Scientific Challenge: Significant questions remain over how chemical and/or biological processes drive calcium carbonate dissolution in the ocean where mineral saturation states and pH levels are high. This studentship will explore the biological processes (e.g., digestion, respiration) and moderators (e.g., mineralogy, organic coatings) that may drive calcium carbonate dissolution in the ocean. The studentship will derive mass-specific dissolution rates to compare abiotic and biotic dissolution mechanisms, field measurements, and rates derived from global biogeochemical models.
Annually, the ocean absorbs ~25% of anthropogenic CO2 emissions through chemical, physical, and biological processes. Marine organisms use calcium carbonate to form skeletons and hard parts, with its formation and sinking linked to removal of alkalinity from the surface ocean, which then decreases the ability of seawater to absorb further CO2. Mechanisms to retain alkalinity in the upper ocean then become important to understand, with our current global models failing to accurately account for these processes, introducing large uncertainties in our ability to understand the present day or future carbon cycle. The remineralisation (dissolution) of calcium carbonate is currently best understood in a purely geochemical sense, with saturation state, pH and mineralogy considered the major drivers of dissolution.
Rates of dissolution at low saturation states (<1) are on the scale of 0.1 to 1% per day, though the average saturation state for the upper ocean is >3. Specific conditions are then needed to drive the dissolution required to balance the ocean’s alkalinity budget. Measurements of calcification and calcium carbonate standing stocks, from in situ or satellite observations, imply a turnover and loss of calcium carbonate on the scale of at least 10% per day, an order of magnitude faster than purely abiotic driven dissolution rates. Biologically driven dissolution is likely to be a major process in the ocean; with several candidate mechanisms suggested. These include zooplankton ingestion and gut digestion, though acidic guts are required, and micro-environments where respiration drives low saturation states. However, zooplankton ingest particles far larger than common calcium carbonate particles in the ocean, and sinking speeds of marine aggregates are too fast to allow significant losses of calcium carbonate before they have sunk out of the upper ocean.
Methodology: The studentship will be networked with several funded research projects examining calcium carbonate formation and dissolution in the ocean, and how these impact the global marine carbon cycle. Individually these projects are examining loss processes or budgetary losses of calcium carbonate; what they lack is an interdisciplinary synthesis of dissolution rate measurements or a quantitative comparison of how moderators such as mineralogy, morphology or organic matter impact these rates. The studentship will take the lead on experiments to examine how respiration of organic matter impacts the dissolution of sedimentary and pelagic calcium carbonate.
There are opportunities for the student to join field work, in UK shelf waters and/or shelf waters of the California Current, where the student can perform further novel in-situ experiments and observations. The studentship will have access to a wide array of observation and experimental facilities, both at Heriot-Watt and provided by project partners, as well as analytical equipment to determine carbonate chemistry, mineralogy (e.g., FTIR, Raman, XRD, SEM), and organic matter composition. Opportunities also exist for the student to undertake, with training provided, global biogeochemical modelling of dissolution rates.
Anticipated outcomes: An improved and up-to-date understanding of biologically mediated calcium carbonate dissolution in the ocean, crucial to inform model parameterizations. Calcium carbonate dissolution is an important consideration in terms of current plans and ideas around adding alkalinity to the ocean (Ocean Alkalinity Enhancement) as an ocean Carbon Dioxide Removal (ocean CDR) scheme.
Eligibility: This project is open to all students, whether home, EU or overseas.
The successful candidate will have at least an upper second-class (2:1) undergraduate honours degree and ideally a Masters Degree in a relevant area.
Essential - A background in marine science or oceanography, with familiarity of seawater chemistry and ocean biogeochemistry. General laboratory skills and/or experience are also needed, including knowledge of analytical techniques though further training will be provided. Desirable - Specific analytical skills, such as analysis of water chemistry or image analysis, coding skills (R, Python) to allow model use.
We recognise that not every talented researcher will have had the same opportunities to advance their careers. We will account for any particular circumstances that applicants disclose (e.g. parental leave, caring duties, disabilities etc.) to ensure an inclusive and fair recruitment process.
How to Apply: To apply you must complete our online application form.
Select PhD Marine Biology as the programme and include the full project title, reference number and supervisor name on your application form. Ensure that all fields marked as ‘required’ are complete.
After entering your personal details, click submit. You will be asked to upload your supporting documents. You must complete the section marked project proposal; provide a supporting statement (1-2 A4 pages) documenting your reasons for applying to this particular project, outlining your suitability and how you would approach the project. You must also upload your CV, a copy of your degree certificate and relevant transcripts and an academic reference in the relevant section of the application form.
You must also provide proof of your ability in the English language (if English is not your mother tongue). We require an IELTS certificate showing an overall score of at least 6.5 with no component scoring less than 6.0, or a TOEFL certificate with an overall score of at least 85, including reading 20, listening 19, speaking 20 and writing 21. Alternatively, if you have received an English-taught Bachelors or Masters degree from one of the countries listed on the UK Government Guidance under ‘Who does not need to prove their knowledge of English’, and it was obtained less than two years from your intended start date, you should provide evidence of your award that clearly states it was delivered and assessed in English language.
Please contact the primary supervisor, Alex Poulton ([Email Address Removed]), for further information or an informal discussion.
Timeline: Applicants must be available to start in September 2024.
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