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

• Published in: Nature (London) Vol. 518; no. 7538; pp. 219 - 222
• Main Authors: Martínez-Botí, M. A., Marino, G., Foster, G. L., Ziveri, P., Henehan, M. J., Rae, J. W. B., Mortyn, P. G., Vance, D.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12-02-2015; Nature Publishing Group
• Subjects: 140/58; 704/106/2738; 704/106/413; Atmosphere; Atmosphere - chemistry; Boron; Boron - analysis; Boron - chemistry; Carbon cycle; Carbon dioxide; Carbon Dioxide - analysis; Climate; Climate system; Deep water; Deglaciation; Environmental aspects; Foraminifera; Freezing; Glaciers; History, Ancient; Humanities and Social Sciences; Hydrogen-Ion Concentration; Ice Cover - chemistry; Identification and classification; Isotopes; letter; multidisciplinary; Natural history; Ocean; Ocean-atmosphere interaction; Oceans; Oceans and Seas; Reservoirs; Science; Seawater - chemistry; Surface water; United Kingdom; Southern Ocean
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14155

The boron isotope pH proxy in sediment-core planktic foraminifera is used as a tracer of oceanic CO
2
outgassing to show that surface waters which derive partly from deep water upwelled in the Southern Ocean became a significant source of carbon to the atmosphere during the last deglaciation.
Oceanic CO
2
loss during the last deglaciation
It is thought that ventilation of a deep ocean carbon reservoir in the Southern Ocean played an important role in the deglacial atmospheric CO
2
rise, but there has been no direct documentation of changes in surface ocean carbon content during deglaciations. Boron isotope pH proxy data, a more direct tracer of oceanic CO
2
outgassing, now show that surface waters — partly derived from deep water upwelled in the Southern Ocean — became a significant source of carbon to the atmosphere 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
1
,
2
, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated
3
. 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
4
,
5
,
6
. 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
6
, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling
4
,
7
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
8
,
9
, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO
2
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.