G01_01

Atmospheric ¹⁴CO₂ time series from Point Barrow, Alaska: ending of the “Bomb Radiocarbon Period” in the Northern Hemisphere

Xu X¹, Walker J², Newman S^3,5, Trumbore S^1,4

¹Keck Carbon Cycle AMS Lab, Department of Earth System Science, University of California, Irvine, Irvine, United States, ²André E. Lalonde AMS Laboratory, University of Ottawa, Ottawa, Canada, ³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA, ⁴Department Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany, ⁵Bay Area Air Quality Management District, San Francisco, USA

The distribution rate and pattern of ¹⁴C produced by nuclear weapons testing during late 1950s – early 1960s has provided a unique opportunity for tracing global carbon cycles, and also for studying atmospheric mixing. The atmospheric ¹⁴CO₂ peaked around 1964, and since then has decreased as excess ¹⁴C cycled between atmospheric, oceanic and terrestrial carbon reservoirs and was diluted by fossil fuel CO₂ addition to the atmosphere. In this century, fossil fuel dilution has become the major factor controlling the rate of decrease in the atmospheric ¹⁴CO₂.

We report a continuous, high precision and high temporal resolution Δ¹⁴CO₂ record from Barrow, Alaska (71°N, 157°W) from 2003 to 2022. Sample collection was through the NOAA/ESRL flask sampling network program, which enables comparison of radiocarbon data with other trace gases and isotopes, including CO and CO₂ mixing ratios and δ¹³C and δ18O of CO₂. There are distinct Δ¹⁴CO₂ seasonal cycles in this record, with a broad minimum around Mar-Apr and a maximum in Sep-Oct with an amplitude of ~7‰. From 2003 to the end of 2019, Δ¹⁴CO₂ decreased linearly by ~4.3‰/year, although it slowed during 2020 which may likely have resulted from reduced fossil fuel CO₂ emission during the global lockdown imposed by the COVID-19 pandemic. By the end of 2021, Δ¹⁴CO₂ at Barrow declined to below 0‰, ending the “Bomb Radiocarbon Period” in the Northern Hemisphere. This iconic event is of global significance because the artificial “aging” of the atmosphere has the potential to affect many radiocarbon applications in the future.