Southern Hemisphere tree rings as proxies to reconstruct Southern Ocean upwelling

Le résumé fourni par la source

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 CO 2 uptake and 75 % of ocean heat uptake between 1861 and 2005. However, the strength of the Southern Ocean sink (air–sea CO 2 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 CO 2 and uptake of anthropogenic CO 2 . Reconstructions of atmospheric radiocarbon (Δ 14 CO 2 ) from Southern Hemisphere tree rings may be a viable method of observing the one-way outgassing flux of natural CO 2 , driven by Southern Ocean upwelling. Here we present 280 tree-ring Δ 14 C measurements from 13 sites in Chile and Aotearoa / New Zealand from 1980 to 2017. These measurements dramatically expand the dataset of Southern Hemisphere atmospheric Δ 14 CO 2 records. We use these records to analyze latitudinal gradients in reconstructed atmospheric Δ 14 CO 2 across the Southern Ocean. Tree rings from Aotearoa / New Zealand's Motu Ihupuku / Campbell Island (52.5° S, 169.1° E) show Δ 14 CO 2 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 CO 2 outgassing from the Antarctic Southern Zone (ASZ) and atmospheric transport to the sampling sites.

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