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Volume 17, issue 23 | Copyright
Atmos. Chem. Phys., 17, 14771-14784, 2017
https://doi.org/10.5194/acp-17-14771-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 12 Dec 2017

Research article | 12 Dec 2017

Sixty years of radiocarbon dioxide measurements at Wellington, New Zealand: 1954–2014

Jocelyn C. Turnbull1,2, Sara E. Mikaloff Fletcher3, India Ansell1, Gordon W. Brailsford3, Rowena C. Moss3, Margaret W. Norris1, and Kay Steinkamp3 Jocelyn C. Turnbull et al.
  • 1GNS Science, Rafter Radiocarbon Laboratory, Lower Hutt, New Zealand
  • 2CIRES, University of Colorado at Boulder, Boulder, Colorado, USA
  • 3National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand

Abstract. We present 60 years of Δ14CO2 measurements from Wellington, New Zealand (41°S, 175°E). The record has been extended and fully revised. New measurements have been used to evaluate the existing record and to replace original measurements where warranted. This is the earliest direct atmospheric Δ14CO2 record and records the rise of the 14C bomb spike and the subsequent decline in Δ14CO2 as bomb 14C moved throughout the carbon cycle and increasing fossil fuel CO2 emissions further decreased atmospheric Δ14CO2. The initially large seasonal cycle in the 1960s reduces in amplitude and eventually reverses in phase, resulting in a small seasonal cycle of about 2‰ in the 2000s. The seasonal cycle at Wellington is dominated by the seasonality of cross-tropopause transport and differs slightly from that at Cape Grim, Australia, which is influenced by anthropogenic sources in winter. Δ14CO2 at Cape Grim and Wellington show very similar trends, with significant differences only during periods of known measurement uncertainty. In contrast, similar clean-air sites in the Northern Hemisphere show a higher and earlier bomb 14C peak, consistent with a 1.4-year interhemispheric exchange time. From the 1970s until the early 2000s, the Northern and Southern Hemisphere Δ14CO2 were quite similar, apparently due to the balance of 14C-free fossil fuel CO2 emissions in the north and 14C-depleted ocean upwelling in the south. The Southern Hemisphere sites have shown a consistent and marked elevation above the Northern Hemisphere sites since the early 2000s, which is most likely due to reduced upwelling of 14C-depleted and carbon-rich deep waters in the Southern Ocean, although an underestimate of fossil fuel CO2 emissions or changes in biospheric exchange are also possible explanations. This developing Δ14CO2 interhemispheric gradient is consistent with recent studies that indicate a reinvigorated Southern Ocean carbon sink since the mid-2000s and suggests that the upwelling of deep waters plays an important role in this change.

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We present a 60-year record of radiocarbon in carbon dioxide (14CO2) from Wellington New Zealand. It records the atmospheric 14C “bomb spike” and decline as bomb 14C moved through the carbon cycle and fossil fuel emissions increased. The bomb peak is lower and 1.4 years later than in the Northern Hemisphere. Since the early 2000s, Wellington 14CO2 has been elevated above the Northern Hemisphere, possibly due to a reinvigorated Southern Ocean carbon sink.
We present a 60-year record of radiocarbon in carbon dioxide (14CO2) from Wellington New...
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