Abstract
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 through 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.
Highlights
IntroductionThe biodiversity of planktonic and nektonic organisms is difficult to explain given the uniform character and vastness of pelagic environments, where genetic isolation seems difficult to maintain (Norris, 2000)
Planktonic foraminifera live stratified across a range of depths spanning from the surface to hundreds of meters down the water column (Rebotim et al, 2017; Meilland et al, 2019)
The late Miocene385 Pliocene peak diversity is present in previous global compilations of total nannofossil diversity (Bown et al 2004; Lowery et al 2020), but here we show that this signal is driven by first a diversification and progressive extinction almost entirely within upper-euphotic taxa
Summary
The biodiversity of planktonic and nektonic organisms is difficult to explain given the uniform character and vastness of pelagic environments, where genetic isolation seems difficult to maintain (Norris, 2000). Planktonic foraminifera live stratified across a range of depths spanning from the surface to hundreds of meters down the water column (Rebotim et al, 2017; Meilland et al, 2019) Properties such as food quantity and quality, oxygen, light and pressure all change markedly across the first few hundreds of meters of the ocean. Depending on such down-column variability in environmental conditions, planktonic foraminifera can actively control their living depth of preference, which remains relatively stable during their adult life-stage (Hull et al, 2011; Weiner et al, 2012; Rebotim et al, 2017; Meilland et al, 2019; Duan et al, 2021). This provides invaluable information about species-specific functional ecology (e.g., feeding strategy) and habitat preferences (e.g., surface versus deep waters), which in combination with biogeographic, taxonomic, biometric, and stratigraphic data have often been used to infer speciation and extinction mechanisms (Norris et al, 1993; Norris et al, 1994; Pearson et al, 1997; Hull and Norris, 2009; Pearson and Coxall, 2012; Woodhouse et al, 2021 ) and reconstruct phylogeny (Aze et al, 2011)
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