Macroevolutionary dynamics and diversity of coccolithophores across long-term cooling periods of the Cenozoic

Jones, Amy P. (2021). Macroevolutionary dynamics and diversity of coccolithophores across long-term cooling periods of the Cenozoic. University of Birmingham. Ph.D.

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Abstract

This thesis addresses the macroevolution of coccolithophores during two significant cooling periods of the Cenozoic: the Eocene-Oligocene Transition (EOT) and the Neogene to Recent. New onshore low-latitude site NKK1, from the Indo-Pacific Warm Pool (IPWP) in the Indian Ocean, documents high-resolution calcareous nannofossil assemblage responses during the EOT. Data from NKK1 consists of detailed high-resolution species relative abundance records documenting the late middle Eocene to early Oligocene, with well-preserved nannofossils. Research on tropical calcareous nannofossil assemblages through the EOT where sediments yield good recovery and well-preserved coccoliths, is uncommon and so NKK1 presents valuable new palaeoceanographic, palaeoecological, palaeoclimatic and biostratigraphic data from calcareous nannofossils across the EOT. Through this time interval species diversity declined coincident with the extinction of rosette-shaped discoasters (\(\sim\)34.44 Ma) termed the Discoaster extinction event (DEE). This interval, during the early EOT, represents the decline of oligotrophic taxa due to increased nutrient supply from the Southern Ocean; additionally, an abundance decrease is observed in small (<3.5 μm) Reticulofenestra morphotype cell sizes through the DEE interval – signifying coccolith size was apparently not limited by atmospheric CO\(_2\).

Additionally, a combination of two datasets offers a global perspective to assess the macroevolutionary drivers influencing coccolithophore lineages during declining atmospheric CO\(_2\) through the late Neogene. The first is a compilation of \(\sim\)2.5 Ma time-slice data from ocean sediments recovered at 9 sites across the Atlantic, Indian, Pacific and Southern Ocean basins that spans the last 15 Ma. The second is a high-resolution (200 ka) dataset from Integrated Ocean Drilling Program Expedition 363, Site U1482, where a continuous sedimentary sequence encompassing the crucial late Miocene to early Pleistocene (7.7-1.7 Ma) interval yields a well-preserved, sub-tropical high diversity calcareous nannofossil assemblage. These two records provide supporting evidence that declining pCO\(_2\) had a significant impact on evolution within the Noelaerhabdaceae family with a correlation between atmospheric CO\(_2\) and a decrease in Reticulofenestra coccolith size observed since the late Miocene. Through the Plio-Pleistocene however, the rise of small Gephyrocapsa spp., an upper euphotic zone species, is correlated with the rise in deep-photic zone dweller, Florisphaera profunda, and a modelled deepening of global nutricline depths. On this basis, we suggest that it is not solely pCO\(_2\) that drives major macroevolutionary turnover in dominance, but that a deepened nutricline exerted as much of a selective pressure as pCO\(_2\) with potential impacts across multiple lineages including the extinction of Discoaster spp., demise in Reticulofenestra groups and the modern dominance of Florisphaera profunda - Gephyrocapsa assemblages.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):