An Introduction to Environmental Chemistry

tation rates are high, associated with nutrient-rich upwelling waters and polar

seas, particularly around Antarctica (Fig. 6.9). Seawater is undersaturated with

respect to silica and it is estimated that 95% of opaline silica dissolves as it sinks

through the water column or at the sediment/water interface. Thus, the preser-

vation of opaline silica only occurs where it is buried in rapidly accumulating sedi-

ment, beneath the sediment/water interface. Subsequent dissolution of opal in

the sediment saturates sediment pore waters with silica. The pore water cannot

readily exchange with open seawater and saturation prevents further opal disso-

lution. High sedimentation rates in the oceans can be caused by high mineral

supply rates from the continents, but are usually caused by high production rates

of biological particles (Section 6.5.4). In high productivity areas, for example parts

of the Southern Ocean bordering Antarctica, diatoms are the common phyto-

plankton species, and this enhances the importance of these regions as silica sinks.

The biological removal of silicon (Si) from seawater is calculated from the opal

content of sediments and rates of sedimentation (Table 6.2).

6.4.6 Sulphides

The oxidation of organic matter proceeds by a number of microbially mediated

reactions once free oxygen has been used up (see Section 5.5 & Table 4.7).

Although small amounts of nitrate (NO 3 - ), manganese (Mn) and iron (Fe) are

206 Chapter Six

(a) (b)

Fig. 6.12(a) Skeleton of siliceous radiolarian Theocorythium vetulum, early Pleistocene, equatorial Pacific. Scale
bar= 75 mm. (b) Siliceous diatom Coscinodiscus radiatus, early Pleistocene, equatorial Pacific. Scale bar= 38 mm.
Photographs courtesy of B. Funnell.