Abstract
The deep ocean naturally releases large amounts of old, pre-industrial
carbon dioxide (CO2) to the atmosphere through upwelling in the Southern
Ocean, closing the global ocean carbon cycle. This Southern Ocean CO2
release is relevant to the global climate, because its changes could
alter atmospheric CO2 levels on long time scales and the present-day
potential of the Southern Ocean to take up anthropogenic CO2. Here,
based on observational data, we show that this CO2 release arises from a
zonal band of subsurface waters between the Subantarctic Front and
wintertime sea-ice edge with a potential partial pressure of CO2
exceeding current atmospheric CO2 levels (∆PCO2) by 175 µatm. This band
of high ∆PCO2 subsurface water conincides with the outcropping of the
27.8 kg m−3 isoneutral density surface that marks the upwelling of
Indo-Pacific Deep Water (IPDW). Vertically, the IPDW layer exhibits a
distinct ∆PCO2 maximum in the deep ocean, which is set by
remineralization of organic carbon and originates from the northern
Pacific and Indian Ocean basins. Below this IPDW layer, the carbon
content increases downwards, whereas ∆PCO2 decreases. Most of this
vertical ∆PCO2 decline results from decreasing temperatures and
increasing alkalinity due to an increased fraction of calcium carbonate
dissolution. These two factors limit the CO2 outgassing from the
high-carbon content deep waters on more southerly surface outcrops. Our
results imply that the response of Southern Ocean CO2 fluxes to possible
future changes in upwelling are sensitive to the subsurface carbon
chemistry set by the vertical remineralization and dissolution profiles.