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.

Abstract Preliminary biostratigraphic investigation of the Greenhorn Limestone at Pueblo, Colorado indicates that the Lincoln, Hartland, and lower Bridge Creek members are part of the Lithraphidites acutum calcareous nannofossil Zone. The middle and upper part of the Bridge Creek Member is assigned to the Quadrum gartneri Zone. Subdivision of these zones is difficult due to the scarcity of several biostratigraphically important taxa. Biohorizons based on secondary taxa do, however, offer promise for intrabasinal and intercontinental correlation. Paleobiogeographic analysis using three time-slices across the basin suggests that fresh-water influx from the basin margins was an important factor in restricting nannofossil occurrence and diversity during the initial stages of the invasion of oceanic surface water masses into the basin (middle Cenomanian). By maximum transgression, however, the effects of fresh-water influx appear to be minimal in the eastern and central portions of the basin.