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 CO(2) and global biological production. Box model studies 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 (pCO2). This early research led to two important ideas: high latitude regions are more important in determining atmospheric pCO2 than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric pCO2 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 CO(2) because it serves as a lid to a larger volume of the deep ocean. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide and in controlling global biological production. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air-sea CO(2) balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO(2) 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.

Climate and marine carbon cycle response to changes in the strength of the Southern Hemispheric westerlies

“…At low latitudes, this water mixes into the thermocline and can eventually be upwelled in the major upwelling zones [Marinov et al, 2006]. The nutrient excess in WINDP or the deficit in WINDM can therefore be transmitted to the low latitudes.…”

Processes and patterns of oceanic nutrient limitation

“…The downward transport of nutrients resulting from the sinking and remineralization of particulate organic material formed in the surface ocean 16 (Fig. 2a) is associated with a flux of carbon, frequently termed the biological pump [90][91][92] . Physical processes also transport biologically unutilized (so-called 'preformed') nutrients into the ocean interior, leading to a decreased efficiency of the biological pump.…”

“…Assuming constant stoichiometry and effective air-sea equilibration of gases in the surface ocean, the biological storage of carbon in the ocean is proportional to the total inventory of nutrients in the interior that arrived through the biological 'remineralized' pathway 90,92 (see Supplementary Information). Consequently, circulation patterns strongly dictate how changes in nutrient limitation can influence atmospheric carbon dioxide concentrations 91 . For example, the high-nitrate lowchlorophyll Southern Ocean currently represents the largest source of preformed macronutrients to the deep ocean 63,91 .…”

“…Consequently, circulation patterns strongly dictate how changes in nutrient limitation can influence atmospheric carbon dioxide concentrations 91 . For example, the high-nitrate lowchlorophyll Southern Ocean currently represents the largest source of preformed macronutrients to the deep ocean 63,91 . As such, glacialinterglacial variations in atmospheric carbon dioxide levels have been linked to altered nutrient biogeochemistry in this region 63,64 .…”

Freshwater fluxes in the Weddell Gyre: results from δ 18 O

“…The Weddell Gyre is an important region in the global climate system due to its prominent role in the formation and export of the deep and bottom waters that flood the global abyss [1], and the associated sequestration of carbon, nutrients and atmospheric gases at depth on climatic time scales [2,3]. These processes are critically sensitive to the freshwater balance of the region: at low temperatures, density (and by extension stratification, circulation and deep water formation) depends almost entirely upon salinity [4] and so will be sensitive to fluctuations in the local freshwater balance caused by changes in glacial discharge, precipitation and the melting/production of sea-ice.…”