Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release

Abstract

Radiocarbon in the atmosphere is regulated largely by ocean circulation, which controls the sequestration of carbon dioxide (CO2) in the deep sea through atmosphere–ocean carbon exchange. During the last glaciation, lower atmospheric CO2 levels were accompanied by increased atmospheric radiocarbon concentrations that have been attributed to greater storage of CO2 in a poorly ventilated abyssal ocean1,2. The end of the ice age was marked by a rapid increase in atmospheric CO2 concentrations2 that coincided with reduced 14C/12C ratios (Δ14C) in the atmosphere3, suggesting the release of very ‘old’ (14C-depleted) CO2 from the deep ocean to the atmosphere3. Here we present radiocarbon records of surface and intermediate-depth waters from two sediment cores in the southwest Pacific and Southern oceans. We find a steady 170 per mil decrease in Δ14C that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. The atmospheric decrease in the radiocarbon signal coincides with regionally intensified upwelling and marine biological productivity4, suggesting that CO2 released by means of deep water upwelling in the Southern Ocean lost most of its original depleted-14C imprint as a result of exchange and isotopic equilibration with the atmosphere. Our data imply that the deglacial 14C depletion previously identified in the eastern tropical North Pacific5 must have involved contributions from sources other than the previously suggested carbon release by way of a deep Southern Ocean pathway5, and may reflect the expanded influence of the 14C-depleted North Pacific carbon reservoir across this interval. Accordingly, shallow water masses advecting north across the South Pacific in the early deglaciation had little or no residual 14C-depleted signals owing to degassing of CO2 and biological uptake in the Southern Ocean.

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Figure 1: Ocean circulation in the South Pacific and Southern Ocean and ambient water-mass patterns at the location of the sediment cores used in this study.
Figure 2: Records of radiocarbon activities, Antarctic temperatures and Southern Ocean upwelling across the last deglaciation.
Figure 3: Records of atmospheric CO 2 , radiocarbon activities, and surface δ 13 C and productivity across the last deglaciation.

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Acknowledgements

We thank the captain and crew of the RV Revelle, and our shipboard colleagues during the Zheng leg 3 (RR0503) cruise funded by the National Science Foundation (NSF), which collected the RR core. Core MD97-2120 was collected through the International Marine Past Global Change Study (IMAGES) program and with the technical support of the Institut Polaire Français Paul Emile Victor (IPEV) who made the research vessel Marion Dufresne available for core retrieval. H.J.S., E.L.S. and T.P.G., and the shore analyses, were supported by NSF awards and the Evolving Earth Foundation, and the Geological Society of America provided support for K.A.R. during her MSc. R.Z. acknowledges support from the MICINN, Spain. A portion of this work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory. We especially thank M. Cook for discussions, continuing input and suggestions throughout this study.

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K.A.R. participated in the RR0503 cruise, sampled cores, prepared sediments, speciated foraminifera for isotopic and radiocarbon analyses, performed all stable isotopic analyses and prepared figures. E.L.S. led the RR0503 cruise, sampled cores, speciated foraminifera for isotopic and radiocarbon analyses, prepared figures and wrote the paper. T.P.G. participated in the RR0503 cruise and performed all radiocarbon analyses. P.S. participated in the RR0503 cruise and identified all tephras. H.J.S. designed the study and, with T.M.H., supervised KAR during her MSc. R.Z. provided the MD core samples and supplementary data for that core. All authors contributed to the interpretation of the results and provided input on the manuscript.

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Correspondence to Elisabeth L. Sikes.

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Rose, K., Sikes, E., Guilderson, T. et al. Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release. Nature 466, 1093–1097 (2010). https://doi.org/10.1038/nature09288

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