Latitudinal Distribution of Atmospheric ∆¹⁴CO₂ over the Southern Ocean

J. Turnbull^1,2, J. Collins¹, E. Behrens³, G. Brailsford³, B. Bukosa³ and S.M. Fletcher³

¹GNS Science, National Isotope Centre, Lower Hutt, New Zealand; +64 4 570 4726, E-mail: j.turnbull@gns.cri.nz
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305
³National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand

The Southern Ocean is a key sink for anthropogenic carbon dioxide (CO₂), yet the processes that govern the rate of uptake remain only partly understood.  We use observations of ∆¹⁴C in CO₂ from shipboard transects across the Southern Ocean to develop latitudinal gradients of ∆¹⁴C in the surface atmosphere.  We present five years of austral summer observations (2016-2020) from ships of opportunity travelling between New Zealand and the Ross Sea, Antarctica, along with long-term measurements from Baring Head, New Zealand and Arrival Heights, Antarctica.  We observe lower ∆¹⁴C in the 50°S to 70°S region, with higher values to the north and south, consistent with upwelling of ¹⁴C-depleted deep waters in this region.  We then combine model ocean simluations of CO₂ and ¹⁴C with simulations from the NAME  III atmospheric dispersion model to predict surface atmosphere ∆¹⁴C and compare with the observations.  Our model simulation does a reasonable job of matching the observations, capturing the spatial pattern and day-to-day variability quite well.  However, the model somewhat underestimates the magnitude of the observed ∆¹⁴C gradient, particularly between 50°S-60°S.  Our results suggest that atmospheric ¹⁴C observations can be used to diagnose the strength of deep water upwelling in the Southern Ocean.

Figure 1. Observed (coloured points, differentiated by year of collection) and modelled (black) ∆∆¹⁴C, the deviation of near-surface atmospheric ∆¹⁴C from Baring Head (41°S) and Arrival Heights (78°S).  Lower values over the Southern Ocean demonstrate exchange of carbon from ¹⁴C-depleted upwelled deep water into the atmosphere.