Major Constituents of the Continental Crust

The composition of seawater

What is a solution?
Last week we looked at the physcial properties of pure water. However, seawater is not pure water. Seawater is a complex "soup" containing
dissolved salts, gases, nutrients, and trace elements.More formally, seawater is a solution in which water, the most abundant ingredient, is the solvent and the salts, gases, and nutrients are solutes.

What does it mean to be dissolved?

We can see salt and sugar disappear when they are placed in water, tea, coffee, etc. But what happens to them?
When a solute (salt) is dissolved it means that the attractive forces between the solute and the solvent (water) particles are great enough to overcome the attractive
forces (intermolecular forces) within the pure solvent and within the pure solvent. Hydrogen bonds holding two water molecules together are broken, and the bonds
holding the solute together are broken, and the molecules change partners to form solvent-solute bonds.

In water, solute particles are held by electrostatic attraction to the polar water molecule. As an example, let's take sodium chloride (table salt), which is an
ionic solid. The ionic bond holding Na and Cl together is overcome by the force of attraction between the polarized water molecule and each ion.
The positively charged Na+ ion is attracted to the negative end of the polar water molecule. The negatively charged Cl- ion is attracted to the postive end of the water molecule.

What are the ingredients in seawater?

Major Ions in Seawater

Constituent	Symbol	g/kg in seawater	% by weight
Chloride	Cl^-	19.35	55.07
Sodium	Na⁺	10.76	30.62
Sulfate	SO₄^2-	2.71	7.72
Magnesium	Mg²⁺	1.29	3.68
Calcium	Ca²⁺	0.41	1.17
Potassium	K⁺	0.39	1.10
Bicarbonate	HCO₃^-	0.14	0.40
Bromide	Br^-	0.067	0.19
Strontium	Sr²⁺	.008	0.02
Boron	B³⁺	.004	0.01
Fluoride	F^-	.001	.01

· Eleven ions account for 99.99% of all the dissolved solids in seawater.

· Chloride, sodium and sulfate alone account for 93.41%.

Constant Proportions
Seawater is a well-mixed solution. Density-driven vertical mixing, wind-driven currents, waves, and tides all stir the ocean. Because of this mixing the ionic
composition of the oceans is the same from place to place, both horizontally and vertically.

Salinity varies quite a bit depending on location and with depth in the water column, so how we can we claim the ionic composition is constant?!

The RATIOS of the major ions given the in above table are constant. Whether salinity is 30 g/kg in the Artic or 40 g/kg in the Red Sea, the major ions exist
in the same proportions. This was verified by the HMS Challenger expedition.

Ratios of Ions in Seawater

Ocean	Na⁺/Cl^-	Mg²⁺/Cl^-	SO₄^2-/Cl^-
Atlantic	0.554	0.067	0.139
Pacific	0.555	0.066	0.140
Mediterranean Sea	0.553	0.068	0.140

There is roughly twice as a much Cl in seawater than Na, no matter where you collect the seawater sample. There is roughly 15 times as much Cl as Mg in seawater, regardless of the overall salt content, etc.

The Law of Constant Proportions states that whatever the salinity of a sample of seawater, the RATIOS between the major ions are constant. This law applies ONLY to the major ions. Ions that follow this law are called conservative. Trace elements, nutrients, and gases do not follow this law.

This behavior makes it very easy to determine the salinity of a seawater sample. We don't have to measure every single ion. Instead, we measure just the chlorine content. We can then use the ratios to infer how much Na, Mg, sulfate, etc. should be present.

Salinity (per mil) = 1.80655 * Chlorinity (per mil)

Sources and sinks of salt

(Refer to Duxbury Figure 5.3):
Dissolved salts are ultimately derived from the continental crust, and move through both the hydrologic cycle and the plate tectonics cycle.
Salt ions are derived from the mechanical and chemical breakdown of continental crust, from volcanic emissions, from ridge emissions, and from the decay
of pre-existing matter in the oceans. The ions are carried to the ocean by rivers, wind, ice, precipitation, and volcanism. They move through biologic and non-biologic processes. Some are returned to land, others become part of the sediment that gets subducted, recycled in the mantle, and eventually returned to the surface at volcanic island arcs or spreading centers.

If sea salts ultimately come from the continental crust, we might expect the composition of seawater to resemble the composition of the continental crust.

Major Constituents of the Continental Crust

Ion	Symbol/charge	% by weight
Oxygen	O^2-	45.20
Silicon	Si⁴⁺	27.20
Aluminum	Al³⁺	8.00
Iron	Fe²⁺ and Fe³⁺	5.80
Calcium	Ca²⁺	5.06
Magnesium	Mg²⁺	2.77
Sodium	Na⁺	2.32
Potassium	K⁺	1.68
Titanium	Ti⁴⁺	0.86
Hydrogen	H⁺	0.14
Manganese	Mn²⁺ and Mn⁴⁺	0.10
Phosphorus	P³⁺	0.10
All others	Trace elements	0.77

Surprisingly, the composition of seawater looks NOTHING like the composition of continental crust! The most abundant ion in seawater, Cl-, is not
even in the top 12 most abundant ions of continental crust.

How can we explain these 2 sets of observations?

Residence Time
The abundance of salts in seawater is due to the rate at which they are supplied to seawater and the rate at which they are removed from seawater.

The amount of time that an ion remains as a dissolved ion in the ocean is the residence time. This concept is quite similar to the "shelf life" of
products in a grocery store. Milk and Cheerios^TM are purchased by customers within hours, even minutes after they are placed on the shelves.
Conversely, generic brand laundry soap is unpopular, and can stay on the shelves for days, even weeks before anyone buys it. It has a long shelf life.
The key idea here is whether there is a consumer who uses the item.

There are many consumers in the ocean, but they can be picky.

Sodium is moderately abundant in rivers, but has no "consumers" in seawater. Sodium is not used for building shells or organic matter. Sodium
remains dissolved in seawater for millions of years until it finds itself stuck to a clay particle, headed for the sediments building up on the seafloor, and
ultimately into a subduction trench.

Nutrients are always in high demand. Nutrients such as nitrate, phosphate, silica and iron (see the nutrient section below) are snapped up almost immediately. Nutrients are the milk and Cheerios^TMof the ocean.

The conservative ions are conservative because they have such long residence times, and because the ocean is well-mixed. The conservative ions are not necessary for biological reactions.

Residence Times of Ions in Seawater

Ion	Residence Time (years)
Chloride	79 million
Sodium	260 million
Magnesium	45 million
Potassium	11 million
Sulfate	8 million
Calcium	8 million
Manganese	1400
Iron	140
Aluminum	100

Trace Elements
Besides the major constituents nearly every other element on the periodic table is present in seawater, but in extremely small amounts. Some are measured in parts-per-billion (1 part in 1 billion, or 1x10^-9) or even parts-per-trillion (1 part in 1 trillion, or 1x10^-12). Precious metals like gold are present in seawater, but extracting it costs more than the value of the gold extracted, so it is not worth it. But during WWI and WWII the U.S., Germany, and Japan all experimented to find ways to do this.

Nutrients
The nutrients are the ions required for plant growth. These are the materials essential for making the hard parts and soft parts of organisms. They are brought to the ocean by rivers and they are consumed almost immediately. These are the fertilizers of the oceans. Nitrogen and phosphorous are the building blocks of amino acids and proteins. On land we use nitrogen (as nitrates) and phosphorus (as phosphates) on crops. In the oceans, the silicate ion (SiO₄^-) is also a nutrient, and is used by algae called diatoms to build opaline silica shells.

The three most important MACRONUTRIENTS are:

Nitrogen as NO₃^-
Phosphorous as PO₄^3-
Silica as SiO₄^-

If we look at a nutrient’s abundance in a vertical slice of ocean, we see that they all have a similar shape (which you will see in class).

Nutrients do NOT follow the law of constant proportions. Their abundance changes over very short time scales and short distances.

When the critters die these nutrients sink to the seafloor. The nutrients are liberated as the critter decomposes. Ocean mixing, in particular UPWELLING brings the nutrients back to the surface water.