5.4: Sources and Sinks of Chemicals Dissolved in Seawater

Seawater is a solution of many different chemical compounds: cations, anions, and both organic and inorganic compounds that are not ionized. Concentrations of the compounds in ocean water are determined by their behavior in global biogeochemical cycles (Fig. 5-5) and by their abundance in the materials of the Earth’s crust. Compounds are both added to ocean water from sources such as river runoff and removed from seawater to sinks such as the seafloor sediments.

Figure 5-5. A simplified biogeochemical cycle.

Biogeochemical Cycles

Continental rocks are weathered and transported to the oceans both as particles and as dissolved ions. The particles are deposited as ocean sediment. Many dissolved elements are used in biological processes and subsequently are incorporated in seafloor sediment in the form of the hard parts and detritus of marine organisms. Elements dissolved in seawater also can be precipitated directly or on particulate matter or they may be adsorbed by (adhere to) mineral grains and detritus and thus be removed to the sediment.

Over millions of years, ocean sediments are compacted by the weight of overlying sediments. Water is squeezed out and minerals precipitate between the grains, cementing the grains together to form sedimentary rock. Vast volumes of sedimentary rock and sediment eventually enter a subduction zone. Some of this material is scraped off the descending lithospheric plate edge and raised to form sedimentary arc islands, continental margin rocks, and exotic terranes (Chap. 4), where it is again weathered and eroded, thus continuing the biogeochemical cycle. Much of the sedimentary rock and sediment is subducted into the mantle, where some of it is melted and ejected through volcanoes as ash or lava. This material either reenters the oceans and returns to the ocean sediment directly, or collects on the land, where it is weathered and eroded to start a new cycle (Fig. 5-5).

Biogeochemical cycles are actually much more complicated than Figure 5-5 indicates. For example, many elements used by marine organisms are rapidly recycled to seawater solution by the decomposition of detritus. Similarly, some elements from oceanic crustal rocks enter seawater in fluids discharged by hydrothermal vents at oceanic ridges (Chap. 15).

Steady-State Concentrations

The concentration of each element in ocean water is determined by the rate at which it enters the ocean water and the rate at which it is removed. If the rate of input exceeds the rate of removal, the concentration will rise, and vice versa. Although global biogeochemical cycles include processes that take hundreds of millions of years, these cycles have been ongoing for billions of years and are thought to be at an approximate steady state. Steady state is achieved when the total quantity of an element in each of the compartments (square boxes) in Figure 5-5 remains approximately the same over time (its input rate equals its removal rate).

To see how a steady state is achieved, consider what would happen if an element’s rate of input to the oceans were increased. The total quantity of an element within the global biogeochemical cycle does not change. Therefore, as the input to the oceans increases and the total quantity of the element in the oceans increases, the total quantity of the element in the other components of the cycle (sediments, rocks, etc.) must decrease (Fig. 5-5). Consequently, less of the element is in the components of the cycle that provide inputs to the oceans, and their inputs to the oceans must decrease. If an element’s input to the oceans increases, so do the rates of the various removal processes in the biogeochemical cycle.

Here’s a simple analogy: If we pour orange juice (the input) into a glass full of water and stir continuously, the glass will overflow. At first, the overflowing liquid (the output) is almost pure water, with only traces of orange juice mixed in it. The input of orange juice has increased, but neither the concentration in the glass nor that in the output reaches a new steady level instantly. As we continue to pour orange juice into the glass, the concentration of juice in the glass increases progressively to full concentration. The concentration of juice in the output also increases progressively in response to the change in concentration in the glass. When the concentrations of orange juice in the glass and in the output match the concentration of orange juice in the input, the system has reached a new steady state. Subsequently, unless we change the concentration of orange juice in the input, or alter the output in some way (such as by allowing the orange juice pulp to settle and accumulate at the bottom of the glass), the input equals the output and the concentration of orange juice within the glass does not vary. Biogeochemical cycles are more complicated than our simple analogy because they have many inputs and outputs, and multiple “glasses” that empty into each other. However, they reach steady state in a similar way, and this steady state can be disturbed only by changes in one or more inputs or outputs.

When the global biogeochemical cycles are approximately at steady state, the rate of input of most elements to the oceans is approximately equal to the rate of removal. However, during Earth’s long history, changes such as plate tectonic movements and climate changes have altered the rates of input or output of the various elements to the oceans so the concentration of elements in the oceans has varied over long timescales.

Residence Time

The concentrations of elements in seawater are determined largely by the effectiveness of the processes that remove them from solution. Concentrations are high if the element is not removed rapidly or effectively from solution in ocean waters and if it is abundant in the Earth’s crust. The effectiveness of removal is expressed by the residence time (CC8), which is a measure of the mean length of time an atom of the element spends in the oceans before being removed to the sediment (or atmosphere).

The relationships of crustal abundance, residence time, and concentration in seawater for several elements are shown in Table 5-4. Note that the effectiveness of the removal processes, as expressed by residence time, is the principal determinant of concentration. Concentrations in this table are expressed both as mg•kg–1 the traditional units, that express weight of element per kg of seawater and, molality units that reflect the relative number of atoms of each element in seawater rather than the total weight of the elements. 

Table 5-4. Crustal Abundance, Oceanic Residence Time, and Seawater Concentration of Several Elements

Element	Crustal Abundance (%)	Ocean Residence Time (years)	Concentration (mg•kg^–1)	Concentration (pmol•kg^–1)
NA (sodium)	2.4	55,000,000	10,780	4.69•10^11
Cl (chlorine as Cl^–1)	0.013	87,000,000	19,360	5.46•10^11
Mg (magnesium)	2.3	13,000,000	1280	5.27•10^10
K (potassium)	2.1	12,000,000	399	1.02•10^10
S (sulfur as sulfate, SO4^2^–)	0.026	8,700,000	898	2.80•10^10
Ca (calcium)	4.1	1,100,000	412	1.02•10^10
Fe (iron)	2.4	200 to 500	0.00003	540
Al (aluminium)	6.0	200	0.00003	1110
Mn (manganese)	0.5	60	0.00002	360
Pb (lead)	0.001	80	0.0000027	13