A Layered Perspective on the Ocean Carbon Cycle

The ocean carbon cycle plays a crucial role in global climate. However, its function and its propensity to change – both in the geological past and in coming centuries – remains highly uncertain. This studentship will develop and utilize a novel “layered” framework to constrain our understanding of the ocean carbon cycle in contemporary and paleo ocean observations, as well as the climate models used by the Intergovernmental Panel for Climate Change (IPCC) to project future climate change.

Describing the ocean in terms of its watermasses (or “layers”) is a concept that goes back to the very nascence of oceanography. Increasingly, the link between watermass distributions and the dynamical processes leading to their formation and destruction is being exploited to derive deep insight into ocean circulation and its role in climate (Groeskamp et al., 2019; Zika et al., 2020). Despite advances for physical oceanography, the framework remains largely unexplored in the context of global biogeochemical cycles.

The utility of a layered framework for understanding the ocean carbon cycle stems from the fact that water in the subsurface ocean, away from the surface boundary layer, conserves its temperature and salinity. Biogeochemical tracers on the other hand evolve in the subsurface due to processes such as respiration. Consequently, within a watermass defined by its temperature and salinity (or density), the coincident evolution of biogeochemical tracers isolates specific biogeochemical processes. Coupled to an understanding of surface ocean processes, the approach provides a unique framework in which to separate physical and biogeochemical drivers.


The transport of biogeochemical tracers across density layers will be calculated using available observations. This will be carried out in collaboration with Dr Sjoerd Groeskamp, at the Royal Netherlands Institute for Sea Research (NIOZ), a world-leading expert in watermass analysis.

The operation of the ocean’s carbon cycle in “density-space” will be interpreted. This will provide a novel perspective on first order questions in ocean biogeochemistry, such as the roles of different “pumps” (e.g. solubility, soft-tissue, CaCO3) in storing carbon in the ocean’s interior and the extent of anthropogenic carbon uptake and ocean acidification.

The student will develop analysis procedures to determine the “density-space” carbon cycle in each of the IPCC models and compare them against this observational benchmark. This will be carried out in collaboration with Dr Andrew Meijers at the British Antarctic Survey, an expert in climate model analysis.

Project Timeline

Year 1

Develop understanding of physical oceanography, biogeochemistry, and the layered framework. Access and quality control available data of physical and biogeochemical measurements.

Year 2

Initial calculations of the “density-space” carbon cycle; evaluate uncertainties due to data sparsity. First manuscript. Begin model data access and analysis.

Year 3

Calculate the density-space carbon cycle in the IPCC models, compare with observations, and develop theory for mismatch.

Year 3.5

Prepare written manuscripts and write thesis.

& Skills

The student will gain specific training in observational oceanography, model data analysis, and numerical modelling, as well as broader education in biogeochemistry, physical oceanography, and climate science. The student will have the opportunity to go on an oceanographic research cruise relevant to (but not essential for) their project. The student will have the opportunity to attend a summer school or workshop on model analysis and intercomparison. Over the course of the PhD the student will gain transferable skills such as coding and software development, scientific writing, statistics, and data analysis, and problem-solving, as well as time management and working towards a long-term goal.

References & further reading

Groeskamp, S, SM Griffies, D Iudicone, R Marsh, AJG Nurser, & JD Zika, ‘The Water Mass Transformation Framework for Ocean Physics and Biogeochemistry’, Annual Review of Marine Science vol. 11, no. 1, 2019, pp. 271-305.
Groeskamp, S, A Lenton, R Matear, BM Sloyan, & C Langlais, ‘Anthropogenic carbon in the ocean-Surface to interior connections’, Global Biogeochemical Cycles vol. 30, no. 11, 2016, pp. 1682-1698.
Iudicone, D, KB Rodgers, Y Plancherel, O Aumont, T Ito, RM Key, G Madec, & M Ishii, ‘The formation of the ocean’s anthropogenic carbon reservoir’, Scientific Reports vol. 6, no. 1, 2016.
Meijers, AJ, ‘The Southern Ocean in the Coupled Model Intercomparison Project phase 5.’, Philos Trans A Math Phys Eng Sci vol. 372, no. 2019, 2014, pp. 20130296.
Naveira Garabato, AC, GA MacGilchrist, PJ Brown, DG Evans, AJS Meijers, & JD Zika, ‘High-latitude ocean ventilation and its role in Earth’s climate transitions.’, Philos Trans A Math Phys Eng Sci vol. 375, no. 2102, 2017.
Zika, JD, JM Gregory, EL McDonagh, A Marzocchi, & L Clément, ‘Recent Water Mass Changes Reveal Mechanisms of Ocean Warming’, Journal of Climate vol. 34, no. 9, 2021, pp. 3461-3479.

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