IAP-25-097
The coral biomineralisation response to temperature and pH change in the Central Pacific over the last 500ka
Tropical corals produce the aragonite skeletons which underpin coral reefs and have a substantial value as resources for fisheries, tourism and land protection. Coral biomineralization is influenced by multiple factors. The aragonite skeletons of tropical corals form at specialist calcification sites. The calcification media at the sites are sourced from seawater but the corals increases the media pH, shifting the dissolved inorganic carbon (DIC) equilibrium in favour of CO32- and promoting the formation of aragonite. Calcification is influenced by both temperature and seawater pCO2 (influencing seawater pH). In addition, coral skeletons contain biomolecules e.g. proteins and lipids, which influence the formation and structure of aragonite.
The aim of this project is to reconstruct environmental change and the coral biomineralisation response to change in the Central Pacific by analysing fossil corals from the Hawaiian Islands. Tropical corals record information on the seawater composition, temperature and pH in their skeletons at the time of their deposition1. The Hawai’ian islands are shield volcanos that formed over a volcanic hotspot and then subsided as they move away from the hotspot. Fringing coral reefs formed around the islands and accreted rapidly to keep pace with the change in relative sea level (as the island subsided). During periods of rapid relative sea level rise the reefs gave up and drowned, forming multiple terraces that are preserved at depths from ~150m to >1000m below present sea level (Figure 1). A large suite of fossil Porites spp. have been collected from the terraces around the Big Island of Hawaii during a recent IODP expedition (IODP 389: Hawaiian Drowned Reefs) and in previous work. Dating demonstrates that the reefs grew episodically but contain corals which grew over the last five glacial cycles. Global sea level and atmospheric CO2 display cyclical variations over this period (Figure 2). The reefs are a unique resource to explore changes in coral biomineralisation in response to climate variations.
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Image Captions
Figure 1. Expedition 389 final site locations from five regions around the island, overlaid on the bathymetric map for offshore Hawai’i.,Figure 2. Plots of a) relative sea level (RSL), m below present day, over the last 500 kyr2 and b) atmospheric pCO2 from the Vostok ice core record3.,Figure 3. pH of coral calcification media pHECM (pH total scale) in 4 different Porites coral genotypes (G4 to G7) cultured at varying seawater pCO2 and temperature (Allison et al., 2021). Hatched bars indicate results at 28°C, and unhatched bars are 25°C. Seawater pCO2 was approximately 180 and 260 µatm in the last Glacial maximum and in the interglacial respectively. Present day seawater pCO2 is ~400 µatm while the 750 µatm treatment represents a future scenario.
Methodology
The student will investigate relationships between coral biomineralisation and climate and seawater pH in the IODP 389 corals using a series of geochemical tools. Coral aragonite δ11B is used as a proxy of coral calcification media pH (see ref 4 for a review) and is influenced by external seawater pH (Figure 3). δ11B analysis of coral skeletons has been used to track regional changes in ocean pH5 and to explore the response of different coral genotypes to ocean acidification6. The student will analyse modern and fossil Hawaiian coral specimens for δ11B to estimate coral calcification media pH. The student will also apply other palaeoproxies e.g. Sr/Ca, Li/Mg, to reconstruct past seawater temperatures and conditions1. The student will estimate calcification rates in the corals from the core x-radiographs which record the annual skeletal density changes in the corals. Recent research by our groups shows that seawater pCO2 and/or temperature also influence the biomolecule compositions of coral skeletons7, their nanograin structure8 and the aragonite lattice9. The student will have access to these techniques to fully explore the biomineralisation response to CO2. Corals will be examined carefully (XRD, petrography) to identify diagenetic alteration which can significantly alter the geochemistry of the primary coral skeleton and overwrite the climate signature10.
This research will indicate how variations in atmospheric CO2 (reconstructed from ice cores, Figure 2b) influence the calcification media pH of massive corals in the central Pacific and will identify how changes in coral calcification media pH relate to coral calcification rate. The PhD research will demonstrate how coral biomineralisation responds to changes in atmospheric CO2 and temperature and will answer key questions. For example, was the response of coral calcification to seawater temperature and pH change different in the past (when change was relatively slow) compared to the present day (when change is rapid). This information is critical in predicting the future of coral reefs in a changing climate.
Project Timeline
Year 1
Develop familiarity with coral biomineralisation processes and palaeoproxies. Screen coral samples for diagenetic alteration in collaboration with Allison and van der Land. Begin d11B analysis of coral samples with Allison and Chen. Apply to and, if successful, attend summer school in development of geochemical and/or microscopy techniques e.g. European Microbeam Society.
Year 2
Focus on d11B and trace element analysis of skeletons. Apply to, and if successful, attend the NERC secondary ion mass spectrometry facility for microbeam analysis of the skeletons. Visit to one of the collaborative partners to develop sampling plans. Undertake Raman spectroscopy and skeletal biomolecule analysis if desired. Attend national conference e.g. MASTS. Plan thesis chapters.
Year 3
Reconstruct environmental conditions at the times of coral growth using geochemical palaeothermometers and resolve relationships between coral biomineralisation processes and local environment. Submit manuscript. Attend international conference e.g. Gordon Research Conference on Biomineralisation.
Year 3.5
Write up and submit thesis. Finalise manuscripts for publication.
Training
& Skills
The student will develop a multidisciplinary skillset gain in geochemical laboratory techniques as well as training and expertise in coral biomineralisation, isotope geochemistry and palaeoclimate reconstruction. The student will join a NERC-funded team studying coral biomineralisation and carbonate palaeoproxy development in the School of Earth and Environmental Sciences at the University of St Andrews. In addition, the student will gain transferable skills in scientific writing, data analysis, statistics, problem solving and presenting. The student will be a member of the Marine Alliance for Science and Technology for Scotland (www.masts.ac.uk), enabling wider access to training and networking opportunities.
References & further reading
1. Thompson DM. Environmental records from coral skeletons: A decade of novel insights and innovation. Wiley Interdisciplinary Reviews: Climate Change.13(1):e745, 2022
2. Grant KM et al., Sea-level variability over five glacial cycles. Nature communications. 25;5(1):5076, 2014.
3. Petit JR et al, Climate and atmospheric history of the past 420 ky from the Vostok ice core, Antarctica, Nature, 399, 429-436, 1999.
4. Allison N, Venn AA, Tambutte S, Tambutte E, Kasemann S, Wilckens F and EIMF, A comparison of SNARF-1 and skeletal δ11B estimates of calcification media pH in tropical coral, Geochimica et Cosmochimica Acta, 355, 184-194, 2023.
5. Shinjo R et al., Ocean acidification trend in the tropical North Pacific since the mid-20th century reconstructed from a coral archive, Mar. Geol., 342, 58-64, 2013
6. Allison N, Cole C, Hintz C, Hintz K, Rae J & Finch A, Resolving the interactions of ocean acidification and temperature on coral calcification media pH. Coral Reefs, https://doi.org/10.1007/s00338-021-02170-2, 2021.
7. Kellock C et al., The role of aspartic acid in reducing coral calcification under ocean acidification conditions, Scientific Reports, https://doi.org/10.1038/s41598-020-69556-0, 2020.
8. Tan CD, Hähner G, Fitzer S, Cole C, Finch A, Hintz C, Hintz K and Allison N, The response of coral skeletal nano-structure and hardness to ocean acidification conditions, Royal Society Open Science, 10, 230248, 2023.
9. Allison N, Ross P, Castillo Alvarez C, Penkman K, Kröger R, Kellock C, Cole C, Clog M, Evans E, Hintz C, Hintz K Finch AA, The influence of seawater pCO2 and temperature on the amino acid composition and aragonite CO3 disorder of coral skeletons, Coral Reefs, 43, 1317-1329, 2024.
10. Allison N et al., Palaeoenvironmental records from fossil corals: the effects of submarine diagenesis on temperature and climate estimates, Geochim. Cosmochim. Acta., 71, 4693-4703, 2007
