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Published February 12, 2015 | metadata_only
Journal Article

Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation


Atmospheric CO_2 fluctuations over glacial–interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial–interglacial atmospheric CO_2 variations invoke changes in deep-ocean carbon storage, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO_2 rise observed in ice-core records. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling, do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO_2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO_2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO_2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial–interglacial ocean–atmosphere exchange of carbon.

Additional Information

© 2015 Macmillan Publishers Limited. Received 22 August; accepted 11 December 2014. We thank the International Ocean Drilling Program for providing samples from ODP Leg 202, R. Gersonde and A. Mackensen for the PS2498-1 core material, J. F.McManus for sharing his unpublished benthic isotope data for ODP1238, and E. J. Rohling, M. P. Hain and C. Beaulieu for discussions. For the calibration of G. bulloides, we thank M. Kucera for providing core-top samples from the archives at the University of Tübingen, H. C. Bostock for samples from the National Institute for Water and Atmospheric Research, Wellington, and B. J. Marshall and R. Thunell for samples from the Cariaco Basin sediment trap time series. J. A. Milton, M. J. Cooper and A. Michalik provided assistance during ICP-MS analyses and sample preparation in the laboratory. C. Alt and M. T. Horigome helped with foraminifera picking. We thank the other members of 'The B-Team' at the National Oceanography Centre Southampton for their contributions. Financial support was provided by the European Community through a Marie Curie Intra-European Fellowship for Career Development to M.A.M.-B., the Universitat Autònoma de Barcelona through a Postdoctoral Research Grant to G.M., the Spanish Ministry of Science and Innovation (PROCARSO project CGL2009-10806) to G.M., P.Z. and P.G.M., a NERC PhD studentship awarded to M.J.H., a NOAA/UCAR Climate and Global Change Postdoctoral Fellowship to J.W.B.R., and NERC grant NE/D00876/X2 to G.L.F. G.M. was also supported by the Australian Laureate Fellowship project FL120100050 (E. J. Rohling).

Additional details

August 20, 2023
August 20, 2023