Abstract Stomata characteristics of two extinct conifer Cheirolepidiaceae species, Pseudofrenelopsis gansuensis and Pseudofrenelopsis dalatzensis , were used to reconstruct the atmospheric carbon dioxide concentration levels during the late Albian of the Cretaceous. A sequence of fossil samples was collected from five beds in the Dalazi Formation in the Zhixin and Luozigou basins of Jilin Province, northeastern China. The stomatal index was used to estimate the palaeo- p CO 2. The average stomatal index value of the two species was 4.1 and the atmospheric CO 2 content was c. 1200 ppmv by the average of Recent standardization and Carboniferous standardization. During the short interval from 104.8 ± 0.5 Ma to 104.0 ± 0.4 Ma in the late Albian, the atmospheric CO 2 content presents a decreasing tendency, with the highest value being c. 1300 ppmv in the Luozigou Basin. In addition, the higher atmospheric CO 2 values of the late Albian are likely linked to a ‘greenhouse climate’ during the time of Oceanic Anoxic Event 1c (OAE1c).

Abstract: The Albian–Cenomanian boundary coincides with a positive stable carbon isotope excursion that previously has been correlated with an oceanic anoxic event (OAE1d); however, there are other secular isotopic excursions associated with the Albian–Cenomanian boundary as well as the Early Cenomanian that are preceded by OAE1d. The focus of this study is Deep Sea Drilling Project (DSDP) Hole 547A (eastern North Atlantic), where multiple isotopic excursions spanning the Albian–Cenomanian boundary and Early Cenomanian have been correlated to the Albian– Cenomanian boundary event (ACBE) and Lower Cenomanian events (LCEs) I to III of the Mont Risou (France), the English Chalk, and Gubbio (Italy) sections using planktonic foraminifera biostratigraphy and stable carbon isotope stratigraphy. Although these excursions are recorded in numerous sections, very little information exists about the triggering mechanisms that led to these perturbations in the carbon cycle. This paper aims to assess the ACBE and LCEs I to III using the calcareous nannofossil assemblages at DSDP Hole 547A to determine stratigraphic changes associated with these excursions. The findings from this study indicate that there were changes in surface water productivity during these excursions, where Biscutum spp. abundances increased at the onset of the excursions, while species evenness and Shannon’s diversity index decreased. Given these findings, it can be inferred that an increase in surface water productivity led to rapid burial of isotopically light carbon, which resulted in the positive stable carbon isotope excursions associated with ACBE and LCEs I to III. It is posited that pulses of intensified mixing occurred along the northwestern African margin during the Late Albian through Early Cenomanian, which caused increased surface water fertility and paleoproductivity. These pulses have been linked to the gradual stabilization of mid-Cretaceous ocean circulation that resulted from plate reorganization and long-term sea-level rise. These events enhanced the connection between different ocean basins, creating a stable circulation mode with well-defined upwelling systems.

Based on its completeness, the Bottaccione-Contessa composite section (BCCS; Gubbio area, Italy) has been analyzed to infer the paleobathymetry throughout the interval spanning the uppermost Albian to the lower Danian. Foraminifera are generally abundant and well preserved and the assemblages are dominated by planktonic foraminifera (planktonic/benthic ratio > 99%). The investigation of the benthic foraminiferal assemblages allows us to infer a lower bathyal depositional environment along most of the BCCS. A somewhat shallower deposition paleodepth is estimated for the Danian part of the BCCS, although this difference could be ascribed to the post–Cretaceous-Paleogene (K-Pg) boundary effect. This study further enhances the application potential of benthic foraminiferal assemblages as a paleobathymetric proxy.