6.4 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 (10^4 to 10^8 years), similar to or longer than the
water itself (around 3.8¥ 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/rratios (see Section 5.2). Other
cations with similar z/rratios 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).The Oceans 191observation indicate the possible variations.
For example, doubling calcium ion (Ca^2 +)
seawater concentrations at present sulphate
ion (SO 42 - ) concentrations would not affect
the sequence, whereas tripling the Ca^2 +
concentration would. Similarly, halving or
doubling present-day potassium ion (K+)
concentrations would result in the formation
of some very unusual bittern salts, not seen
in the geological record.
Ideas about variations in sodium ion (Na+)
and chloride ion (Cl-) concentrations are
based on ancient halite inventories. The total
volume of known halite deposits amounts
to about 30% of the NaCl content of the
present oceans. If all of this salt were added
to the present oceans, the salinity of
seawater would increase by about 30%,
setting an upper limit. However, the ages
of major halite deposits are reasonably
well dispersed through geological time,
suggesting that there was never a time when
all of these ions were dissolved in seawater.
Setting lower limits on Na+and Cl-
concentrations in seawater can be estimated
by considering the larger evaporite deposits
in the geological record. For example, in
Miocene times (5–6 million years ago) about
28 ¥ 1018 mol of NaCl was deposited in the
Mediterranean–Red Sea basins. This volume
of salt represents just 4% of the present mass
of oceanic NaCl. This suggests that periodic
evaporate-forming events are only able to
decrease Na+and Cl-concentrations of
seawater by small amounts. It has been
suggested that the salinity of seawater has
declined in ‘spurts’ from 45 to 35 g l-^1 over
the last 570 million years. During this time
the formation of Permian-aged salts alone
(280–230 million years ago) may have caused
a 10% decrease in salinity, possibly
contributing to the extinction of many
marine organisms at the end of this period.
Overall, these types of constraints suggest
that the major ion chemistry of seawater has
varied only modestly (probably by no more
than a factor of 2 for each individual ion)
during the last 900 million years or so
(slightly less than a quarter of geological
time).