The exact cause of well-documented natural cycles in CO2 is hotly debated. The rapidity and scale of the changes however points toward variations in the amount of carbon stored in the ocean as being the main culprit. By studying these natural variations of CO2 we gain a better understanding of the carbon cycle and hence how the climate system and CO2 interact.
1. What processes are responsible for changing atmospheric CO2 on glacial-interglacial timescales?
2. Are there particular regions of the ocean, and particular processes, that are key in driving change? - what is the importance of the Southern Ocean?
3. How do carbon cycle feedbacks operate in different climate states?
How do we do it?
The ocean holds around 60 times the CO2 of the atmosphere. On geological timescales the ocean therefore drives atmospheric CO2 change. The processes responsible leave tell tale fingerprints in the carbonate system of the surface and deep ocean.
The boron isotope pH proxy is a sensitive tracer of ocean pH. Ocean pH is a component of the ocean carbonate system and so by knowing pH we place quantitative constraints on how much CO2 is stored in the ocean at any one time.
We use the boron isotope pH proxy in planktic foraminfera, benthic foraminifera and deep sea coral (picture left) to reconstruct the pH of the ocean in the past to reveal what processes and which parts of the ocean are key in driving glacial-interglacial CO2.
Gray, W.R., Rae, J.W.B., Wills, R.C., Shevenell, A.E., Taylor, B., Burke, A., Foster, G.L., Lear, C.H. (2018) Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean, Nature Geoscience, doi:10.1038/s41561-018-0108-6. News and Views by Sam Jaccard and Eric Galbraith
Stewart, J.A., Anagnostou, E., Foster, G.L., (2016), An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite, Chemical Geology, 447, 148-160 (doi.org/10.1016/j.chemgeo.2016.10.029.
Martínez-Botí, M.A., Marino, G., Foster, G.L., Ziveri, P., Henehan, M.J., Rae, J.W.B., Mortyn, P.G. and Vance, D. (2015) Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation. Nature, 518, (7538), 219-222. (doi:10.1038/nature14155).
Naik, Sushant S., Divakar Naidu, P., Foster, Gavin L. and Martínez-Botí, Miguel A. (2015) Tracing the strength of the southwest monsoon using boron isotopes in the eastern Arabian Sea. Geophysical Research Letters, 42, (5), 1450-1458. (doi:10.1002/2015GL063089).
Foster, G.L. and Sexton, P.F. (2014) Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial. Geology, 42, (11), 1003-1006. (doi:10.1130/G35806.1).
Rae, James W.B., Sarnthein, Michael, Foster, Gavin L., Ridgwell, Andy, Grootes, Pieter M. and Elliott, Tim (2014) Deep water formation in the North Pacific and deglacial CO2 rise. Paleoceanography, 29, (6), 645-667. (doi:10.1002/2013PA002570).