Investigating the Impact of Past and Future CO2 Emissions on the Distribution of Radiocarbon in the Ocean

Abstract:

The ocean is a significant sink for carbon dioxide from fossil fuel burning, absorbing roughly a third of human CO₂ emitted over the industrial period. This has implications not only for climate but also for the chemical and isotopic composition of the ocean. Human activities have increased the ocean radiocarbon content through nuclear bomb tests in the 1950s-60s, which released a large amount of radiocarbon (¹⁴C) into the atmosphere, but fossil fuel emissions are decreasing the radiocarbon content through the release of ¹⁴C-depleted CO₂. Here, we use the ECCO-v4 ocean state estimate to examine the changing nature of the air-sea flux of radiocarbon and its spatial distribution in the ocean in response to past and future CO₂ emissions, the latter taken from the the Representative Concentration Pathway (RCP) database used in IPCC simulations. In line with previous studies we find that the large air-sea gradient of ¹⁴C induced by nuclear bomb testing led to rapid accumulation of radiocarbon in the surface ocean. Surface fluxes of ¹⁴C have considerably weakened over the past several decades and in some areas ¹⁴C is being returned to the atmosphere. As fossil fuel emissions continue to reduce the atmospheric ¹⁴C/C ratio (∆¹⁴C), in most RCP scenarios the total ocean ¹⁴C inventory starts decreasing by 2030. With strong emissions, the Δ¹⁴C of surface waters is driven to increasingly negative values and in RCP 8.5 by 2100 much of the surface ocean has apparent radiocarbon ages in excess of 2000 years, with subtropical gyres more depleted in ¹⁴C than the Southern Ocean. Surface waters become significantly more negative in Δ¹⁴C than underlying waters. As a result, turning conventional tracer oceanography on its head, recently ventilated waters are characterized by more negative Δ¹⁴C values. Similar patterns can be expected for CFCs in the ocean as atmospheric concentrations decrease over the next several decades. Our results have a number of implications, notably for current and planned ocean observation programs, as well as ongoing efforts to exploit radiocarbon to quantify changes in ocean ventilation in response to anthropogenic climate change.