IAP-24-094
Anthropogenic ocean carbon uptake and acidification – establishing the “hockey stick” for ocean CO2
Ocean acidification is described as “the other CO2 problem”1, with impacts on a diverse array of marine life and the ecosystems and communities they support. Yet compared to global warming, our understanding of this problem – in particular with respect to its historical context – is in its infancy. Paleoclimate-based reconstructions of temperature have proved critical to our understanding of global warming, underpinning the famous “hockey stick” shaped graph, where a relatively short instrumental record of warming is placed against the paleo context of pre-industrial conditions. Yet observations of ocean pH are substantially more limited than those of temperature: the longest continuous pH records date from only 19832, and until the 2000s such records are available from only a handful of sites, confounding efforts to place observed acidification in the context of natural variability, or to examine its impacts on calcifying organisms. Might natural variations in surface ocean pH, for instance due to variable upwelling, be of similar magnitude to anthropogenic acidification, such that organisms might be reasonably well adapted to this degree of change? Or is acidification already well-outside the norm in all regions of the ocean? And can impacts be seen in marine calcifying organisms, on timescales beyond those of culture experiments? The only way to directly address these critical questions is by reconstructing the paleo record of recent ocean acidity, which is the central aim of this PhD project. This is important, in addition, for understanding the history and future of the ocean carbon sink3, which has taken up roughly a third of anthropogenic CO2 emissions to date, and for establishing a baseline against which future proposals to enhance ocean carbon uptake artificially may be evaluated4.
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Image Captions
Figure 1: Records of anthropogenic carbon uptake from carbon isotopes (top panel) and its apparent impact on calcifiers (bottom panel) in high resolution sediment cores (Pallacks et al., 2023). Equivalent records of ocean acidity itself – the central and yet still missing piece of this story – will be produced in this project.
Methodology
To transform our understanding of anthropogenic carbon uptake by the ocean, and resulting ocean acidification, this project will create detailed reconstructions of ocean pH spanning the common era – the past 2000 years. This will be achieved using the boron isotope proxy for ocean pH5 in the shells of foraminifera, supplemented by equivalent data from banded corals. Pilot work6 with high resolution sediment cores demonstrates the ability to pick up signals of anthropogenic carbon uptake using its carbon isotope fingerprint (sometimes called the “Suess effect”) measured in foram shells and indicates an associated change in foram calcification, through size-normalised foram shell weights (Figure 1). What’s missing is an equivalent record of ocean acidification itself, critical given that pH decline and associated decrease in carbonate saturation state (Omega) are the means through which anthropogenic carbon uptake may impact calcifying organisms. At present such data include only a very short instrumental record and a handful of boron isotope datapoints6, supplemented by model-derived estimates based on atmospheric CO2 change. Recent advances in boron isotope analysis pioneered in our lab7 now give an opportunity to transform this situation, by allowing us to make high precision measurements on small, high-resolution samples, opening the door to pH reconstructions with sub-decadal resolution spanning multiple centuries. These will be developed in this project from regions spanning different carbon uptake regimes and degrees of natural upwelling and variability.
To examine the potential impact of acidification on key calcifying organisms, pH records will be compared to size-normalised shell weights from foraminifera, and measures of calcification from coccolithiphores. To probe deeper into patterns of calcification change, discrete samples will be “x-rayed”, using micro-CT or synchrotron analysis to examine structural changes internal to these shells.
Depending on student interest, data generation may be complemented by work with Earth system models, using model-data reanalysis to improve global quantification of oceanic uptake of anthropogenic CO2.
Project Timeline
Year 1
Training in high-precision boron isotope analysis, including sample preparation, purification, and analysis by multi-collector inductively-coupled-plasma mass spectrometry (MC-ICPMS). Complementary lab-based training in trace element analysis by ICPMS and identification of foraminifera. Examination of patterns of recent carbon uptake from ocean data and models. Generation of an initial pH reconstruction.
Year 2
Generation of pH reconstructions from additional sites. Measurement of x-ray data on calcification change. Presentation of initial results at a national conference (e.g. the Geochemistry Group Research in Progress meeting or the Challenger marine science conference) and write up first paper.
Year 3
Depending on student interest, further data generation to build global coverage, or work on data assimilation in an Earth system model. Presentation of findings at an international conference (e.g. Goldschmidt or AGU) and write up additional papers.
Year 3.5
Write, submit and defend PhD thesis
Training
& Skills
Training in geochemistry techniques for the measurement of trace metal concentrations and novel isotope systems: triple quadrupole inductively coupled plasma mass spectrometry (QQQ-ICPMS) and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS)
Training in calcifier morphometrics and microanalytical techniques
Training in Earth system modelling
Training in use of big data – for ocean carbon datasets
References & further reading
1. Doney, S.C., Fabry, V.J., Feely, R.A. and Kleypas, J.A., 2009. Ocean acidification: the other CO2 problem. Annual review of marine science, 1(1), pp.169-192.
2. Bates, N.R., Astor, Y.M., Church, M.J., Currie, K., Dore, J.E., González-Dávila, M., Lorenzoni, L., Muller-Karger, F., Olafsson, J. and Santana-Casiano, J.M., 2014. A time-series view of changing surface ocean chemistry due to ocean uptake of anthropogenic CO₂ and ocean acidification. Oceanography, 27(1), pp.126-141.
3. Landschützer, P., Gruber, N., Bakker, D.C. and Schuster, U., 2014. Recent variability of the global ocean carbon sink. Global Biogeochemical Cycles, 28(9), pp.927-949.
4. Siegel, D.A., DeVries, T., Doney, S.C. and Bell, T., 2021. Assessing the sequestration time scales of some ocean-based carbon dioxide reduction strategies. Environmental Research Letters, 16(10), p.104003.
5. Rae, J.W.B., 2018. Boron isotopes in Foraminifera: Systematics, biomineralisation, and CO2 reconstruction. Boron isotopes: The fifth element, pp.107-143.
6. Pallacks, S., Ziveri, P., Schiebel, R., Vonhof, H., Rae, J.W., Littley, E., Garcia-Orellana, J., Langer, G., Grelaud, M. and Martrat, B., 2023. Anthropogenic acidification of surface waters drives decreased biogenic calcification in the Mediterranean Sea. Communications Earth & Environment, 4(1), p.301.
7. Trudgill, M., Nuber, S., Block, H.E., Crumpton‐Banks, J., Jurikova, H., Littley, E., Shankle, M., Xu, C., Steele, R.C. and Rae, J.W., 2024. A simple, low‐blank batch purification method for high‐precision boron isotope analysis. Geochemistry, Geophysics, Geosystems, 25(3), p.e2023GC011350.
