An Introduction to Environmental Chemistry

190 Chapter Six

Box 6.2 Salinity and major ion chemistry of seawater

on geological timescales

The evidence that the salinity and major ionic

composition of seawater have remained

reasonably constant over at least the last 900

million years comes from ancient marine

evaporite deposits. Evaporites are salts that

crystallize from evaporating seawater in

basins largely cut off from the open ocean.

Over the last 900 million years, marine

evaporites have normally begun with a

gypsum-anhydrite section (CaSO 4 .2H 2 O-

CaSO 4 ), followed by a halite (NaCl) sequence.

Bittern salts (named due to their bitter taste)

have precipitated from the final stages of

evaporation and have variable composition,

including magnesium salts, bromides,

potassium chloride (KCl) and more complex

salts, depending on conditions of evaporation

(Fig. 1).

The order of precipitates is the same as

that seen in modern marine evaporites and

can be reproduced by experimental

evaporation of seawater. This sequence of

salt precipitation sets limits on the possible

changes of major ion compositions in

seawater, since changes beyond these limits

would have resulted in different sequences

of salt formation.

Calculations demonstrating the actual

limits on changes in the major ion chemistry

of seawater imposed by the evaporite

precipitation sequence are beyond the scope

of this book. However, some simple

Evaporation of

1 km of seawater

100

75

50

25

0

Percentage of original liquid remaining

Salinity

increases

Bittern salts

ppt’s

Bittern salts

Resulting thickness

of salts from 1 km

of 35 g l–1 seawater

35 g l–1

CaCO 3

CaSO 4. 2H 2 O

NaCl 12 m

1 m

20 cm

2.5 m

CaSO 4. 2H 2 O

ppt’s

80 g l–1

220 g l–1

350 g l–1

NaCl ppt’s

CaCO 3

precipitates

Fig. 1Successional sequence and approximate thickness of salts precipitated during the evaporation of 1 km

of seawater. After Scoffin (1987), with kind permission of Kluwer Academic Publishers.

(continued)