6.5 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 (N 2 ) 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 (CO 2 ) and oxygen (O 2 ), are intimately involved in bio-
logical 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 ele-
ments 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 mmol l-^1
concentrations or less) in seawater. The concentration of some dissolved metals
in seawater is very small—typically a few nanomoles per litre (nmol l-^1 ). Sampling
so as to avoid contamination and measuring such tiny concentrations, in the pres-
ence 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 mgl-^1 ),
whereas in surface waters particulate matter concentrations are relatively high
(generally 10–100mgl-^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 resuspen-
sion 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, gen-
erated 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 typically216 Chapter Six