J I

Fig. 10-21 (a) is an idealized map of the patterns of deep water flow (solid lines) and surface water flow (dashed lines). The large circles designate the sinking of NADW (North Atlantic Deep Water) in the Norwegian Sea and the recooling of water along the perimeter of the Antarctic Continent; the dark circles indicate the distributed upwelling which balances this deep water generation, (b) is an idealized vertical section running from the North Atlantic to the North Pacific showing the major advective flow pattern (thin lines) and the rain of particles (thick wavy lines). The combination of these two cycles leads to the observed distribution of nutrients. (Reproduced with permission from W. S. Broecker and T.-H. Peng (1982). "Tracers in the Sea," p. 34, Eldigio Press, Palisades, NY.)

plankton = 36 and the surface water content is 10 000 pM. The predicted deep ocean concentration is 10090 resulting in a deep ocean enrichment ratio of 1.01. These features are summarized for several elements in Table 1014. Three elements (P, N, Si) show nearly complete depletion in surface water, reflecting the fact that Si limits diatom growth, while P and N limit the remaining organisms. Si shows the largest enrichment in the deep Pacific relative to the deep Atlantic suggesting that biological filtering is more efficient for Si than P. Hard parts of organisms undergo destruction at greater average depths than do the soft parts.

Three other elements (C, Ba, Ca) show a similar deep Pacific to surface distribution but of smaller magnitude (less than 10). Note that the present cycle requires less than 100% efficient surface removal, otherwise nutrients would be all in the Pacific after a one-way trip.

There are two important consequences of this superposition of biological cycling on the ocean circulation pattern that show up in the sediments.

1. It leads to lower diatom productivity in the Atlantic relative to the Pacific.

2. It leads to a tilting of the depth of CaC03 preservation in the sediments. The deep Pacific is more corrosive to CaC03 than the deep Atlantic (more C02 from respiration) and thus CaC03 is found in sediments 1500 m deeper in the Atlantic than in the Pacific.

10.4.4.2 Long-term processes composition control of

In the previous section we considered only internal cycling. The questions we want to turn to now are:

1. What controls surface water concentrations?

2. What controls the P content of deep water and thus the deep-water content of other elements?

Broecker's (1971) approach was to form groups of elements that appear to be controlled by similar processes. We will follow that approach here while examining the important factors and time scales. The groups presented will differ from Broecker's in that we will include new information on hydrothermal processes not available at the time Broecker wrote his paper (Edmond et al., 1979; McDuff and Morel, 1980). The groups used are kept as close as possible to Broecker's original list.

10.4.4.2.1 Group la (e.g., CI). Elements in this group have long oceanic residence times. These elements are soluble and not reactive. The original source was degassing of the Earth's interior, which is either very slow now or complete. The main property of this group is geologic removal by formation of soluble salts in evaporite deposits.

Table 10.14 Concentration distributions in the sea for elements used in significant amounts by marine organisms"

Element

[S]

[DA]

[DP]

[DP]/[S]

([DP]-[S])/([DA]-[S])

Ref.

Pfc

<0.02

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