PhD studentship: Recognizing the importance of biology in the ocean's carbonate pump

Dr Alex Poulton (Heriot-Watt University)
36-month PhD studentship, starting September 2023.

About the Studentship

Recognition of the central role of ocean biology in the biological carbon pump led to a revolution in our understanding of how it functions, as well as its magnitude, efficiency and variability. In a relative sense, the carbonate counter pump - whereby calcium carbonate (CaCO3) formation in the upper ocean and its export (sinking) to depth reduces the surface ocean's capacity to absorb CO2, with dissolution of this material at depth - is best understood in a purely geochemical sense. The role of diversity is however clear in the carbonate counter pump; for example, pelagic calcifiers (e.g., coccolithophores, pteropods, fish) produce different CaCO3 polymorphs, which vary in propensity to dissolve, so that this fundamental difference dictates the depth where their CaCO3 dissolves (and alkalinity is returned). However, parallel to these basic biological differences are other more complex ecological ones, for example nutritional strategy, trophic role, depth habitat, biogeography, and the mechanisms of CaCO3 formation. Such diversity in physiology, ecology and behaviour are likely to have a strong, but as yet unquantified, impact on the carbonate pump, how the pump functions overall, as well as dictating its magnitude, efficiency and variability (in time and space) - and importantly, its sensitivity and response to future environmental change (ocean acidification, warming, deoxygenation).

The goal of this studentship is to change our perspective of the carbonate pump by recognising the role of biodiversity and ecology in its magnitude, efficiency and variability. Information exists in the literature reflecting what is known about the different organisms involved in the carbonate pump, however this information has not been compiled together into a holistic perspective of the carbonate pump. This studentship will put this information and existing data together under a common schematic of the carbonate counter pump to deliver a data-driven and statistically robust synopsis of the carbonate pump in terms of quantitative estimates of contributions, biological comparisons and sensitivity to change. In this way this project will fill gaps in biological knowledge of the carbonate pump, identify key research questions to address missing knowledge, and better inform model parameterisations that seek to represent the carbonate pump. An opportunity to include field work in this analysis in summer 2025 may be included, where a contemporary assessment of the different calcifiers from the Canary Islands to South Africa is under discussion. Other opportunities include working with biogeochemical modellers based in Paris (LSCE) and the University of Exeter, to understand how carbonate production is parameterised in current models and/or how to better represent calcifier diversity in these models.

The studentship will work within the recently funded EU project Ocean-ICU ('Improving Carbon Understanding';, which involves 30 research institutes across Europe addressing knowledge gaps in the ocean's carbon-cycle. Ocean-ICU recognises our limited ability to measure and model the ocean's carbon-sink due to a simplistic understanding of the biological processes (diversity, physiology, trophic interactions) involved, as well as the potential impacts of future oceanic variability (pH, temperature, oxygen) related to our changing climate. Ocean-ICU will improve appreciation of these processes, evaluate their significance, and transfer those that are important into new and existing models that inform the IPCC and contribute to resolving observed model-data mismatch of ocean carbon sink estimates and future predictions. There is currently considerable interest in how calcifier diversity impacts the fate of their material within the water column, and whether this ecology could help to explain the discrepancies between modelled and observed profiles of alkalinity in the ocean.

Working as part of Ocean-ICU, the student will be involved in:

  • A new compilation of the carbonate pump, recognising the different organisms involved (from bacteria to fish);
  • Production of a meta-analysis of the sensitivity of these different contributors to climate stressors (pH, oxygen, temperature);
  • Combination of these two elements in a 'risk assessment' of the carbonate pump to climate change.

Tiny (~0.02 mm) single-celled coccolithophores, with their green chloroplasts and reflective outer calcium carbonate scales (coccoliths), highlighting their dual nature in the ocean's carbonate and biological carbon pumps.

Project Timeline

Year 1 (2023-2024): Data synthesis of production, stocks, fluxes, dissolution and burial of carbonate from different calcifiers. Literature review of calcifier biology and ecology related to their roles in the carbonate budget. Expert discussion of specific calcifier ecology and biogeochemistry through participation in international meetings, workshops and conferences.

Year 2 (2024-2025): Literature review and data synthesis of calcifier sensitivity to environmental stressors (pH, temperature, deoxygenation) to enable a meta-analysis of impacts and responses. Examination of model parameterisations and outputs related to the carbonate (alkalinity) budget and its sensitivity to environmental change will be included as part of Ocean-ICU activities. Potential cruise participation (summer 2025) in the South Atlantic Ocean.

Year 3: Combination of syntheses gained on calcifier roles in the carbonate (alkalinity) pump with that of their relative sensitivities to climate change to produce a 'risk assessment' of the carbonate pump.

Training & Skills

The knowledge, facilities and expertise available within the supervisory team and across their institutions provide a combination of world-leading field, analytical and laboratory capabilities, technical support and research activity that ideally fits this PhD project, maximising the training that will be available. This project will equip the student with a range of skills, including data synthesis and meta-analysis, fieldwork, analytical science, numeracy and translation of science for wider audiences (e.g., the public and policymakers).

Host-institute scholar support: The Lyell Centre has a large research student cohort (>40) that will provide peer-support throughout the studentship, including participation in the annual post-graduate research conference. All project supervisors are also highly research-active, and the scholar will interact with all members of their research groups through lab-group meetings at the Lyell Centre, Exeter University and LSCE, providing an opportunity to learn about other techniques and research areas which may be applicable to their research. Additionally, the supervisors are all based in research-active institutes that span a broad range of ecological, environmental and global geoscience research, exposing the scholar to a range of other research areas. Active participation in these research groups will provide the opportunity to discuss cutting-edge and emerging topics in the field, review recent high-impact papers and present research to academics with a common research interest in an informal and supportive atmosphere.

Where required, and to maintain continued professional development, the scholar will be supported to attend specialist training courses directly aligned to the project, such as in meta-analysis and evidence-based synthesis, analytical ocean biogeochemistry and elemental cycling. The project supervisors will also support and encourage the scholar's attendance on transferable skills training such as data management, scientific writing and science communication.


The PhD stipend (£19,494) is for 3 years (36 months), and there is an international fee available to support applicants from the UK and abroad.


Enquiries to

All applicants must have or expect to have a first or upper second-class BSc, MSc, MSci or equivalent degree by 31/08/23 to allow registration in September 2023. Selection will be based on academic excellence and research potential, and all short-listed applicants will be interviewed (in person or via Teams).

Closing date: 30/06/23. All successful candidates must commence studies by 01/09/23.

Further Reading

Ziveri et al. (2023). Pelagic calcium carbonate production and shallow dissolution in the North Pacific Ocean. Nature Communications, 14, 805,

Sulpis et al. (2021). Calcium carbonate dissolution patterns in the ocean. Nature Geoscience, 14, 423-428,

Manno et al. (2018). Threatened species drive the strength of the carbonate pump in the northern Scotia Sea. Nature Communications, 9, 4592,

posted: 05 April 2023     Please mention EARTHWORKS when responding to this advertisement.