Naaq Claq NaClSeqn

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 (SO4-) 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 SO4-removal by evaporation (Table 6.2) is plausible, albeit poorly constrained. Again, the SO4- estimate dictates an equal removal of Ca2+ ions to form CaSO4.2H2O.

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 Mg2+ (Table 6.3). To balance this removal, clay minerals exchange a significant amount of Ca2+ to the oceans, adding an extra 8% to the river flux (Table 6.3). These modern values are,

Table 6.3 Additions to the river flux from ion exchange between river-borne clay and

seawater.

From Drever et al. (1988).

Laboratory studies

Amazon

Average*

Percentage of

(1012molyr1)

(1012molyr-1)

(1012molyr-1)

river flux'

Na+

-1.58

-1.47

-1.53 ± 0.06

26

K+

-0.12

-0.27

-0.20 ± 0.08

17

Mg2+

-0.14

-0.49

-0.31 ± 0.08

7

Ca2+

0.86

1.05

0.96 ± 0.10

8

*The averages are based on modern suspended sediment input to the oceans, which is probably double the long-term input rate (see Section 4.4). These values are thus halved when used in Table 6.2. ^ Corrected for sea-salt and pollution inputs. Minus sign indicates removal from seawater.

*The averages are based on modern suspended sediment input to the oceans, which is probably double the long-term input rate (see Section 4.4). These values are thus halved when used in Table 6.2. ^ Corrected for sea-salt and pollution inputs. Minus sign indicates removal from seawater.

however, thought to be double the long-term values due to the effects of abnormally high postglacial suspended-solid input rates (see introductory text to Section 6.4). The modern values are halved in Table 6.2 to account for this.

6.4.4 Calcium carbonate formation

It is surprisingly difficult to calculate whether seawater is supersaturated or under-saturated with respect to CaCO3. There are a number of different approaches to the problem all based on equilibrium relationships that describe the precipitation (forward reaction) or dissolution (back reaction) of CaCO3.

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