ATMOSPHERIC CARBON DIOXIDE AND ITS RELATION TO CARBON CYCLE PERTURBATIONS DURING OCEAN ANOXIC EVENT 1D: A HIGH RESOLUTION RECORD FROM DISPERSED PLANT CUTICLE

Past geological greenhouse intervals are associated with Ocean Anoxic Events (OAEs), which result from an increase in marine primary productivity and/or an increase in the preservation of organic matter. The end point is widespread black shale deposition combined with a long-term atmospheric positive δ¹³C excursion and an increase in the burial of ¹²C. Some OAEs show a negative δ¹³C excursion preceding the positive excursion, indicating a perturbation in the global carbon cycle prior to the initiation of these events.

The Rose Creek (RCP) locality, southeastern Nebraska, is the only known terrestrial section that preserves OAE1d (Cretaceous, Albian-Cenomanian Boundary) and has abundant charcoal and plant cuticle. These features allow for a combined carbon isotope and stomatal index (SI) analysis to determine both changes in the cycling between carbon pools (C isotope analysis) and changes in paleo-CO₂ via changes in SI.

Preliminary (and ongoing) SI data analysis using dispersed cuticle of Pandemophyllum kvacekii (an extinct Laurel) collected at 30 cm intervals indicate changes in SI consistent with changes in CO_2. Fitting our samples to a published RCP δ¹³C profile, pre-excursion CO₂ concentrations are high. CO₂ decreases to lower concentrations in the basal 1.2 m of the RCP section, where δ¹³C_bulk shows a negative excursion and δ¹³C_charcoal remains at pre-excursion values. CO₂ concentrations become higher toward the top of the negative δ¹³C excursion, where δ¹³C_bulk and δ¹³C_charcoal are at their most negative values, and drop as the negative carbon excursion terminates. Using published transfer functions, we estimate that pre-excursion CO₂ concentrations were a maximum of 900 ppm. In the basal 1.2 m of RCP, CO₂ drops to a maximum of 480 ppm, and rises to a maximum of 710 ppm near the top of the negative excursion. As δ¹³C values rise towards pre-excursion values, CO₂ declines to a maximum of 400 ppm. The trend in SI is comparable to the trend in δ¹³C_charcoal and follows recognized patterns, while SI shows partial divergence from δ¹³C_bulk. These data, while preliminary, highlight the importance of considering isotope substrate when investigating carbon cycle perturbations.