The Southern Ocean biogeochemical divide

Abstract

Modelling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air–sea balance of CO₂ and global biological production^{1,2,3,4,5,6,7,8}. Box model studies^1,2,3,4 first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure ($p_{{\text{CO}}_{\text{2}}}$). This early research led to two important ideas: high latitude regions are more important in determining atmospheric $p_{{\text{CO}}_{\text{2}}}$ than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric $p_{{\text{CO}}_{\text{2}}}$ are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling pre-industrial atmospheric CO₂ because it serves as a lid to a larger volume of the deep ocean^5,6. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide⁷ and in controlling global biological production⁸. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air–sea CO₂ balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO₂ and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.