Box 63 Residence times of major ions in seawater

The total volume of the oceans is 1.37 x 1021l and the annual river discharge to the oceans is 3.6 x 1016l yr-1. The residence time of water in the oceans is therefore:

Input

3.8 x 1016

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).

Table 1 Residence times of major ions in seawater.

Residence time (106yr)

Mg2+

Ca2+

HCO-

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 x river concentration = 3.6 x 10-6 x 0.23 x 10-3molyr-1 = 8.28 x 1012molyr-1 eqn. 2

Inventory = water content of the oceans x ocean concentration = (1.37 x 1021) x (470 x 10-3)mol = 644 x 1018 mol eqn. 3

Residence time =

644 x 1018mol

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 different 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.

Sources

Rivers

Atmosphere

Hydrothermal (MOR)

*

Seawater

(see text)

Fig. 6.7 Simple box model summarizing material inputs to seawater. Style of arrow indicates relative importance of input: bold, high; pecked, low. MOR, mid-ocean ridges.

Fig. 6.7 Simple box model summarizing material inputs to seawater. Style of arrow indicates relative importance of input: bold, high; pecked, low. MOR, mid-ocean ridges.

Identifying removal mechanisms for a specific component is difficult because removal processes are usually slow and occur over large areas. Some removal processes are very slow—operating on geological timescales of thousands or millions of years —and impossible to measure in the present oceans. The requirement to study element cycles on geological timescales is further complicated by processes like climate change and plate tectonics which affect the geometry of ocean basins and sealevel. These large-scale geological processes can have significant effects on removal processes of major ions from the oceans.

The effects of the geologically recent glacial-interglacial oscillations during the Quaternary period (the last 2 million years) are particularly relevant. Firstly, the rapid rise in sealevel over the last 11 000 years, following the melting of polar ice accumulated during the last glacial period, has flooded former land areas to create large, shallow, continental shelves, areas of high biological activity and accumulation of biological sediments (Section 6.2.4). Also, the unconsolidated glacial sediments which mantle large areas of northern-hemisphere (temperate-arctic zone) land surfaces are easily eroded. This results in high particulate concentrations in rivers, which carry material to estuaries and continental shelves. This enhanced sediment supply results in correspondingly high detrital sediment-seawater interactions, increasing the importance of removal processes such as ion exchange.

Table 6.2 Simplified budget for major ions in seawater. All values are in 1012molyr Data from Berner and Berner (1987).

Removal/source*

Table 6.2 Simplified budget for major ions in seawater. All values are in 1012molyr Data from Berner and Berner (1987).

Removal/source*

Ion

River input

Sea-air fluxes

Evaporites

CEC-clay

CaCO3

Opaline silica

Sulphides

MOR

Cl-

5.8

1.1

4.7

Na+

8.3

0.9

4.7

0.8

1.6

Mg2+

5.0

0.1

0.6

4.9

SO2-

3.2

1.2

1.2

K+

1.1

0.1

-0.8

Ca2+

11.9

1.2

-0.5

17

-4.8

HCO-

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.

*Minus sign indicates a source.

CEC-clay, cation exchange on estuarine clay minerals; MOR, mid-ocean ridge and other seawater-basalt interactions.

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 following 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 majority 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 approximately half (47%) of the water volume is evaporated, CaCO3 precipitates (see eqn. 6.4). With continued evaporation and an approximately four-fold increase in salinity, CaSO4.2H2O (gypsum) precipitates:

0 0

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