Paleoceanographic changes across the Jurassic/Cretaceous boundary: The calcareous phytoplankton response |
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| Institution: | 1. Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan;2. Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;3. Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, PR China;2. State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences, Wuhan, Hubei, 430074, P.R. China;3. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;2. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;3. Geological Survey of Canada, 1500 - 625 Robson St., Vancouver, B.C. V6B 5J3, Canada;4. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;5. Institute of Geology and Paleontology, Tohoku University, Aoba-ku, Sendai 980-8578, Japan |
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| Abstract: | 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. |
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