Past ocean oxygen and pH changes

Biogeochemical Cycles



The oceans are expected to become less oxygenated and more acidic in relation to future warming and anthropogenic greenhouse gas emissions. These changes in seawater chemistry will negatively impact marine ecosystem and global biogeochemical cycles.
To understand the natural, longer-term cycling of seawater oxygen concentrations and pH across the geological record of environmental change, several novel proxy methods were recently developed. Until now individual projects have focussed on reconstructing either oxygenation or pH, even though deoxygenation and ocean acidification seldom occur in isolation.

This project will consolidate recent progress made in such methodologies, using novel trace element ratios to concomitantly reconstruct seawater oxygen and pH during key warm intervals of the last ca. 3.6 million years, allowing to assess environmental conditions during intervals that were warmer, similar to, and colder than today. This may include the Piancenzian (2.6 to 3.6 Ma), and more recent interglacials of the Pleistocene.

The project will utilise geochemical information contained in the calcite shells of microscopic fossil foraminifera (Fig. 1). Bottom water and surface water oxygen reconstructions will be based on of carbon isotope gradients between different species of benthic foraminifera (e.g. Hoogakker et al., 2015), complemented by morphology and trace element measurements, and planktonic foraminifera iodine/calcium ratios (Lu et al., 2016). Bottom and surface water pH will be reconstructed using the boron isotope composition (11B) of benthic planktonic foraminifera (Foster & Rae 2016). These methods have provided several high profile reconstructions (e.g. de la Vega et al., 2020, Hoogakker 2018).

The tropical Pacific holds the worlds’ largest oxygen minimum zone (Fig. 2), which are of low pH. Reconstructions will focus on this area, utilizing sample material from the International Ocean Discovery Program (

The student will be trained in combining oxygen and pH reconstructions from critical periods with Earth System Model (cGENIE) simulations to identify and assess links between climate variability and ocean chemistry changes.

Click on an image to expand

Image Captions

Fig 1. Photographs of examples of planktonic (floating surface and subsurface waters) and benthic (seafloor dwelling) foraminifera: a – planktonic Globigerina bulloides, b – benthic Uvigerina.

Fig. 2. Top: Oxygen concentrations in the Pacific at 125 meters water depth. Bottom: Pacific surface water (top 50 meters) pH (adapted from Raven et al., 2005).


Marine carbonate samples are available from the IODP. Sample processing, and foraminifera identification, picking and cleaning will be carried out at Heriot-Watt’s Lyell Centre, while benthic foraminifera stable isotopes (for bottom water oxygen and age model reconstructions) will be measured at BGS Keyworth using IRMS. Benthic foraminifera B/Ca (for bottom water pH) will be analyzed at the Australian National University in collaboration with advisor Dr Jimin Yu, while planktonic foraminifera will be analysed for boron isotopes and trace elements (for surface water pH) in the St Andrews Isotope Geochemistry (STAiG) lab. Planktonic foraminifera I/Ca ratios (for surface water oxygenation) will be measured at BGS Keyworth.

The project allows flexibility, with the opportunity to focus on approaches, time intervals, and techniques of particular interest to the student.

Project Timeline

Year 1

Training in foraminifera identification & cleaning, clean laboratory methods and mass spectrometry, initial measurements and training, literature review.

Year 2

Generate records across key warm intervals. cGENIE modelling. First manuscript.

Year 3

Finalize data sets including higher resolution intervals, prepare written manuscripts and write thesis.

Year 3.5

Preparation of written manuscripts; finalization thesis.

& Skills

The student will gain specific training in foraminifera identification, mass spectrometry, clean lab chemistry, and geochemical modelling, as well as broader education in geochemistry, oceanography, and climate science. To expand their network and skills, the student will participate in the Urbino Summerschool in Paleoclimatology in their first year, and spend two months at partner Dr Yu’s laboratory at the Australian National University in their second year.
Over the course of the PhD the student will gain transferable skills such as scientific writing, statistics and data analysis, and problem-solving, as well as time management and working towards a long-term goal.
The student will furthermore benefit from partnerships with Prof Leng (BGS), Prof Ridgwell (University of California Riverside), and Dr Pichevin (University of Edinburgh).

References & further reading

De la Vega et al. (2020), Sci. Rep., 10:11002.
Foster & Rae (2016), AnnRev, 44, (207-237).
Hoogakker et al. (2015), Nat. Geosci., 15, 40-44.
Hoogakker et al. (2018), Nature, 562, 410-414.
Lu et al., (2016), Nat. Comm. 7:11146.
Raven et al. (2005) Royal Society Policy document 12/05.

Further Information

For further information, please contact:
Dr Babette Hoogakker
The Lyell Centre
Heriot-Watt University
EH14 4AS
+44 (0)1314513912

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