Minor chemical components in seawater

6.5.1 Dissolved gases

Gases dissolve from the atmosphere into the oceans according to the Henry's law constant (see Box 3.4). In the absence of biological processes the ocean surface waters would therefore be saturated with all atmospheric gases. Some of these gases, such as argon (Ar) and helium (He), are chemically inert, while others such as nitrogen gas (N2) are available as a nutrient source to only a few specialized nitrogen-fixing organisms and are hence effectively inert in seawater, and thus very close to saturation. We will not consider these species further. Other gases, such as carbon dioxide (CO2) and oxygen (O2), are intimately involved in biological cycles, similar to those already discussed in freshwater systems (see Section 5.5). We will consider these gases later when discussing the impact of biological cycles on ocean chemistry (Section 6.5.4).

6.5.2 Dissolved ions

Seven major ions dominate the chemistry of seawater, but all of the other elements are also present, albeit often at extremely low concentrations. The major ions in seawater are little affected by biological processes or human activities because seawater is a vast reservoir and the major ions have long residence times. By contrast, complex cycling processes and involvement in biological systems typify the behaviour of dissolved trace elements (components present at |mmoll-1 concentrations or less) in seawater. The concentration of some dissolved metals in seawater is very small—typically a few nanomoles per litre (nmoll-1). Sampling so as to avoid contamination and measuring such tiny concentrations, in the presence of major ions with millimolar concentrations, is difficult. These difficulties prevented routine analysis of trace metals in seawater until the 1970s, although reliable nutrient measurements were available earlier.

Particulate matter concentrations in the deep ocean are low (a few |lgl-1), whereas in surface waters particulate matter concentrations are relatively high (generally 10-100 |lgl-1) dominated by material produced by biological processes in the euphotic zone (Fig. 6.18). Similarly high values can be encountered within tens or hundreds of metres of the deep ocean floor, caused either by resuspension of deep-sea sediments, or from hydrothermal fluid plumes, sourced from hydrothermal vents (Section 6.4.7 & Fig. 6.18). Apart from this region near the seafloor, particulate matter in the oceans is predominantly of organic origin, generated by primary production in surface seawater. The euphotic zone, where this production occurs, has variable depth, generally around 100 metres in clear open-ocean waters. Since the oceans are on average almost 4000 metres deep, the primary production that drives global biological cycling throughout the oceans occurs in a shallow surface zone.

Dissolved metals in seawater have various sources, for example the dissolution of redox-sensitive metals from reducing ocean floor and mid-ocean ridge hydrothermal sediments. Hydrothermal sediments, for example, are typically

Table 6.7 Source of dissolved manganese to the oceans (109molyr '). After Chester (2000).

Rivers

Atmosphere

Hydrothermal (MOR)*

Source

5

0.5

11-34

*An unknown but potentially significant amount of manganese emitted from mid-ocean ridge (MOR) hydrothermal sources is precipitated near black smokers (i.e. immediately removed), so this estimate is not directly comparable to the other inputs.

*An unknown but potentially significant amount of manganese emitted from mid-ocean ridge (MOR) hydrothermal sources is precipitated near black smokers (i.e. immediately removed), so this estimate is not directly comparable to the other inputs.

iron- and manganese-rich. These sediments derive from FeS particles that precipitate in the 'black smoke' plume as the hot and acidic hydrothermal water is rapidly cooled and mixed with alkali seawater. The particles fall out both close to the vent and some kilometres from it. It is currently difficult to quantify the global metal fluxes from these processes, although estimates have been made. In the case of manganese, mid-ocean ridge hydrothermal sources can be up to hundreds of mmolkg-1, 106 times higher than ambient seawater, and thus may be significant to the global oceanic budget (Table 6.7). Modern atmospheric fluxes of some metals are larger than river inputs (Table 6.8), caused by various combustion processes—coal burning, metal smelting and automobile engines. The shift towards a larger industrial atmospheric source for some metals may increase their concentrations in open-ocean waters, since riverine metal inputs are often removed in estuaries (Section 6.2).

The chemistry of dissolved metals in seawater can be grouped into three classes, which describe the behaviour of the metal during chemical cycling. These classes —conservative, nutrient-like and scavenged —have been recognized by the shapes of concentration profiles when plotted against depth in the oceans.

Table 6.8 Comparison of total atmospheric and riverine inputs to the world's oceans (109mol yr-1). Based on Duce et al. (1991).

Element

Riverine*

Atmospheric

Nitrogen (ex N2)

1500-3570

2140

Cadmium

0.0027

0.02-0.04

Copper

0.16

0.25-0.82

Nickel

0.19

0.37-0.48

Iron

19.7

580

Lead

0.01

0.43

Zinc

0.09

0.67-3.5

Total input—dissolved plus particulate. Estimates are based on data available in early 1990s and so include significant amounts of material mobilized by human activity.

^Dissolved input only; particulate components are assumed to sediment out in estuaries and the coastal zone.

Total input—dissolved plus particulate. Estimates are based on data available in early 1990s and so include significant amounts of material mobilized by human activity.

^Dissolved input only; particulate components are assumed to sediment out in estuaries and the coastal zone.

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