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The fossil record of marine microplankton provides insights into the evolutionary drivers which led to the origin of modern deep-water plankton, one of the largest components of ocean biomass. We use global abundance and biogeographic data, combined with depth habitat reconstructions, to determine the environmental mechanisms behind speciation in two groups of pelagic microfossils over the past 15 Myr. We compare our microfossil datasets with water column profiles simulated in an Earth system model. We show that deep-living planktonic foraminiferal (zooplankton) and calcareous nannofossil (mixotroph phytoplankton) species were virtually absent globally during the peak of the middle Miocene warmth. The evolution of deep-dwelling planktonic foraminifera started from subpolar–mid-latitude species, during late Miocene cooling, via allopatry. Deep-dwelling species subsequently spread towards lower latitudes and further diversified via depth sympatry, establishing modern communities stratified hundreds of metres down the water column. Similarly, sub-euphotic zone specialist calcareous nannofossils become a major component of tropical and sub-tropical assemblages during the latest Miocene to early Pliocene. Our model simulations suggest that increased organic matter and oxygen availability for planktonic foraminifera, and increased nutrients and light penetration for nannoplankton, favoured the evolution of new deep-water niches. These conditions resulted from global cooling and the associated increase in the efficiency of the biological pump over the last 15 Myr. [ABSTRACT FROM AUTHOR]

The fossil record of marine microplankton provides insights into the evolutionary drivers which led to the origin of modern deep-water plankton, one of the largest component of ocean biomass. We use global abundance and biogeographic data combined with depth habitat reconstructions to determine the environmental mechanisms behind speciation in two groups of pelagic microfossils over the past 15 million years. We compare our microfossil datasets with water column profiles simulated in an Earth System model. We show that deep-living planktonic foraminiferal (zooplankton) and calcareous nannofossil (mixotroph phytoplankton) species were virtually absent globally during the peak of the middle Miocene warmth. Evolution of deep-dwelling planktonic foraminifera started from subpolar-midlatitude species during late Miocene cooling, via allopatry. Deep-dwelling species subsequently spread towards lower latitudes and further diversified via depth sympatry, establishing modern communities stratified hundreds of meters down the water column. Similarly, sub-euphotic zone specialist calcareous nannofossils become a major component of tropical and sub-tropical assemblages during the latest Miocene to early Pliocene. Our model simulations suggest that increased organic matter and oxygen availability for planktonic foraminifera, and increased nutrients and light penetration for nannoplankton, favored the evolution of new deep water niches. These conditions resulted from global cooling and the associated increase in the efficiency of the biological pump over the last 15 million years. [ABSTRACT FROM AUTHOR]

Theory suggests that the ocean’s biological carbon pump, the process by which organic matter is produced at the surface and transferred to the deep ocean, is sensitive to temperature because temperature controls photosynthesis and respiration rates. We applied a combined data-modeling approach to investigate carbon and nutrient recycling rates across the world ocean over the past 15 million years of global cooling. We found that the efficiency of the biological carbon pump increased with ocean cooling as the result of a temperature-dependent reduction in the rate of remineralization (degradation) of sinking organic matter. Increased food delivery at depth prompted the development of new deep-water niches, triggering deep plankton evolution and the expansion of the mesopelagic “twilight zone” ecosystem. [ABSTRACT FROM AUTHOR]

We combine stable isotope, calcareous nannoplankton, and benthic foraminiferal records for Ocean Drilling Program Sites 1262 (paleodepth ~3,500 m) and 1263 (paleodepth ~1,500 m) on Walvis Ridge (SE Atlantic), to document the marine biotic response to Eocene Thermal Maximum 3, in the early part of the Early Eocene Climate Optimum, ~3.1 Myr after the Paleocene/Eocene Thermal Maximum. Bottom water warming may have decreased the vertical thermal gradient at both sites, but more at Site 1263 than at Site 1262. Floral and faunal changes were more muted at Site 1262 than at shallower Site 1263, indicating that carbonate dissolution was not the most important cause of biotic effects. Assemblage changes were more pronounced in benthos than in plankton. Calcareous nannofossils underwent minor ecological changes, possibly related to the presence of warmer waters, especially at Site 1263, and dissolution-resistant taxa increased in abundance. Benthic foraminiferal diversity decreased at both sites, but benthic foraminiferal accumulation rates declined dramatically at Site 1263, remaining stable at Site 1262. Ocean circulation may have changed during ETM3, resulting in the presence of a warmer (intermediate) water mass at Site 1263. More pronounced warming may have caused enhanced remineralization of organic matter, so less food reached the benthos. The biotic response to the X-event was less pronounced than that to earlier and more severe hyperthermal events, the Paleocene/Eocene Thermal Maximum and Eocene Thermal Maximum 2. The extent of the biotic response reflects the severity of the environmental disturbance but varies by location (e.g., paleodepth on Walvis Ridge). [ABSTRACT FROM AUTHOR]

Planktonic foraminifera are widely used in biostratigraphic, palaeoceanographic and evolutionary studies, but the strength of many study conclusions could be weakened if taxonomic identifications are not reproducible by different workers. In this study, to assess the relative importance of a range of possible reasons for among-worker disagreement in identification, 100 specimens of 26 species of macroperforate planktonic foraminifera were selected from a core-top site in the subtropical Pacific Ocean. Twenty-three scientists at different career stages – including some with only a few days experience of planktonic foraminifera – were asked to identify each specimen to species level, and to indicate their confidence in each identification. The participants were provided with a species list and had access to additional reference materials. We use generalised linear mixed-effects models to test the relevance of three sets of factors in identification accuracy: participant-level characteristics (including experience), species-level characteristics (including a participant's knowledge of the species) and specimen-level characteristics (size, confidence in identification). The 19 less experienced scientists achieve a median accuracy of 57 %, which rises to 75 % for specimens they are confident in. For the 4 most experienced participants, overall accuracy is 79 %, rising to 93 % when they are confident. To obtain maximum comparability and ease of analysis, everyone used a standard microscope with only 35 × magnification, and each specimen was studied in isolation. Consequently, these data provide a lower limit for an estimate of consistency. Importantly, participants could largely predict whether their identifications were correct or incorrect: their own assessments of specimen-level confidence and of their previous knowledge of species concepts were the strongest predictors of accuracy. [ABSTRACT FROM AUTHOR]