Abstract
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 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.
Highlights
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)
We find T. crassaformis by the late Pliocene at our investigated tropical and sub-tropical sites (2.5 Ma; Sites U1338, U1489, 872, U1490, 242, and U1482; Figs. 10 and 11), with oxygen isotope values ranging from 1.0 ‰ to 2.0 ‰, which translate to depth habitats of 400–600 m (Boscolo-Galazzo et al, 2021; Fig.1)
The late Miocene–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 first by a diversification and by progressive extinction almost entirely within the 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 metres 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 metres 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
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