Coevolution of the Oceans and Biotic Carbon Storage

Abstract

Marine plankton constitute the base of the food chain, support ocean biodiversity, and are vital to nutrient and carbon cycling. Planktic foraminifera and coccolithophores are calcifying plankton which, through the burial of their exoskeleton, contribute to the largest store of carbon on Earth. The amount of carbonate stored by these calcifiers is a function of their size, weight, and abundance. However, there is uncertainty about how plankton will respond to climate change. I use a suite of methods, including statistical modelling, the fossil record, and ecosystem modelling, to deepen our understanding of the vulnerabilities (and resilience) of plankton to environmental change.
I use statistical modelling to investigate the environmental controls on foraminifera weight (Chapter 2) — a strongly debated proxy for past CO2. I find complex species-specific and spatially heterogenous results, which call into question the use of this proxy. My research highlights the need for a deeper understanding of foraminiferal biology and ecology, and a unified approach for shell weight reconstruction.
I develop a new high-throughput method which uses imaging flow cytometry to rapidly reconstruct coccolith size from sediment (Chapter 3). I quantify size, weight/thickness, and abundance of planktic foraminifera and coccolithophores over the Cenozoic (Chapter 4) and detect stability in carbonate accumulation through time, despite major changes in climate, palaeogeography, and ocean
circulation. Coccolithophores contribute less to total carbonate towards the modern, driven by the compounding impacts of CO2, competition with diatoms, and decreased diversity, whereas, planktic foraminifera exhibit resilience. I expand my analyses to the wider plankton system and use EcoGEnIE 1.2 to model how plankton functional groups, and their size and biomass, changed through the Cenozoic (Chapter 5). Total plankton biomass increases over the Cenozoic despite a shift to smaller size, and zooplankton consistently contribute more to total biomass than phytoplankton. I evidence resilience of plankton through ecological redundancy, with biomass losses offset by shifts to different size-classes and/or
successful traits. CO2 controls the magnitude of biomass within an individual timeslice, but changing palaeogeography controls the trend in biomass through time.

Date of Award	9 Dec 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Daniela N Schmidt (Supervisor) & Paul J Valdes (Supervisor)