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 280 tree-ring Δ14C measurements from 13 sites in Chile and Aotearoa / New Zealand from 1980 to 2017. These measurements dramatically expand the dataset of Southern Hemisphere atmospheric Δ14CO2 records. We use these records to analyze latitudinal gradients in reconstructed atmospheric Δ14CO2 across the Southern Ocean. Tree rings from Aotearoa / New Zealand's Motu Ihupuku / Campbell Island (52.5° S, 169.1° E) show Δ14CO2 was on average 3.1±3.3 lower than atmospheric background, driving a latitudinal gradient among Aotearoa / New Zealand sites between 41.1 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.