Abstract
The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.
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Data availability
The data produced in this study are available in Extended Data Tables and will also be made available at the NOAA (https://www.ncdc.noaa.gov/paleo/study/25230) and Pangaea data repositories.
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Acknowledgements
This work was supported by NERC Standard Grant NE/N003861/1 to J.W.B.R. and L.F.R., an NOAA Climate and Global Change VSP Fellowship to J.W.B.R, NERC Standard Grant NE/M004619/1 to A.B. and J.W.B.R., a NERC Strategic Environmental Science Capital Grant to A.B. and J.W.B.R., Marie Curie Career Integration Grant CIG14-631752 to A.B., an ERC consolidator grant to L.F.R., NSF grant OCE-1503129 to J.F.A., and NERC studentships to B.T. and E.L.
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J.W.B.R., A.B., and L.F.R. designed the study. A.B., L.F.R., T.C., and T.L. collected and uranium–thorium dated the coral samples. J.W.B.R., B.T., E.L., C.C., R.G., J.A.S., and D.C.N. made boron isotope analyses. J.W.B.R., A.B., L.F.R., and J.F.A. developed the interpretation and all authors contributed to the preparation of the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Deep Southern Ocean CO2 chemistry and atmospheric CO2 over the last 40,000 years.
Green triangles and blue diamonds show lower and upper cell deep-sea coral δ11B data, respectively. Individual subsamples are shown as small open symbols and mean values as larger filled symbols. Error bars on individual subsamples are equivalent to 2 s.d. analytical reproducibility and error bars on mean coral values represent 2 s.e. uncertainty on the mean of replicate subsamples (see Methods). Approximate pH values are given based on coral δ11B using the calibration in Extended Data Fig. 5, but uncertainty on this calibration is large (inset error bar), given the paucity of modern deep-sea coral data from low pH waters. Instead we focus on the δ11B values themselves, which provide a proxy of carbonate chemistry in their own right54. Synchronized ice core CO2 data36 are shown in grey symbols: circles from Dome C, dots from WAIS, stars from Taylor Dome, triangles from TALDICE, pluses from EDML, diamonds from Byrd, and squares from Siple Dome. Grey bands highlight intervals of CO2 rise.
Extended Data Fig. 2 Deep Southern Ocean CO2 chemistry and atmospheric CO2 over the last 40,000 years, highlighting the depths of upper cell corals.
Symbols and data are as plotted in Extended Data Fig. 1, but with the addition of the lower panel and annotations showing the depth in metres of each upper cell coral sample. No systematic offset is seen between samples from different depths. The only signal that occurs simultaneous with a change in depth is the decrease at ~11.5 kyr ago, but the jump back up to higher δ11B values following this event occurs without a change in depth, giving confidence that the excursion is not a depth-related signal. Furthermore the large excursion at ~14.7 kyr ago occurs without a significant change in depth. Note that all of the lower cell corals come from within 17 m water depth of each other.
Extended Data Fig. 3 Records of Southern Ocean biogeochemistry and CO2 over the last 40,000 years.
Data are plotted as in Fig. 2, but with opal flux23, a proxy for upwelling, deep sea coral δ15N62, a proxy for surface ocean nitrate consumption, and authigenic uranium concentrations16, a proxy for bottom water redox. The opal flux and authigenic uranium records combine two sediment cores: TN057-13-4PC in the younger part of the record (pluses) and TN057-14PC in the older part of the record (crosses). The opal flux records from each core are shown on separate scales. The coral δ15N data are grouped into samples from the Antarctic zone (AZ, blue) and Subantarctic zone (SAZ, red); smoothed fits to the data are shown, as provided in the original study62. Intervals of low CO2 during the last ice age are associated with low upwelling, an efficient biological pump, low oxygen water rich in respired carbon, and low-pH carbon-rich water in the deep Southern Ocean.
Extended Data Fig. 4 Deglacial records of Southern Ocean CO2 chemistry and opal fluxes, and climate over Antarctica and Greenland.
Data are plotted as in Figs. 2, 3, but with opal flux23, a proxy for upwelling, surface ocean-atmosphere CO2 difference, based on δ11B in planktic foraminifera26, and radiocarbon data4,25 from corals within these sample groupings, shown as 14C age offsets compared to the contemporaneous atmosphere. Intervals of rising CO2 in the atmosphere are associated with input of waters rich in CO2 and nutrients to the upper reaches of the Southern Ocean. Radiocarbon ages reflect the competing influences of upwelling of 14C-depleted waters and improved ventilation over the deglaciation.
Extended Data Fig. 5 Boron isotope calibration for modern D. dianthus.
Data are from open ocean sites44,56,60, with two additional recent (<1,650 years ago) samples from the Southern Ocean from this study. Water column δ11B of borate (B(OH)4−) values are as previously published or are calculated from carbonate chemistry data from nearby GLODAPv2 sites for the new samples, as described47,54. Note that the sensitivity of δ11B in carbonates to pH is based on the pH sensitivity of δ11B of borate. pH itself is not easily shown on a plot like this, as the relationship between δ11B of borate and pH is also somewhat influenced by water temperature, salinity, and depth54. A power law function was fitted to the data using Matlab’s curve fitting toolbox (solid line: δ11BCoral = −1.8214 × δ11BB(OH)4−−12.22 + 27.03; R2 = 0.57). Dashed lines show the 95% confidence intervals and give a measure of calibration uncertainty as shown in the error bar in Extended Data Fig. 1, although data from a given site may be able to record relative changes in pH more sensitively, as seen in many paleo-proxies.
Extended Data Fig. 6 Replicate subsamples from a D. dianthus septum.
To test for the potential influence of microstructural variability in composition, a coral septum was divided into four areas, which were then split into chunks of approximately 0.2, 0.8, 3.2 and 11 mg. These were then individually crushed, cleaned, and analysed. This sample treatment was designed to preserve heterogeneity between subsamples, although note that the clustering of subsamples of a given size from a certain area of the coral may lead to that group recording a slightly different signal (as seen in the 3.2 mg group). The lines in the middle panel show the mean and 2 s.d., excluding one outlier in the 11 mg group. δ11B is correlated with Mg/Ca and U/Ca, showing the influence of internal variability in coral composition.
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Rae, J.W.B., Burke, A., Robinson, L.F. et al. CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales. Nature 562, 569–573 (2018). https://doi.org/10.1038/s41586-018-0614-0
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DOI: https://doi.org/10.1038/s41586-018-0614-0
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