An Introduction to Environmental Chemistry

The residence times in Box 6.3 are based on riverwater being the only input

of ions to the oceans. This is a simplification as there are also inputs from the

atmosphere and from hydrothermal (hot water) processes at mid-ocean ridges

(Fig. 6.7). For major ions, rivers are the main input, so the simplification in Box

6.3 is valid. For trace metals, however, atmospheric and mid-ocean ridge inputs

are important and cannot be ignored in budget calculations (Section 6.5).

The long residence times of the major ions compared with the water mean

that seawater is a more concentrated solution than riverwater. However, the dif-

ferent ionic ratios of seawater and riverwater show that the oceans are not simply

the result of riverwater filling the ocean basins, even if the resulting solution has

been concentrated by evaporation. Although major ion residence times are all

long, they vary over four orders of magnitude, showing that rates of removal for

specific ions are different. Processes other than evaporative concentration must

be operating.

192 Chapter Six

Box 6.3 Residence times of major ions in seawater

The total volume of the oceans is 1.37¥ 1021 l

and the annual river discharge to the oceans

is 3.6¥ 1016 lyr-^1. The residence time of water

in the oceans is therefore:

eqn. 1

Applying this approach to the data in Table

6.1, it is straightforward to calculate the

residence times of the major ions (Table 1),

assuming that:

1 dissolved salts in rivers are the dominant

sources of major ions in seawater;

2 steady-state conditions apply (see Section

3.3).

Inventory

Input = years

¥

¥ =¥

137 10

38 10 38 10

21

16 4

.

..

The first assumption is probably valid, since

the other sources listed in Table 6.2 do not

greatly alter the results derived by

considering rivers alone. The issue of steady

state cannot be verified for very long

(millions of years) timescales, but the

geological evidence does suggest that the

concentration of major ions in seawater has

remained broadly constant over very long

time periods (Box 6.2). As an example of the

residence time calculation, consider sodium

(Na+):

Input=discharge in rivers¥river concentration

eqn. 2

Inventory=water content of the oceans¥ocean

concentration

eqn. 4

Residence time mol

78 yr

= ¥

¥

=¥

644 10

828 10

10

18

12 1

6

. mol yr

=¥( )¥¥( )

=¥

1 37 10 470 10-

644 10

21 3

18

. mol
mol

=¥ ¥ ¥

=¥

---

36 10 023 10

828 10

631

12 1

..

.

mol yr

mol yr

Table 1Residence times of major ions in seawater.

Ion Residence time (10^6 yr)

Na+ 78

Mg^2 + 14

K+ 13

Ca^2 + 1.1

HCO 3 - 0.09

SO 42 - 12

Cl- 131

eqn. 3