Pelagic sediments are boring as they result from monotonous processes: after death, calcareous and/or siliceous phytoplankton and zooplankton (with minor contributions of clayey particles) very slowly settle on the ocean floor. Pelagic sedimentation, therefore, closely corresponds to the productivity of surface waters while being controlled by ocean chemistry, fertility, temperature and depth-size of the basin. The biological pump extracts nutrients and carbon from the photic zone to form organic matter which, however, fails to reach the deep ocean, unless exceptionally unusual conditions are established. Some phytoplanktonic organisms, though, have invented biomineralization and many mineralized parts accumulate at the seafloor as oozes, later diagenetically transformed into pelagic limestone and, more sporadically, chert.
Arguably, the modern ocean originated in the Early Triassic when a group of phytoplankton learned, by chance or by necessity, to calcify. Since then, coccolithophores developed the ability to secrete a variety of coccoliths/nannoliths and coccospheres. Coccolithogenesis, in a sense, continued to take snapshots that we can use to assess the functioning and dynamics - at various time resolutions- of the ocean, the largest and oldest ecosystem on our planet. My talk will try to provide data and interpretations of good and bad times for Mesozoic coccolithophores, with the ultimate goal of sharing with you my understanding of what a "normal" ocean was, what was its resilience to global perturbations, and which were the tipping points.In Jurassic and Cretaceous oceans calcareous nannoplankton were already widespread from coastal to open oceanic settings and of enough abundance and diversity to be rock-forming. Their variations somehow correlate with environmental global change, although getting from correlation to causality is not always straight forward. Mesozoic ocean anoxic events (OAEs) represent some of the most dramatic disruptions of the global carbon cycle and the geological records of OAEs have been thoroughly investigated to understand how the Earth system has overcome such extreme stress. Quantitative studies reveal major shifts in nannofossil assemblages with species-specific variations in size and major decreases in abundance, especially of the dominant rock-forming taxa. The absence/rarity of calcareous nannofossils at the peak of the OAE perturbation is primarily interpreted as the result of a major change in ocean alkalinity (and development of acidification) that possibly hindered biocalcification. However, none of the nannoplankton forms experiencing a calcification crisis got extinct: they recovered when the paleoenvironment returned to a pre-perturbation state, although slowly and partially.
Calcareous nannoplankton evolution is marked by spectacular speciation episodes (some of them anticipating and accompanying OAEs) in absence of extinctions. Furthermore, Jurassic and Cretaceous nannoplankton underwent accelerated originations during times of prolonged stability that, apparently, may have triggered innovative ways of coccolith/nannolith calcification. After decades of research devoted to environmental perturbations, we know very little about the unstressed ocean. Yet to understand/model how to stop and/or reverse the current global change, we should first know the characteristics of a calm, stable, normal ocean. What concentrations of atmospheric CO2? What fluctuations in chemistry, fertility, temperature? What variations in marine biota? The answers are written in the boring pelagic limestones!