Southern Hemisphere tree-rings as proxies to reconstruct Southern Ocean upwelling

Abstract. The Southern Ocean plays a key role in regulating global climate and acting as a carbon sink. This region, defined as south of 35° S, is accountable for 40 % of all oceanic anthropogenic CO₂ uptake, and 75 % of ocean heat uptake between 1861 and 2005. However, the strength of the Southern Ocean sink (air-sea CO₂ flux) is variable – weakening in the 1990s and strengthening again in the 2000s. Typical methods of constraining the flux must grapple with two opposing forces: outgassing of natural CO₂ and uptake of anthropogenic CO₂. Reconstructions of atmospheric radiocarbon (Δ¹⁴CO₂) from Southern Hemisphere tree-rings may be a viable method of observing the one-way outgassing flux of natural CO₂, driven by Southern Ocean upwelling. Here we present more than 400 tree-ring ∆¹⁴C measurements from 13 sites in Chile and New Zealand from the 1980s to the present. These measurements dramatically expand the dataset of Southern Hemisphere atmospheric Δ¹⁴CO₂ records. We use these records to analyse latitudinal gradients in reconstructed atmospheric Δ¹⁴CO₂ across the Southern Ocean. Tree-rings from New Zealand’s Campbell Island (52.5S, 169.1E) show Δ¹⁴CO₂ was on average 3.3±3.5 ‰ lower than atmospheric background, driving a latitudinal gradient among New Zealand sites between 41.1° S and 52.5° S, whereas samples from similar latitudes in Chile do not exhibit such a strong gradient. We demonstrate that the gradient is driven by the combination of CO₂ outgassing from the Antarctic Southern Zone (ASZ) and atmospheric transport to the sampling sites.