An Introduction to Environmental Chemistry

cloud of iron, zinc, lead and copper sulphides and iron oxides on injection into

cold, oxic oceanic bottom waters. This sulphidic plume of particles identifies

clearly the location of the hot springs and gives rise to their colloquial name—

‘black smokers’ (Plate 6.1, facing p. 138).

The high temperatures encountered in hydrothermal circulation cells at mid-

ocean ridges increase substantially the rate and extent of chemical reaction

210 Chapter Six

Zone of equation 6.17

Descending

dense, cold

seawater

Seawater

Heating

Ascending

warm, buoyant

hydrothermal

fluid

Heat source

Hot springs and

black smokers

~ 2 km

Fig. 6.15Hydrothermal convection at a mid-ocean ridge.

Table 6.5Estimates of ocean volume circulation times through hydrothermal systems.

Modified from Kado et al. (1995).

Axial hydrothermal convection Off-axis convection Recycling through

(350°C fluid) (20°C fluid) ‘black smoke’ plume*

3.3¥ 107 yr 5.5¥ 105 yr 2.8¥ 103 yr water

2.4¥ 105 yr reactive

elements

* Extrapolated from study of the Endevor Ridge. Although the ocean volume is cycled through the plume

in 2.8¥ 103 years, the rate of reaction of the plume with seawater is much slower. To strip the reactive

elements from seawater it is necessary to cycle the ocean volume approximately 100 times through the

plume (2.4¥ 105 yr).