Chemical cycling of major ions

The concept of residence times was introduced when discussing atmospheric gases (see Section 3.3), but it is applicable to most other geochemical systems, including the oceans. Residence times of the major ions in seawater (Box 6.3) are important indicators of the way chemical cycling operates in the oceans. These residence times are all very long (104 to 108 years), similar to or longer than the water itself (around 3.8 x 104 years) and very much longer than those calculated for atmospheric gases (see Section 3.3). Long residence times mean there is ample opportunity for ocean currents to mix the water and constituent ions thoroughly. This ensures that changes in ion ratios arising from localized input or removal processes are smoothed out. It is the long residence times of the ions that create the very constant ion ratios in seawater. The long residence times result from the high solubility of the ions and hence their z/r ratios (see Section 5.2). Other cations with similar z/r ratios will also have long oceanic residence times (e.g. caesium ion (Cs+)), but these are not major ions in seawater because of their low crustal abundances. Chloride is an interesting exception as it is abundant in sea-water, has a long residence time and yet has a low crustal abundance. Most of this Cl- was degassed from the Earth's mantle as hydrogen chloride (HCl) very early in Earth history (see Section 1.3.1) and has been recycled in a hydro-sphere-evaporite cycle since then (Section 6.4.2).

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