Re-Emergence of Excess Bomb Radiocarbon in Upwelling Waters with High-Latitude Origins

The quantity of radiocarbon (¹⁴C) in the atmosphere was nearly doubled by nuclear weapons testing in the 1960s. Since then, the terrestrial biosphere and the ocean have absorbed most of the excess ¹⁴C from the atmosphere, although atmospheric radiocarbon activity (∆¹⁴C) continues to decline due to ongoing emissions of ¹⁴C-free CO₂ from combustion of fossil fuels. The large transient decline in atmospheric ∆¹⁴C combined with gas exchange at the surface and spatially variable time scales of ocean mixing have led to large ∆¹⁴C gradients in the surface ocean between upwelling- and downwelling-dominated regions. These gradients continue to evolve over time. We examine the rate of change of surface ocean ∆¹⁴C between CLIVAR (2000-2011) and WOCE era (1990s) or other slightly earlier (1980s) datasets and find spatial patterns that reveal mixing between ¹⁴C-enriched mode waters, ¹⁴C-depleted deep waters and surface waters that are well-equilibrated with the atmosphere. The ∆¹⁴C of mode water reaching equatorial upwelling regions has increased between the WOCE and CLIVAR time periods, and the greater contribution of ¹⁴C to the low-latitude surface ocean appears to have significantly offset the ∆¹⁴C decline otherwise imparted by air-sea gas exchange with the atmosphere. Consequently, ∆¹⁴C gradients between low-latitude upwelling regions and gyre centers have weakened proportionally more than between gyre centers and regions where pre-industrial water still upwells, such as the Southern Ocean. Properly accounting for the re-emergence of water with post-industrial characteristics is important to constrain earth system models that seek to explain DIC, pH and other anthropogenically perturbed tracers in the surface ocean. Because of the history of ∆¹⁴C in the atmosphere, ocean ∆¹⁴C is a useful tracer for this purpose.