An Introduction to Environmental Chemistry

Modern surface seawater is demonstrably supersaturated with respect to

CaCO 3 , and the presence of carbonate sediments in rocks of all ages suggests that

it has been throughout much of Earth history. Oceanic pH is unlikely to have

fallen below 6 as such a shift requires a 1000-fold increase in atmospheric pCO 2

over its present value of 10-3.6atm. An atmospheric pCO 2 of this magnitude may

have occurred very early in Earth history, but the existence of 3.8 billion-year-

old CaCO 3 sediments shows that seawater was still close to saturation with calcite.

Achieving calcite saturation at these high pCO 2 values implies that oceanic

Ca^2 +concentrations were at that time 10 times larger than modern values. Sea-

water pH has probably never exceeded 9 in the geological past, because at that

pH sodium carbonate would be a much more common mineral precipitate than

calcite. There is no evidence in ancient marine sediments that sodium carbonate

has ever been a common marine precipitate.

6.4.5 Opaline silica

Opaline silica (opal) is a form of biologically produced silicon dioxide

(SiO 2 .nH 2 O) secreted as skeletal material by pelagic phytoplankton (diatoms) and

one group of pelagic zooplankton (radiolarians) (Fig. 6.12). Opaline silica-rich

sediments cover about one-third of the seabed, mainly in areas where sedimen-

The Oceans 205

1

2

3

4

5

Depth (km)

123456

1

2

3

4

5

Supersaturated

Under-

saturated

Lysocline

CCD

Lysocline

CCD

Ω calcite

Increasing rate of

Rate ofCaCO CaCO 3 dissolution

3

dissolution

Degree of saturation

(Ω) for calcite

Basaltic

seafloor

Sea-

bed

Thick CaCO 3

Foraminifera ooze

skeletons

may survive

Red

clay

Relationship between

seafloor topography

and sediment type

CaCO 3

supply

Coccoliths

(foraminifera(

+

+

+

+

+

+

+

+

+

+

+ +

+

+ +

+

+ +

+

+

+

+ +

}

Fig. 6.11Schematic diagram showing depth relationship between degree of saturation for calcite in seawater
and rate of CaCO 3 dissolution. At 4 km depth, as seawater approaches undersaturation with respect to calcite,
rate of dissolution of sinking calcite skeletons increases. The lysocline marks this increased rate of dissolution.
Below the lysocline only large grains (foraminifera) survive dissolution if buried in the seabed sediment. Below
the calcite compensation depth (CCD; see text) all CaCO 3 dissolves, leaving red clays.