Atmospheric Carbon Dioxide and Its Relation to Carbon Cycle Perturbations during Ocean Anoxic Event 1d: A High Resolution Record From Dispersed Plant Cuticle
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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 in the deep ocean combined with a long-term atmospheric positive d13C excursion and an increase in the burial of carbon 12. Some OAEs show a negative d13C excursion preceding the positive excursion, indicating a perturbation in the global carbon cycle prior to the initiation of these events. The Rose Creek Pit (RCP) locality, southeastern Nebraska, is currently 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 analysis of carbon isotopes and stomatal index (SI) to determine changes in the cycling between carbon pools (carbon isotope analysis) and their relation to paleo-CO2 via changes in SI. To do this, RCP SI data were calculated from the cuticle of Pandemophyllum kvacekii (an extinct laurel) and related taxa, and fitted to d13C curves derived from fossil gymnosperm charcoal and vitrain, as well as other published charcoal and bulk organic d13C profiles from RCP and nearby sediment cores. Absolute values of CO2 were estimated using three published transfer functions based on species of extant Lauraceae. This study represents the first attempt to quantify variation in CO2 during OAE1d. SI indicates changes in CO2 coincident with changes in d13C. Pre-excursion pCO2 was relatively low (330–615 ppm) and was associated with a slight positive shift in d13C (~1‰) immediately preceding the negative excursion recorded in other RCP d13C curves. Those curves record a negative excursion (avg. ≈ 2.21‰) beginning at or slightly below the floor of RCP. This study records a more modest negative shift of ~1.2–1.47‰ because sampling began after the beginning of the negative excursion. All RCP chemostratigraphy shows that negative excursion lasts through ~3.3 m of the section. During this negative excursion, pCO2 increases from the pre-excursion values to a high of ~380–800 ppm. After the negative excursion, all RCP chemostratigraphic curves and pCO2 values show a slow return to pre-excursion values. Despite the finer sampling intervals of this study compared to others at RCP and in surrounding areas, fossil gymnosperm charcoal and vitrain d13C curves do not record the positive excursion found in foraminifera and carbonate d13C curves during OAE1d. This study confirms that d13C of fossil wood, whether coalified or charcoalified, and SI from dispersed cuticle can reliably capture carbon cycle perturbations and changes in atmospheric CO2 around OAEs.