201 OCN Chemical Oceanography Class Notes, Fall 2001 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 some 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 (it was cold!) and that 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 will start by looking at what the salt in seawater is made of. Salt in seawater: Major ions Anyone who has been swimming in the ocean knows that its salt tastes like the table salt, sodium chloride, that you put on your food. So it will come as no surprise to find out that the main chemical ingredient of seawater is sodium chloride. In fact, as Table I shows, 86% of the dissolved material in seawater is sodium chloride, followed in decreasing order of concentration by magnesium, sulphate, calcium and potassium. These six chemical elements together account for 99.4% of all the dissolved chemicals in seawater (Note that the percentages in table 1 only add up to 99% because the numbers are rounded).
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If we add up the weights in the table below, we can see that in each kg (a kilogram is about 2 lbs.) of seawater there are about 35 grams (a little bit more than an ounce) of salt dissolved. The amount of salt dissolved in the water is also known as its salinity. A fairly typical amount of salt in seawater is 35 grams of salt per 1 kilogram (1,000 grams) of seawater; we say this water has 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 in fact, the average of all the river water in the world contains only about 0.1 gram per kg. So 35 grams per kg is quite a lot, enough dissolved material to affect the physical properties of the water. For example, the freezing point of pure water is 0C (32F) while that of seawater is about 1.8C (28F). Table 1
Major ions in seawater Symbol ClNa+ Mg2+ SO42Ca2+ K+ Name 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 in 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 its temperature of maximum density. For fresh water this occurs at about 4C (40F) --i.e., several degrees above its freezing point. However, for seawater though, the temperature of maximum density occurs at the freezing point of seawater (-1.8) this 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 in Table 1 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
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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 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 99.4% of the salt of the 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 99.4% of the dissolved salts in seawater, the other 0.6% is made up of all the other elements and compounds (see the web 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. 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, e.g. the nutrients and tritium, will be discussed in more detail in future lectures.
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Table 2
Minor ions, nutrients and other components of interest to us Minor ions Symbol
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Name
Amount/kg moles 2.32 mmol 0.84 mmol 90 mol 73 mol 67 mol gms 0.142 0.0674 0.0079 0.00445 0.00128
HCO3 BrSr2+ H2BO3 F-
Bicarbonate Bromide Strontium Borate Fluoride
Nutrients SiO2 PO43NO3Silica Phosphate Nitrate 0-170 mol 0-3.2 mol 0-44 mol Other chemical species Cd2+ 3H Cadmium Tritium 0-1nmol measured relative to hydrogen.
So, why are some ions present in seawater 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 from place to place? 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 chemicals are delivered to the ocean is with the water that flows into the oceans. The table outlines below the various water flows into the oceans as well as giving the volume of the oceans.
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Table 3
Water flows into the oceans Volume of the oceans: 1.37 x 1021 litres. Volume of river water flowing into the oceans: 3.74 x 1016 litres/year B A
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: Time taken to fill the ocean from rain to the surface ocean
= A/B = 36,000 yrs. = A/C = 3,600 yrs.
Time taken to fill the ocean from hydrothermal circulation = A/D = 8,000,000 yrs.
From the table above we can se that would take 36,000 years to replace all of the 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 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 be 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)
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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 chemicals 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) Chloride (Cl-) Sodium (Na+) Sulphate (SO42-) Magnesium (Mg2+) Calcium (Ca2+) Potassium (K+) Bicarbonate (HCO3-) Bromide (Br-) Borate (H2BO3-) Strontium (Sr2+) * 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 in seawater and the most abundant ion in seawater (Cl) 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 will dissolve in rainwater 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 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 seawater and as such we should not count it as a new source of chloride to the oceans. Earth's crust (as elements) Silicon (Si) Aluminium (Al) Iron (Fe) Calcium (Ca) Sodium (Na) Potassium (K) Magnesium (Mg) Titanium (Ti) Manganese (Mn) Phosphorous (P) River water (as ions) Bicarbonate (HCO3-) Calcium (Ca2+) Silicate (SiO2) Sulphate (SO42-) Magnesium (Mg2+) Potassium (K+) Sodium (Na+) Iron (Fe3+)
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Figure 1. The relative abundance of selected ions in average river water and seawater.
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 time known as the residence time of the element. The residence time of an element is ultimately determined by the rate at which the element is removed from seawater, not the rate at which it is added. What residence time means, for example, is that if an atom of sodium is released from a rock through weathering and 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 c...