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Friday, August 27, 2010

Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release by Kathryn A. Rose et al., Nature 466 (2010)

Nature, 466, 1093-1097 (26 August 2010); doi: 10.1038/nature09288

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

Kathryn A. Rose1,7,8, Elisabeth L. Sikes2,8, Thomas P. Guilderson3,4, Phil Shane5, Tessa M. Hill1, Rainer Zahn6 & Howard J. Spero1
  1. Department of Geology, University of California, Davis, California 95616, USA
  2. Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA
  3. Lawrence Livermore National Laboratory, Livermore, California 94550, USA
  4. Institute of Marine Sciences, University of California, Santa Cruz, California 95064, USA
  5. School of Environment, University of Auckland, Auckland 1142, New Zealand
  6. Institució Catalana de Recerca i Estudis Avançats, ICREA, Universitat Autònoma de Barcelona, Institut de Ciència i Tecnologia Ambientals, Departmento de Geologia, 08193 Bellaterra, Spain
  7. Present address: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA.
  8. These authors contributed equally to this work.
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 CO2levels were accompanied by increased atmospheric radiocarbon concentrations that have been attributed to greater storage of CO2in a poorly ventilated abyssal ocean1, 2. The end of the ice age was marked by a rapid increase in atmospheric CO2concentrations2 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 CO2released 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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