Changes in CO2 during OAE1d: A Comparison to Other Carbon Cycle Perturbations of the Mesozoic
Abstract:Ocean Anoxic Events (OAEs) and their associated carbon cycle perturbations are hallmarks of the Cretaceous. Quantification of these short-term disruptions to the carbon cycle is needed to understand the nature and climatic tipping points of greenhouse gas-initiated OAEs. The Rose Creek Pit (RCP) locality, Dakota Formation, southeastern Nebraska, offers a unique opportunity to make these comparisons for OAE1d through a combination of δ13C analysis and stomatal index (SI).
RCP is already known to contain the negative δ13C excursion of OAE1d in charcoal and bulk organic matter. We produced new δ13C curves from fossil gymnosperm charcoal (δ13C gym) and vitrain (δ13Cvit) collected at 30cm intervals at RCP. These new curves reproduce the negative δ13C isotopic excursion found in the earlier chemostratigraphic curves and confirm that RCP contains the record of the carbon isotope excursion associated with OAE1d. In the same stratigraphic interval, SI as a proxy for atmospheric CO2 was calculated from dispersed leaf cuticle of Pandemophyllum kvacekii, new morphotypes of Pandemophyllum, and the related species Pabiania variloba. Average SI responds to the negative δ13C excursion (~3‰) by a gradual decrease from 8.2 to 5.1. Both δ13C and SI return to pre-excursion values above the level of the excursion.
Furthermore, we calculate a coeval CO2 increase of 141−470 ppm during the negative excursion on the basis of published transfer functions, a value smaller than those calculated for other Mesozoic carbon cycle perturbations assessed with SI (K-T and Tr-J Boundaries, OAE2, TOAE; 179−2000+ ppm). We estimate that between 282−940 Gt of carbon was added to the atmosphere during OAE1d, much less than that estimated for other events (358−4000+ Gt C). Calculated changes in radiative forcing (∆F ≈1.1−4.7 W/m2) and global temperatures (∆T ≈ 0.9−3.8 ⁰C) resulting from increased CO2 also suggest that OAE1d had a smaller global effect than other events (∆F ≈ 2.5−9.9 W/m2 and ∆T ≈ 2.0−7.9 ⁰C).