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Millennial-scale changes in atmospheric CO2 levels linked to the Southern Ocean carbon isotope gradient and dust flux

Abstract

The rise in atmospheric CO2 concentrations observed at the end of glacial periods has, at least in part, been attributed to the upwelling of carbon-rich deep water in the Southern Ocean1,2. The magnitude of outgassing of dissolved CO2, however, is influenced by the biological fixation of upwelled inorganic carbon and its transfer back to the deep sea as organic carbon. The efficiency of this biological pump is controlled by the extent of nutrient utilization, which can be stimulated by the delivery of iron by atmospheric dust particles3. Changes in nutrient utilization should be reflected in the δ13C gradient between intermediate and deep waters. Here we use the δ13C values of intermediate- and bottom-dwelling foraminifera to reconstruct the carbon isotope gradient between thermocline and abyssal water in the subantarctic zone of the South Atlantic Ocean over the past 360,000 years. We find millennial-scale oscillations of the carbon isotope gradient that correspond to changes in dust flux and atmospheric CO2 concentrations as reported from Antarctic ice cores4,5. We interpret this correlation as a relationship between the efficiency of the biological pump and fertilization by dust-borne iron. As the correlation is exponential, we suggest that the sensitivity of the biological pump to dust-borne iron fertilization may be increased when the background dust flux is low.

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Figure 1: The South Atlantic overturning circulation.
Figure 2: The palaeocenaographic records of MD02-2588.

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Acknowledgements

We thank the International Marine Past Global Changes Study (IMAGES) and Institut Polaire Français Paul Emile Victor (IPEV) for making the RV Marion Dufresne available and for technical support. We are indebted to G. Uenzelmann-Neben for generously providing geophysical survey data. J. Becker and H. Medley are thanked for technical assistance. We acknowledge financial support from the UK Natural Environment Research Council (NERC) and NERC Radiocarbon Laboratory (I.R.H. and P.D.), the Spanish Ministerio de Educación y Ciencia (MEC) (R.Z. and P.D., postdoctoral research grant EX-2004–0918), The Seventh Framework Programme PEOPLE Work Programme Grant 238512 (I.R.H., R.Z. and M.Z., Marie Curie Initial Training Network ‘GATEWAYS’, www.gateways-itn.eu) and the Climate Change Consortium of Wales (M.Z. and I.R.H.; www.c3wales.org).

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I.R.H. and R.Z. collected the core material. P.D. performed preparation for foraminiferal stable isotope analysis. Stable isotope analyses were carried out in Cardiff University under the guidance of I.R.H. All authors contributed to data analysis and interpretation. M.Z. wrote the manuscript with contributions from all other authors.

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Correspondence to Martin Ziegler or Ian R. Hall.

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The authors declare no competing financial interests.

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Ziegler, M., Diz, P., Hall, I. et al. Millennial-scale changes in atmospheric CO2 levels linked to the Southern Ocean carbon isotope gradient and dust flux. Nature Geosci 6, 457–461 (2013). https://doi.org/10.1038/ngeo1782

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