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 (δ¹³C_vit) 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).