Despite these complications, the main removal mechanisms of major ions from
seawater are known (Table 6.2). Quantifying the importance of each mechanism
is less easy and the uncertainty of data in Table 6.2 should not be forgotten.
The amount of removal is compared with the riverine inputs resulting in a
geochemical ‘budget’ that helps constrain the quality of the data. In the follow-
ing section we outline the important removal processes for major ions in
seawater.6.4.1 Sea-to-air fluxes
Sea-to-air fluxes of major ions are caused by bubble bursting and breaking waves
at the sea surface. These processes eject sea-salts into the atmosphere, the major-
ity of which immediately fall back into the sea. Some of these salts are, however,
transported over long distances in the atmosphere and contribute to the salts in
riverwater (see Section 5.3). These airborne sea-salts are believed to have the
same relative ionic composition as seawater and their flux out of the oceans is
estimated by measuring the atmospheric deposition rates on the continents. In
terms of global budgets, airborne sea-salts are an important removal process
only for Na+and Cl-from seawater; removal of other major ions by this route is
trivial.6.4.2 Evaporites
Evaporation of seawater will precipitate the constituent salts, the so-called evap-
orite minerals, in a predictable sequence (Box 6.2). This sequence starts with the
least soluble salts and finishes with the most soluble (see Box 4.12). If approxi-
mately half (47%) of the water volume is evaporated, CaCO 3 precipitates (see194 Chapter Six
Table 6.2Simplified budget for major ions in seawater. All values are in 10^12 mol yr-^1. Data
from Berner and Berner (1987).
Removal/source*
River Sea–air CEC– Opaline
Ion input fluxes Evaporites clay CaCO 3 silica Sulphides MOR
Cl- 5.8 1.1 4.7 —— — — —
Na+ 8.3 0.9 4.7 0.8 —— — 1.6
Mg^2 + 5.0 —— 0.1 0.6 —— 4.9
SO 42 - 3.2 — 1.2 —— — 1.2 —
K+ 1.1 —— 0.1 —— — -0.8
Ca^2 + 11.9 — 1.2 -0.5 17 —— -4.8
HCO 3 - 30.6 —— — 34 — -2.4 —
Si 5.8 —— ——7.0 — -1.1
* Minus sign indicates a source.
CEC–clay, cation exchange on estuarine clay minerals; MOR, mid-ocean ridge and other seawater–basalt
interactions.