Impact of the Middle Jurassic diversification of Watznaueria (coccolith-bearing algae) on the carbon cycle and δ~(13)C of bulk marine carbonates

摘要

During the Mid Mesozoic Revolution, thought to have started 200 Ma ago (Late Triassic), the production of calcium carbonate in the ocean shifted from platform and epicontinental seas to the open ocean, concurrently with the diversification of coccolithophorids. In this regard, the radiation of the coccolith genus Watznaueria during the Middle Jurassic is thought to represent one of the most important steps of this diversification. Nevertheless, the timing of this diversification remains poorly constrained, and its possible impact on global carbon budgets remains unclear. In this study, we present new records of nannofossil fluxes and carbon stable isotope composition from sedimentary deposits of Lower Bajodan age from the Cabo Mondego (Portugal) reference section to further address the possible impart of this diversification on the Middle Jurassic global carbon cycle. Our results show that calcareous nannofossil fluxes increase markedly from the upper part of the Aalenian to the Early Bajocian. coinciding with a 0.75‰ positive shift in carbon isotope compositions of bulk carbonate. Reconstructions of mass accumulation rates indicate that nannofossil fluxes increased by two orders of magnitude (from 10~9 to 10~(11) nannofossils/m~2/yr) during the corresponding time interval, mainly related to the rise of Watznaueria genus, whose relative abundance jumped from 2% to 20% of the total rock composition. The calculated amount of carbon derived from calcareous nannofossils deposited in the Early Bajocian seas was, however, 10 to 20 times lower than current levels. Mass balance calculations indicate that the increase of nannofossil flux throughout the studied interval was most likely not the main cause of the accompanying isotopic perturbation, suggesting a limited role of the Early Bajocian diversification on the global carbon cycle. Our results show that while the diversification of Watznaueria throughout the Bajocian caused a major increase in the flux of pelagic carbonate to the deep ocean, it was most likely quantitatively insufficient to have a large impact on the global biogeochemistry of the oceans.