An Introduction to Environmental Chemistry

cold, oxygen-rich water, displacing the bottom water in its path. The displaced

bottom water is forced to move upwards slowly, setting up an oceanic circulation

(Figs 6.27 & 6.28).

The deep mixing at high latitudes only occurs in two locations: in the North

Atlantic and around Antarctica. Deep mixing does not occur in the North Pacific,

mainly because a physical sill, related to the Aleutian Arc, prevents water

exchange between the Arctic and the Pacific (Fig. 6.28). This asymmetry in deep

mixing drives a global ocean circulation, in which surface water sinks in the North

Atlantic, returns to the surface in the Antarctic and then sinks again and enters

the Pacific and Indian Oceans (Fig. 6.27). The deep flow tends to concentrate at

the western edge of ocean basins, but allows a slow diffusion of water through-

out the ocean interiors. This slow, deep-water flow is compensated by a poleward

return flow in surface waters (Fig. 6.29). A ‘parcel’ of seawater takes hundreds of

years to complete this global ocean journey, during the course of which the deep

water continually acquires the decay products of sinking organic matter from

surface seawater. Waters in the North Pacific have more time to acquire these

decay products because they are the ‘oldest’, in the sense of time elapsed since

they were last at the surface and had their nutrients removed by biological

processes. As a result the deep waters of the Pacific Ocean have higher macronu-

trient concentrations than those in the Atlantic Ocean (Fig. 6.20). The waters of

the North Pacific also have the lowest dissolved oxygen concentrations and high

230 Chapter Six

Americas

Atlantic

Pacific

Americas

Antarctica

Indian

Africa

Europe–Asia

Source

Recooling

Fig. 6.27Idealized map of oceanic deep-water flow (solid lines) and surface-water flow

(dashed lines). Open circles represent areas of water sinking and dark circles areas of

upwelling. After Broecker and Peng (1982).