Ocean circulation and its effects on trace element distribution

The preceding discussion of trace elements in seawater has assumed that the oceans have a uniform, warm, nutrient-depleted surface mixed layer and a static deep zone. In fact, at high latitudes surface seawater is cold enough to destroy any density stratification, mixing the oceans to depths of up to 1000 m. This dense surface water sinks and flows slowly into the centre of the oceans as a layer of

Fig. 6.26 Chlorophyll concentrations as indicators of phytoplankton growth, inside and outside an iron-enriched patch of seawater in the Southern Ocean. After a few days chlorophyll concentrations had increased markedly in the iron-enriched water indicating phytoplankton growth. After Boyd et al. (2000), reprinted with permission from Nature. Copyright (2000) Macmillan Magazines Limited.

Fig. 6.26 Chlorophyll concentrations as indicators of phytoplankton growth, inside and outside an iron-enriched patch of seawater in the Southern Ocean. After a few days chlorophyll concentrations had increased markedly in the iron-enriched water indicating phytoplankton growth. After Boyd et al. (2000), reprinted with permission from Nature. Copyright (2000) Macmillan Magazines Limited.

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 throughout 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

Antarctica

Fig. 6.27 Idealized 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).

Antarctica

Fig. 6.27 Idealized 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).

Fig. 6.28 Global oceanic deep-water circulation. Major flow routes are marked by stippled ornament. Deep mixing in the North Pacific is prevented by the topography of the seabed around the Aleutian Arc. Modified from Stommel (1958), with permission from Elsevier Science.

dissolved CO2 concentrations, since oxygen has been used to oxidize greater amounts of organic matter to CO2. Overall, the supply of dissolved oxygen to seawater is adequate to oxidize the sinking organic matter and, apart from a few unusual areas in the oceans, oxygen concentrations in the bottom waters are adequate to support animal life. The higher dissolved CO2 concentration in the Pacific results in a shallower calcite compensation depth (CCD) in the Pacific, relative to the Atlantic Ocean (Section 6.4.4).

The slower regeneration of silicon compared with nitrogen and phosphorus (Section 6.5.4) means that relatively more silicon is regenerated in deep waters, producing steeper interocean concentration gradients (Table 6.9 & Fig. 6.20). Similarly, other elements with nutrient-like behaviour, such as zinc and cadmium, have higher concentrations in the North Pacific compared with other oceans. By contrast, scavenged elements have concentrations that are lower in the deep waters of the Pacific compared with the North Atlantic, because of the longer time available for their removal by adsorption on to sinking particulate matter (Table 6.9).

This pattern of global oceanic circulation has probably existed since the end of the last glaciation, 11 000 years ago. Before this, the circulation pattern is thought to have been different, due to changes in glacial climatic regime and changes in polar ice volume. It is unclear whether changes in ocean circulation provoked climatic change at this time or vice versa. Despite the uncertainty, it is clear that ocean circulation and global climate are intimately linked.

Fig. 6.29 World ocean-surface currents viewed from the Antarctic region. Modified from Spilhaus (1942), with permission from the American Geographical Society.
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