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

We present 60 years of Delta(CO2)-C-14 measurements from Wellington, New Zealand (41 degrees S, 175 degrees 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 Delta(CO2)-C-14 record and records the rise of the C-14 bomb spike and the subsequent decline in Delta(CO2)-C-14 as bomb C-14 moved throughout the carbon cycle and increasing fossil fuel CO2 emissions further decreased atmospheric Delta(CO2)-C-14 . 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 parts per thousand 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. Delta(CO2)-C-14 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 C-14 peak, consistent with a 1.4-year interhemispheric exchange time. From the 1970s until the early 2000s, the Northern and Southern Hemisphere Delta(CO2)-C-14 were quite similar, apparently due to the balance of C-14-free fossil fuel CO2 emissions in the north and C-14-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 C-14-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 Delta(CO2)-C-14 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.