Box 62 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 (CaSO4.2H2O-CaSO4), 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-

Bittern salts + +

NaCl

CaSO4.2H2O

CaCO3

Bittern salts PPt's

Fig. 1 Successional 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.

Bittern salts + +

NaCl

CaSO4.2H2O

CaCO3

12 m

Bittern salts PPt's

Fig. 1 Successional 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)

observation indicate the possible variations. For example, doubling calcium ion (Ca2+) seawater concentrations at present sulphate ion (SO4-) concentrations would not affect the sequence, whereas tripling the Ca2+ concentration would. Similarly, halving or doubling present-day potassium ion (K+) concentrations would result in the formation of some very unusual bittern salts, not seen in the geological record.

Ideas about variations in sodium ion (Na+) and chloride ion (Cl-) concentrations are based on ancient halite inventories. The total volume of known halite deposits amounts to about 30% of the NaCl content of the present oceans. If all of this salt were added to the present oceans, the salinity of seawater would increase by about 30%, setting an upper limit. However, the ages of major halite deposits are reasonably well dispersed through geological time, suggesting that there was never a time when all of these ions were dissolved in seawater.

Setting lower limits on Na+ and Cl-concentrations in seawater can be estimated by considering the larger evaporite deposits in the geological record. For example, in Miocene times (5-6 million years ago) about 28 x 1018mol of NaCl was deposited in the Mediterranean-Red Sea basins. This volume of salt represents just 4% of the present mass of oceanic NaCl. This suggests that periodic evaporate-forming events are only able to decrease Na+ and Cl- concentrations of seawater by small amounts. It has been suggested that the salinity of seawater has declined in 'spurts' from 45 to 35 gl-1 over the last 570 million years. During this time the formation of Permian-aged salts alone (280-230 million years ago) may have caused a 10% decrease in salinity, possibly contributing to the extinction of many marine organisms at the end of this period.

Overall, these types of constraints suggest that the major ion chemistry of seawater has varied only modestly (probably by no more than a factor of 2 for each individual ion) during the last 900 million years or so (slightly less than a quarter of geological time).

0 0

Post a comment