An Introduction to Environmental Chemistry

eqn. 6.4). With continued evaporation and an approximately four-fold increase
in salinity, CaSO 4 .2H 2 O (gypsum) precipitates:

eqn. 6.2

Once 90% of the water (H 2 O) has been evaporated, at dissolved salt concentra-
tions around 220 g l-^1 , NaCl precipitates:

eqn. 6.3

and, in addition, some magnesium (Mg) salts begin to crystallize; if evaporation
continues, highly soluble potassium (K) salts precipitate (Box 6.2).
The problem with invoking evaporation as a removal mechanism for ions in
seawater is that there are currently very few environments in which evaporite salts
are accumulating to a significant extent. This is because enormous volumes of
seawater need to be evaporated before the salts become concentrated enough to
precipitate. Clearly this cannot occur in the well-mixed open oceans, where net
evaporative water loss is roughly balanced by resupply from continental surface
waters (river flux). This implies that evaporative concentration of seawater can
only occur in arid climatic regions within basins largely isolated from the open
ocean and other sources of water supply. There are no modern examples of such
basins; modern evaporite deposits are two to three orders of magnitude smaller
than ancient deposits and are restricted to arid tidal flats and associated small salt
ponds; for example, on the Trucial Coast of the Arabian Gulf. Note, however,
that large evaporite deposits do exist in the geological record, the most recent
example resulting from the drying out of the Mediterranean Sea in late Miocene
times (about 5–6 million years ago).
As Cl-has a very long oceanic residence time, the sporadic distribution of
evaporate-forming episodes (Box 6.2), integrated over million-year timescales,
results in only quite small fluctuations in the salinity of seawater. However, the
lack of major evaporate-forming environments today suggests that both Cl-and
sulphate (SO 42 - ) are gradually accumulating in the oceans until the next episode
of removal by evaporite formation.
In Table 6.2 the amount of Cl-removed from seawater by evaporation has
been set to balance the input estimate. This is acceptable because there are no
other major Cl-sinks, after allowing for sea-to-air fluxes and burial of pore water
(Section 6.4.8). The Cl-removal term dictates that the same amount of Na+is
also removed to match the equal ratios of these ions in NaCl. The figure for SO 4 2-
removal by evaporation (Table 6.2) is plausible, albeit poorly constrained. Again,
the SO 42 - estimate dictates an equal removal of Ca^2 +ions to form CaSO 4 .2H 2 O.

6.4.3 Cation exchange

Ion-exchange processes on clay minerals moving from riverwater to seawater
(Section 6.2.3) remove about 26% of the river flux of Na+to the oceans and are
significant removal processes for K+and Mg^2 +(Table 6.3). To balance this
removal, clay minerals exchange a significant amount of Ca^2 +to the oceans,
adding an extra 8% to the river flux (Table 6.3). These modern values are,

Na()+aq+Cl()-aq ªNaCl()s

Ca^2 ()aq+ ++SO^24 - ()aq 22 H O 24 ()lsªCaSO.H O 2 ()

The Oceans 195