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Abstract: Nannoplankton play a significant role in the global carbon cycle; changes in the composition of the assemblage can influence primary productivity, carbon burial, and ultimately climate. Here counts of nannofossils in Eocene samples document one of the largest nannoplankton assemblage turnovers of the Cenozoic Era. This event is marked by the global expansion of Reticulofenestra at the termination of the early Eocene climatic optimum (EECO), and continues through the progressive cooling to the earliest Oligocene glaciation. To understand the mechanisms driving this assemblage shift we use multivariate statistical techniques and carbon and oxygen isotope records from localities worldwide. We find that the turnover in the global nannofossil assemblage is tied to thermal destratification of the ocean and a coincident decrease in the efficiency of the oceanic biologic pump at the height of the EECO. We propose that increased vertical mixing and increased nutrient supply to surface waters during the EECO caused a shift in the trophic structure towards enhanced productivity, influencing the global nannoplankton population. [Copyright &y& Elsevier]

Abstract: Calcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. <3.5μm, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the “nannoconid crisis”, but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the “nannoconid decline” observed in several Tethys sections.The normalized ratio between low and high fertility taxa (Fertility Index) was used to characterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremian–lower Aptian sediments on the Galicia Margin. [Copyright &y& Elsevier]

• A carbonate layer is an important global feature of the Cretaceous-Paleogene boundary. • Micrite is composed of diverse crystal morphologies suggesting a range of origins. • Crystals resemble precipitates from solution and carbonation of vaporized CaO. • Crystals associated with fossil organics resemble calcite precipitated by bacteria. • The layer likely represents global microbial blooms in supersaturated oceans. Microcrystalline calcite (micrite) dominates the sedimentary record of the aftermath of the Cretaceous–Paleogene (K–Pg) impact at 31 sites globally, with records ranging from the deep ocean to the Chicxulub impact crater, over intervals ranging from a few centimeters to more than seventeen meters. This micrite-rich layer provides important information about the chemistry and biology of the oceans after the impact. Detailed high-resolution scanning electron microscopy demonstrates that the layer contains abundant calcite crystals in the micron size range with a variety of forms. Crystals are often constructed of delicate, oriented agglomerates of sub-micrometer mesocrystals indicative of rapid precipitation. We compare the form of crystals with natural and experimental calcite to shed light on their origin. Close to the crater, a significant part of the micrite may derive from the initial backreaction of CaO vaporized during impact. In more distal sites, simple interlocking rhombohedral crystals resemble calcite precipitated from solution. Globally, we found unique calcite crystals associated with fossilized extracellular materials that strikingly resemble calcite precipitated by various types of bacteria in natural and laboratory settings. The micrite-rich layer contains abundant bacterial and eukaryotic algal biomarkers and most likely represents global microbial blooms initiated within millennia of the K–Pg mass extinction. Cyanobacteria and non-haptophyte microalgae likely proliferated as dominant primary producers in cold immediate post-impact environments. As surface-water saturation state rose over the following millennia due to the loss of eukaryotic carbonate producers and continuing river input of alkalinity, "whitings" induced by cyanobacteria replaced calcareous nannoplankton as major carbonate producers. We postulate that the blooms grew in supersaturated surface waters as evidenced by crystals that resemble calcite precipitates from solution. The microbial biomass may have served as a food source enabling survival of a portion of the marine biota, ultimately including life on the deep seafloor. Although the dominance of cyanobacterial and algal photosynthesis would have weakened the biological pump, it still would have removed sufficient nutrients from surface waters thus conditioning the ocean for the recovery of biota at higher trophic levels. [ABSTRACT FROM AUTHOR]