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OCN 201 Chemical Oceanography Class Notes, Fall 2014
The origin of sea salt
Chris Measures,
Department of Oceanography
Introduction
Everyone knows that the sea is salty but what exactly is the salt in the sea made of
and how did it get there? Chemical Oceanography is the study of the chemicals that
make up the salt in the ocean. Chemical Oceanographers are interested in knowing what
kind of chemicals are present in the ocean, how they get there and how their distributions
can be used to help us understand the ocean. The distribution of the dissolved chemicals
in the oceans can be used to tell us about the cycle of biological life in the ocean. They
can also be used to track water as it moves from one part of the ocean to another. We can
use the distribution of some dissolved chemicals at the bottom of the ocean to show us
where hot water from submarine hydrothermal springs is coming out of the bottom of the
ocean. And we can use the distribution of chemicals in sediments at the bottom of the
ocean to tell us about the history of the Earth -- such as what the weather was like 20,000
years ago and whether a massive meteorite from outer space crashed into the Earth 63
million years ago, wiping out the dinosaurs and many of the other forms of life, and
giving the mammals (the group we are part of) our chance to make it as a dominant
species on the planet.
While all of these things can be used to explain why it is important that we study
Chemical Oceanography, perhaps the most important thing to realise is that if there were
no dissolved chemicals in the ocean i.e., if the oceans were made up of distilled water,
there would be no living creatures in the ocean. If there were no salt in the ocean its
physical properties would be very different and it would not circulate the way that it does
now. Chemical Oceanography is then central to all oceanography. Also since the climate
of the Earth is controlled primarily by the circulation of the ocean, we can see that
chemical oceanography the study of the chemicals dissolved in seawater is directly
related to the study of the climate of our planet.
We will cover each of these topics in more detail over the next few lectures but
first of all we start by looking at what the salt in sea water is made of.
Salt in sea water: Major ions
Anyone who has been swimming in the ocean knows that its salt tastes like the
table salt, sodium chloride, that you may put on your food. So it will come as no surprise
to find out that the main chemical ingredient of sea water is sodium chloride. In fact, as
table 1 shows, 86% of the dissolved material in sea water is sodium chloride, followed in
decreasing order of concentration by magnesium, sulphate, calcium and potassium.
These six chemical elements together account for 99.8% of all the dissolved chemicals in
sea water (the percentages in the table below only add up to 99% because the numbers
are rounded).
If we add up the weights in the table below we can see that in each kg (a kilogram
is approx. equal to 2 lbs) of seawater there are about 35 grams (there are about 28 grams
in one ounce) of salt dissolved. The amount of salt dissolved in the water is also known
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as its salinity. A fairly typical amount of salt in sea water is 35 grams of salt per 1
kilogram (1,000 grams) of sea water this is a salinity of 35. This is quite a large amount
of material, for example rivers rarely have more than 1 gram per kg of dissolved salts -the average of all the river water in the world contains about 0.1 gram per kg. 35 gram
per kg is a large enough amount of dissolved material to affect the physical properties of
the water. For example, the freezing point of pure water is 0˚C (32 ˚F) while that of
seawater is about -1.8˚C (28 ˚F).
Table 1
Major ions in seawater
Symbol
Name
ClNa+
Mg2+
SO42Ca2+
K+
Chloride
Sodium
Magnesium
Sulphate
Calcium
Potassium
% of
total
55
31
4
8
0.5
0.5
Amount/kg
mmoles
gms
546
19.344
469
10.773
53
1.294
28
2.712
10.3
0.412
10.2
0.399
This depression of freezing point comes solely because of the amount of salt
dissolved in the water. Another interesting property of the water that is affected by the
dissolved salt is the temperature of maximum density. For fresh water this occurs at
about 4˚C (40 ˚F) -- i.e., several degrees above its freezing point. However, for sea water
though the temperature of maximum density occurs at the freezing point of seawater (-1.8
˚C). The temperature of maximum density has a tremendous affect on the circulation of
the ocean, since it affects how and where the surface waters in the ocean are made dense
enough to sink down to form the new deep waters of the ocean. We will discuss this in
more detail later when we talk about the deep circulation of the ocean.
The six chemicals listed above that make up most of the salt in seawater have a
special name. They are called the major ions of seawater.
Marcet's Principle
The major ions have an interesting property which is that the ratios of the ions to each
other is almost exactly the same wherever you go in the open ocean (away from
coastlines where rivers are running into the ocean).
What this means is that even though the total amount of salt dissolved in seawater can
vary from 32 to 39 grams per kilogram, the ratio of each major ion to each other major
ion stays exactly the same. For instance the ratio of sodium to chloride (in molar units) is
0.859 in seawater of salinity 32 and in seawater of salinity 35. This means that the
concentrations of all of these ions changes together in unison. The fact that the ratios
stay constant is known as Marcet’s Principle, after the scientist who discovered it.
Marcet's Principle states that:
“Regardless of how the salinity may vary from place to place, the ratios
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between the amounts of the major ions in the waters of the open ocean are
nearly constant.”
Because of this property of constant relative proportions, these ions are often
referred to as conservative ions.
The constancy of the ratio of the major ions is extremely valuable to physical
oceanography. The density of seawater determines how it circulates. Density is the mass
per unit volume of the seawater and is determined by both the temperature of the water
and its salinity (the amount of salt dissolved in it). Since we know that the 6 major ions
account for 99.8% of the salt in seawater and they occur in the same relative proportions
(Marcet’s Principle), it is possible for Physical Oceanographers to measure just one of
those six ions and temperature to determine density.
Minor and trace species
While the 6 major ions make up 99.8% of the dissolved salts in sea water, the
other 0.2% is made up of all the other elements and compounds (see the sheet on
Elements, Compounds and Ions). The minor and trace species, as all these other
chemicals are known, usually have highly variable concentrations in the ocean both from
region to region as well as from the surface to the deep.
Table 2
Minor ions, nutrients and other components of interest to us
Minor ions
Symbol
-
HCO3
BrSr2+
H2BO3
F-
Name
Amount/kg
moles
2.32 mmol
0.84 mmol
90 µmol
73 µmol
67 µmol
Bicarbonate
Bromide
Strontium
Borate
Fluoride
gms
0.142
0.0674
0.0079
0.00445
0.00128
Nutrients
SiO2
PO43NO3-
Silica
Phosphate
Nitrate
0-170 µmol
0-3.2 µmol
0-44 µmol
Other chemical species
Cd2+
3H
Cadmium
Tritium
0-1nmol
measured relative to hydrogen
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This variability is what makes them of great interest to Chemical Oceanographers.
They are often referred to as non-conservative ions, in contrast to the conservative,
major ions. A complete list of all the constituents in seawater would be extremely long
and boring, instead included here is a list of just a few. Some of these, the nutrients, will
be discussed in more detail in the next lecture.
So, why are some ions present in sea water in high concentrations while others are
present in only low concentrations?
Also, why are some ions present in constant relative proportions and others vary
considerably?
Are these properties connected?
We will try to find the answer to these questions by looking at where the chemicals in the
oceans are coming from.
Sources of chemicals to the sea
It seems reasonable to assume that a likely way in which ions are delivered to the
ocean is with the water that flows into the oceans. The table below outlines the various
water flows into the oceans as well as giving the volume of the oceans.
Table 3
Water flows into the oceans
Volume of the oceans:
1.37 x 1021 litres.
A
Volume of river water flowing into the oceans:
3.74 x 1016 litres/year
B
Volume of rain falling onto the surface of the ocean:
3.8 x 1017 litres/year
C
Water coming into the oceans from submarine hydrothermal vents:
1.7 x 1014 litres/year
D
Time taken to fill the oceans from rivers:
= A/B = 36,000 yrs.
Time taken to fill the ocean from direct rain
= A/C = 3,600 yrs.
Time taken to fill the ocean from hydrothermal = A/D = 8,000,000 yrs.
From the table above we can see that would take 36,000 years to replace all the
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water in the oceans from the rivers flowing into it. We calculate this number by
dividing the volume of the ocean (1.37 x 1021 litres) by the yearly river flow into the
oceans (3.74 x 1016 litres/year). In a similar manner we can calculate the time taken to
fill the oceans from rain and also from the hydrothermal vents that are found on the
bottom of the ocean.
It would take 8 million years to fill the oceans with water from hydrothermal
springs so we would not expect them to be major sources of dissolved chemicals to the
ocean (we will see later though that for a few elements hydrothermal springs can very
important sources).
Although rain falling on to the surface ocean is the most important source of
water to the ocean, it does not bring many chemicals into the ocean with it since most
rain is just water that evaporated from the nearby surface ocean and then rained back in
again, it is thus water that is recycling rapidly. (However we will see later that this is a
very important route for iron to get into the ocean)
It is the rain that falls onto the continents weathering (breaking down) the rocks
and forming the rivers that flow back to the oceans that brings most of the chemicals to
the oceans.
Since rivers will get their dissolved ions from rocks on the continents, we will
compare the composition of seawater with that of average rocks.
Table 4
Most common ions in seawater and river water and the elemental composition of the
Earth’s crust* listed in order of abundance
Seawater
(as ions)
Earth’s crust
(as elements)
River water
(as ions)
Chloride (Cl-)
Sodium (Na+)
Sulphate (SO42-)
Magnesium (Mg2+)
Calcium (Ca2+)
Potassium (K+)
Bicarbonate (HCO3-)
Bromide (Br-)
Borate (H2BO3-)
Strontium (Sr2+)
Silicon (Si)
Aluminium (Al)
Iron (Fe)
Calcium (Ca)
Sodium (Na)
Potassium (K)
Magnesium (Mg)
Titanium (Ti)
Manganese (Mn)
Phosphorous (P)
Bicarbonate (HCO3-)
Calcium (Ca2+)
Silicate (SiO2)
Sulphate (SO42-)
Magnesium (Mg2+)
Potassium (K+)
Sodium (Na+)
Iron (Fe3+)
* Excluding oxygen
From the table we can see that while four elements are common to both lists, the
top three elements in the crust of the Earth do not even make it into the top 10 elements
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in seawater and the most abundant ion in seawater is not even in the top 10 elements of
the Earth’s crust. Why is this?
One explanation could be simply that just because something is very abundant in
rocks does not mean that it has to dissolve in rain water and end up in the ocean.
Therefore, we will look directly at the composition of river water.
In figure 1 on the left is shown the average composition of river water presented
as a series of bars. The height of the bar indicates the amount of each of the elements
present. If we compare this to the figure on the right representing seawater we can see
that for the major ions in seawater the pattern is very different. Note that the river water
concentrations have been corrected for cyclic salts. Chloride in river waters is a cyclic
salt because virtually all of it comes from sea spray blown off the ocean. This chloride
then is just from recycled sea water and as such we should not count it as a new source of
chloride to the oceans.
Figure 1. The relative abundances of selected ions in average river water and seawater.
Residence time
If the oceans do not look chemically like the rivers that are feeding them then
there must be some other process controlling the chemistry of the oceans. There is, it is
the average amount of time that an element will spend in the ocean once it reaches it.
This is known as the residence time of the element. Residence time is ultimately
determined by the rate at which the ion is removed from seawater.
What the residence time means, for example, is that if an atom of sodium is
carried down a river into the ocean, then on average it will stay in the ocean for 237
million years before it is removed* into the sediments (*exactly how this happens will be
covered later).
We can calculate residence times for all the elements in the ocean, provided we
have a way of estimating the amount in the ocean and the amount entering the ocean
every year. (Instead of the amount entering into the oceans each year we could, if we
knew it, use the amount being removed since we assume that they must be equal, or else
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the concentrations of the elements would be changing over time).
Figure 2 shows a plot of the residence times of a large number of elements in the
oceans. The first thing to note from this figure is the enormous range of the residence
times shown on the bottom axes. Calculated residence times range from about 50 years
for iron (Fe) to 100 million years for chloride (Cl). For comparison the oceans
themselves are about 3.5 billion years old, this is estimated from the time the Earth
cooled below 100˚C, thus allowing liquid water to form.
The second thing to notice from the figure is the residence time of the major ions.
They all have residence times greater than one million years.
The third thing to notice on the figure is the line that denotes the stirring time of
the ocean, at 500 years. The mixing time is the time it takes for water to sink from the
surface water in the North Atlantic into the deep water, be carried around the bottom of
the ocean, return to the surface in the North Pacific and flow back to the surface of the
North Atlantic -- the average time it takes to stir the ocean once.
Figure 2. The relationship between seawater concentration and residence time for
selected elements.
The actual number you will find for this varies (between 500 and 1500 years)
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depending on the book you read, we will assume for the purposes of this class that the
mixing time is 1,000 years.
We are now in a position to answer the questions posed originally:
Why are some ions present in seawater in high concentrations while others are
present in only low concentrations?
All the elements that are present in high concentrations in seawater have long
residence times. They have time to accumulate in the oceans and so their concentration
can increase. Elements with short residence times are removed quickly, they do not have
time to build up to a high concentration.
Why are some ions present in constant relative proportions and others vary
considerably?
If an element has a residence time of one million years it will go through one
thousand stirrings between the time it enters the ocean and the time it leaves, enough to
be very well mixed. If the residence time of an element is only 100 years it will not even
get one stirring cycle between arrival and removal and will not be well mixed.
Are these properties connected?
Yes. The residence time of an element in the ocean determines both its
concentration and how well mixed it is.
In the real world, what is really controlling the residence time of the elements in
the ocean, is the process of removal not that of supply. What we will look at next is the
main route by which chemicals are removed from the ocean the growth of organisms in
the surface waters of the ocean, their death and the subsequent sinking of their remains
into the deep waters. This growth and death process is also one of the main mechanisms
(apart from movement of the water itself) by which elements are redistributed within the
oceans. The elements that are most affected by the growth and death process in the
surface oceans are the nutrients, the chemicals that form the basis of life. As was pointed
out at the beginning of this section, without dissolved chemicals the ocean would be a
lifeless desert.
© Copyright 2014, University of Hawai'i Department of Oceanography
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