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

Jocelyn C Turnbull,India Ansell,Gordon W Brailsford,Margaret W Norris,Rowena C Moss,Kay Steinkamp,Sara E Mikaloff Fletcher

doi:10.5194/acp-17-14771-2017

Abstract

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.

Highlights

Measurements of radiocarbon in atmospheric carbon dioxide ( 14CO2) have long been used as a key to understanding the global carbon cycle
Natural 14C production is roughly balanced by radioactive decay, which mostly occurs in the carbon-rich and slowly overturning ocean carbon reservoir and to a lesser extent in the faster cycling terrestrial carbon reservoir
We calculated F14C from the original measurement data recorded in our databases and updated a handful of records where transcription errors were found. 14C is derived from F14C and corrected for radioactive decay since the time of collection; this is slightly different from 14C as defined by Stuiver and Polach (1977), which is corrected to the date of measurement. 14C has been recalculated using the date of collection for all results, resulting in changes of a few tenths of per mil in most 14C values relative to those reported by Currie et al (2011) and Manning et al (1990)

Summary

Introduction

Measurements of radiocarbon in atmospheric carbon dioxide ( 14CO2) have long been used as a key to understanding the global carbon cycle. The first atmospheric 14CO2 measurements were begun at Wellington, New Zealand in 1954 (Rafter, 1955; Rafter and Fergusson, 1959), aiming to better understand carbon exchange processes (Otago Daily Times, 1957). The penetration of bomb-14C into the oceans has been used to understand ocean carbon uptake processes (Oeschger et al, 1975; Broecker et al, 1985; Key et al, 2004; Naegler et al, 2006; Sweeney et al, 2007). Terrestrial biosphere carbon residence times and exchange processes have been widely investigated using bomb-14C Stratospheric residence times, cross-tropopause transport and interhemispheric exchange can be examined with atmospheric 14CO2 observations (Kjellström et al, 2000; Kanu et al, 2015)

Methods

Results

Conclusion