Routledge, Claire Marie; (2021) Plankton evolution and the Paleogene/Neogene transition. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis documents calcareous nannoplankton diversity, population dynamics and community structure from the early Oligocene through early Miocene, across the significant Warmhouse to Coolhouse transitional interval in Earth history when ice sheets were first established on Antarctica, using sections recovered by Integrated Ocean Discovery Program (IODP) Expedition 342 from the North Atlantic Ocean. This period of dramatic climatic and palaeoceanographic change saw major extinctions and compositional changes in plankton, and a previous lack of long-term, high-resolution quantitative records have limited attempts to document the structure of this biotic change and to test whether climate played a significant role. Here, rapidly deposited deep-sea sections from IODP Sites U1406 and U1411 in the North Atlantic are used to generate a high-resolution record of quantitative nannofossil assemblage diversity data. These sections provide continuous and expanded stratigraphy with excellent calcareous microfossil preservation that is relatively unique for Paleogene deep-sea sediments, ensuring that these records are of highest quality. An unprecedented ~23 million year, high-fidelity whole-assemblage record from the middle Eocene to early Miocene was produced by merging this study’s record with a comparable nannoplankton dataset from the same IODP expedition sites. Together this composite section shows that diversity was almost constantly declining from the middle Eocene into the lower Miocene and that with community composition shifts were largely responding to decreasing temperatures and increased nutrient availability. These changes are dominated by shifts in the overwhelmingly dominant reticulofenestrid group, but subtle signals are also present in temperature and productivity sensitive taxa such as discoasters, sphenoliths and Clausicoccus. The Oligocene itself is characterised by continued and progressive loss of warm-water taxa, but also significant loss of cool-water forms. As the Oligocene progresses, significant new nannoplankton groups emerge, most notably the Umbilcosphaera jafari Group, Calcidiscus leptoporus Group and Helicosphaera carteri Group, representing a significant shift towards the community composition that characterises the modern ocean. This study also demonstrates that nannoplankton abundances are paced by orbital cycles in the late Oligocene and Oligocene–Miocene transition, with a significant obliquity control that may represent seasonality-driven productivity cycles. Finally, using summed coefficient of variation analysis across comparable, high-resolution datasets from the first 44 million years of the Cenozoic, the study shows that nannoplankton community variability is relatively stable across this entire interval, except for an interval of exceptional volatility immediately following the Cretaceous/Paleogene mass extinction. Background levels are comparable in the Paleocene, Eocene and Oligocene and above-background events are largely linked to, and scale to, levels of significant climate change, such as the Eocene hyperthermals, the Eocene–Oligocene icehouse to greenhouse transition and intensification of glacial/interglacial cycles in the mid-Oligocene. Overall, this research documents long-term, high-resolution nannoplankton assemblage composition for a relatively understudied Oligocene interval bookmarked by two transient cooling events. The resolution and length of the 23 million year record is the first of its kind, allowing the cyclic drivers of community change on orbital timescales to be resolved and for the long-term variability in assemblages occurring at the same magnitude throughout the Paleogene to be observed, highlighting the resilience of the plankton to climatic and environmental perturbations.

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