| Summary: | During the Cretaceous the construction of Large Igneous Provinces (LIPs), forming gigantic oceanic plateaus, affected ecosystems at global scale. LIP volcanism was coeval with episodes of oxygen depletion in the oceans with consequent burial of massive amounts of organic matter known as Oceanic Anoxic Events (OAEs). Under these conditions, biota were forced to face excess CO2 and global perturbations in the ocean-atmosphere system. In the open ocean, coccolithophores are important carbonate rock-forming organisms, extremely sensitive to changes in physical-chemical parameters of surface waters. They are an ideal tracer for detecting the direct/indirect impacts of submarine volcanism on transient responses and evolution of calcifying biota. We investigated calcareous nannoplankton assemblages across the early Aptian OAE1a and the latest Cenomanian OAE2, associated to the Ontong Java Plateau (OJP) and the Caribbean Plateau (CP), respectively. Massive submarine volcanism of OJP triggered a disruption in the oceanic carbonate system: excess CO2 arguably induced ocean acidification that was detrimental to marine calcifiers, with temporary failure, but no extinctions, of rock-forming nannoconids and production of dwarf and malformed coccoliths. Similarly, during OAE2 the excess CO2 from CP volcanism affected nannoplankton calcification inducing some coccolith dwarfism. Hydrothermal plumes during construction of both OJP and CP introduced biolimiting metals that fertilized the global ocean. However, some toxic metals might have disturbed the functioning of some intolerant coccolithophorid species. There is a causal link between intervals of LIP submarine volcanism and changes in nannoplankton composition, abundance and biocalcification through OAE1a and OAE2. Changes in ocean chemistry, structure, and fertility during formation of oceanic plateaus might explain observed tempo and mode of nannoplankton evolution: major origination episodes might result from magmas especially enriched in biogeochemically important ...
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