Paleoceanographic changes across the Jurassic/Cretaceous boundary: The calcareous phytoplankton response

Calcareous nannoplankton experienced rapid diversification at the Jurassic/Cretaceous boundary, including the appearance of several highly successful and long-ranging Cretaceous genera. This study examines, at high-resolution, calcareous nannofossil assemblages and bulk-rock oxygen and carbon isotopes of lower Tithonian to lower Berriasian (∼151 to 142 Ma) sediments recovered in Deep Sea Drilling Project Hole 534A, central Atlantic Ocean. The results show major changes in assemblage composition and abundance of three genera (Conusphaera, Nannoconus, and Polycostella). Conusphaera dominates the nannolith assemblage in the late middle Tithonian ("Conusphaera world"), while the nannoconid dominance ("Nannoconus world") began in the Berriasian. The acme peak of the genus Polycostella in the late Tithonian partially superimposes that of Conusphaera. Although these genera are indicators of warm, oligotrophic surface waters, stable isotope data suggest that the individual taxa may flourish in slightly different ecological regimes. Nannoconus flourished under warmer and possibly more nutrient-depleted surface waters than Polycostella, a genus that thrived in relatively cooler waters. These findings imply paleoceanographic changes across the J/K boundary interval with a cooling in the late Tithonian, followed by a temperature increase in the Berriasian. Because the transfer of these heavily calcified nannolith taxa to the seafloor is more efficient than that of average coccolithophorids, the J/K evolutionary event changed carbonate cycling and burial in the oceans.