temperatures, and the peak eustatic highstand of the Mesozoic. Two well-known perturbations in the global carbon cycle mark this interval – the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) and the Mid-Cenomanian Event (MCE). Although studies during the past two decades have arrived at a consensus that the Caribbean Large Igneous Province (LIP) likely played a key role in triggering OAE2, arguably the most significant perturbation of the Late Cretaceous, the detailed environmental developments during the Mid-Late Cenomanian leading up to it have only recently been the focus of investigations. This study, based on previous studies of the Middle Cenomanian – Early Turonian climate, tectonics, sea level, and carbon isotope chemostratigraphy, tests plausible environmental scenarios to explain the behavior of the Middle Cenomanian to Early Turonian carbon cycle, via isotope-mass balance calculation in a simple carbon cycle box model. The model experiments successfully reproduce two distinctive features observed in the Mid-Late Cenomanian δ13C curves - 1) decoupling of δ13Ccarb and δ13Corg reflecting increasing isotope fractionation in response to steadily rising pCO2, driven by enhanced volcanic degassing of mantle-derived CO2, which likely preceded the presumed peak volcanism of the Caribbean LIP; and 2) a long-lived, secondary positive excursion that documents enhanced organic carbon burial in shallow shelf seas that expanded during global sea-level rise and highstand. We demonstrate a plausible combination of environmental forcings that pre-conditioned the mid-Cretaceous ocean-atmosphere system for a massive perturbation, OAE2.