Three-dimensional simulations of the impact of Southern Ocean nutrient depletion on atmospheric CO 2 and ocean chemistry

Abstract

Surface nutrient concentrations in the Southern Ocean are an important indicator of the atmosphere-ocean chemical balance that played a key role in ice-age reduction of atmospheric pCO2 and would play a role in any Fe fertilization scenario for increasing oceanic uptake of anthropogenic CO2. The response of the ocean and atmosphere to a scenario of extreme depletion of Southern Ocean surface nutrients by an increase in the organic matter flux to the deep ocean is examined with a three-dimensional model of ocean circulation coupled to a one-box model of the atmosphere. After 100 yr, the increase in the organic matter flux is 6–30 Gt C yr‒1—about twice the global new production determined by the same model for the present ocean. The removal of nutrients from surface waters of the Southern Ocean reduces the nutrient content of the near-surface and intermediate depth waters of the entire ocean, resulting in a 0.5–1.9 Gt C yr‒1 reduction of low-latitude new production. The deep circumpolar waters, enriched in nutrients by regeneration of organic matter, spread into the deep and bottom waters of the remainder of the ocean, giving an overall downward shift of nutrients from surface and intermediate to circumpolar and deep waters. The oceanic total C distribution is also shifted downward, resulting in uptake of atmospheric CO2 of 46–85 ppm (98–181 Gt C) in the first 100 yr. The oxygen content shifts upward in the water column, approximately mirroring the downward shift of nutrients. Some of the oxygen shifted to the upper ocean escapes to the atmosphere. As a consequence, the global average oceanic content of oxygen, presently 168 µmol kg‒1, is reduced by 6–20 µmol kg‒1, with anoxia developing in the southwestern Indian Ocean.