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

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.