An Introduction to Environmental Chemistry

system still adjusting to rapid changes in Pb^2 +input. Although it is regrettable

that humans have caused global-scale lead pollution, chemical oceanographers

can at least make use of the information as these transient pollutants act as

tracers to allow a better understand of material cycling in the oceans (see also

Box 7.1).

In the deep oceans, particle-rich hydrothermal plumes (Plate 6.1, facing p.

138), and the iron- and manganese-rich hydrothermal sediments that fall out

from them, cause intense scavenging of some elements from seawater, making

mid-ocean ridge environments important sinks for some metals. It is calculated

that the entire ocean is stripped of its reactive constituents by hydrothermal

plumes in about 2.4¥ 105 years (Table 6.5). To do this, the volume of the oceans

has to pass through the hydrothermal plumes about 100 times and this means

that scavenging in the plume is significant for elements that have oceanic resi-

dence times (Box 6.3) in excess of 10 000 years. Scavenging in the plume is a

226 Chapter Six

Table 6.10Removal fluxes of elements by hydrothermal plumes (mol yr-^1 ). Based on data in

Elderfield and Schultz (1996), with permission from the Annual Review of Earth and Planetary

Sciences.

Hydrothermal plume % removal of

Element removal flux River input flux river input flux

V 4.3¥ 108 5.9¥ 108 73

P 1.1¥ 1010 3.3¥ 1010 33

As 1.8¥ 108 8.8¥ 108 20

Cr 4.8¥ 107 6.3¥ 108 8

1700 1890 1910 1930 1950 1970 1990

40

30

20

Cd/Ca (nmol mol 10

–1

)

Florida Keys

(M. annularis)

Year

Fig. 6.24Lead concentrations in dated year bands from a coral collected from the Florida

Keys. After Shen and Boyle (1987), with permission from Elsevier Science.