OS 101—Marine Environment
Winter 2007
The Chemistry of Seawater
Chapter 5-6
I.
The Dissolving Power of Water
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II.
water is referred to as the ‘universal solvent’
Because of its dipolar nature and ability to form Hydrogen bonds, it
can ‘surround’ charged ions and hasten dissolution of more ions
This is caused by the formation of hydration spheres
As an example, the ionic strength of NaCl is reduced by a factor of
80x in water!
Ocean Salinity
- A salt is a compound formed by replacing the proton donor (H+) in
an acid with another charged particle, usually a metal
i. For example, HCl + Na  NaCl + H+
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salinity is the amounts of total salts in a liquid by weight
i. if we took 1000 g (one liter) of seawater, and let all the water
evaporate, we would have about 3.5 g salt left behind
ii. Therefore, saltwater is about 96.5% water, and 3.5% salt
iii. We don’t usually refer to salts as percent, but as parts per
thousand
1. Symbol is ppt, or parts per thousand, or ‰
2. So, seawater is on average 35 ppt
- Rule of Constancy of Composition: pretty much anywhere we go in
the world’s oceans, there are the same proportions of salts, even though
the ppt
changes.
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Only six elements make up 99% of salts in seawater:
i. Chloride (Cl-)
ii. Sodium (Na+)
iii. Sulfur (SO4-2)
iv. Magnesium (Mg+2)
v. Calcium (Ca+2)
vi. Potassium (K+)
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III.
Because of this constancy of composition, we need only measure
one element or ion to estimate salinity.
Chloride (Cl-) is the easiest to measure, so we used to measure Cli. Salinity ‰ = 1.80655 x Clii. Modern instruments, we rely on the fact that the more ions,
the better conductor water is, so we measure conductivity
iii. We don’t actually measure salinity (weight), or Cl- (ions) any
more…because of that, we switched to the Presumed
Salinity Units (psu) designation, since we don’t actually
measure it!
iv. Most recently, we switched to the Presumed Salinity Scale
(pss) because there aren’t actually any units associated with
conductivity
Besides the 6 major elements, there are many minor ones. The
next group, found at 1-100 parts per million (ppm), include Bromine,
Carbon, Strontium, Boron, Silicon, Fluorine
Then, < 1ppm, Nitrogen, Lithium, Rubidium, Phosphorous
Sea Water Density
- Density is simply mass per unit volume, typically g/cm3
- It is affected by temperature (expansion/contraction), salinity (adds
weight as salts), and Pressure (expansion/contraction)
- Because of salts, seawater has a density 2-3% higher than pure
water…therefore the density increases from 1.0000 to 1.022-1.030
- In a stable ocean, density always increases with depth (water
sinks)
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We define density as Sigma (σ)
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The temperature of maximum density in freshwater is
3.98°C…which means that as water cools, it sinks
You can decrease the temperature of maximum density (and the
freezing point) by adding salts…because of that, seawater almost
never freezes, because it will sink from the surface before it gets
cold enough
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Density is very easy to measure in the laboratory, but very difficult
to measure in the ocean. So, we measure the 3 properties that
control density:
i. Temperature
ii. Salinity
iii. Pressure
IV.
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Water is essentially uncompressible, so we often ignore pressure—
however, when we think of thousands of meters of seawater, it
becomes important.
i. Sigma-T refers to the density in situ, at ambient, T, S, P
ii. Sigma-Theta is corrected for pressure
1. As pressure increases, there’s slight compression of
water
2. This causes adiabatic heating (heating without an
input of energy)…bring it to the surface, temperature
drops, and becomes more dense
3. Therefore, Sigma-T is always less than Sigma-Theta
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We usually convert density numbers to a more convenient scale:
i. Specific Gravity = ((density/.999974) – 1)*1000
ii. Easier to use a T-S diagram
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Temperature and Salinity are Conservative Properties
i. This means they don’t change, except by mixing, once they
leave the boundaries (surface or bottom of the ocean)
ii. Provides a “fingerprint” for different water masses
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In contrast, Non-Conservative Properties can change even
though they are not in contact with the boundaries…they are
affected by biology.
Vertical Profiles
- Look at the distribution of temperature, salinity, and density with
depth:
i. It becomes obvious that density is controlled by temperature
ii. In a stable ocean, get layers of density…water is free to
circulate in areas of constant density (i.e. mixed layer), but is
kept from mixing across boundaries
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Regions where the slope changes rapidly are clines:
i. Pycnocline—density slope
ii. Thermocline—temperature slope
iii. Halocline—salinity slope
iv. Nutricline—nutrient slope
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Isopycnal surfaces are layers where density doesn’t change (in 3dimensions)
Isothermal, Isohaline surfaces are same idea, but for temperature
and salinity
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V.
Acidity and Alkalinity
- Acid: a compound which, when dissolved in water, forms hydrogen
ions
- Base: (or alkaline compound), is a hydrogen ion acceptor
pH = -log[H+], where [H+] designates the hydrogen ion concentration
Since pH is defined as an algorithm, only the exponent of the
concentration needs to be used to describe the solution. A pH of 8 means
that there are 1x10-8 moles H+ per liter.
H20 + H20  H30 + (H+)
H2O  H+ + OHTherefore, water is a strong acid/base pair, and annihilates each
other…the pH ends up being exactly neutral.
Another example: H20 + HCl  H2O + H+ + Cl-, so it’s more acidic
Alkalinity: amount of H+ ions that can be absorbed, or neutralized, by a
solution. It buffers the acidity.
Seawater is dominated by CO3-2, HCO3-, and OH
A = [HCO3] + 2 [CO3] +[OH] –[H]
- Seawater pH is about 8.2-8.5, so has about 7x10-9 mols/L H+
- Alkalinity is about 0.0023 mol/L HCO3 equivalents, or about 2x10-3
- Therefore, seawater has 3 million times more buffering capacity than
acidity!
VI.
Chemistry versus Depth
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Oxygen is abundant in the near surface, because it is in equilibrium
with the atmosphere, and photosynthesis produces oxygen
Carbon Dioxide is also at equilibrium in the surface
With depth, you go through the oxygen minimum zone, at about
1000m…this is where most organic matter is converted back into
CO2
Therefore, CO2 has a mirror-image relationship with depth
pH is controlled by CO2, so it drops as CO2 increases
VII.
Sources of Salts
- In the ancient atmosphere, there was much more CO2, sulfur,
chlorine because of volcanic activity
- These form HCl, HCO3, CO3, H2SO4
- The atmosphere and oceans were much more reducing, which
helped to dissolve away the rock minerals
- In the modern ocean, the concentration of salts is dependent on
how reactive they are…if they aren’t very reactive, the
concentration stays extremely constant (Na, Cl), while if they are
very reactive, they show strong gradients (Fe)
- Biogeochemical Cycles: describes the sources, sinks, and fluxes of
elements through the oceans, atmosphere, and land…same as the
hydrologic cycle, except instead of water, we’re looking at other elements
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Major Biogeochemical Cycles:
i. Carbon
1. Actually driven by carbonate chemistry and the
balance between the oceans and atmosphere, and
by the effects of biology on the inorganic carbon
2. “Solubility Pump” is simply the “pumping” of carbon
from the surface to depth by the change in solubility
as pressure increases and temperature decreases
3. “Biological Pump” is the packaging of inorganic
carbon as organic material, which is rapidly exported
to depth as organic matter (often fecal pellets).
Sometimes referred to as the “fecal express”
4. Carbonate Compensation Depth (CCD) is the point at
which carbonate is soluble in seawater. Below that
depth, carbonate will dissolve if exposed to seawater
ii. Phosphorous
1. Geologists think of phosphorous as being the single
most limiting element in the ocean for biology
2. Primary source of phosphorous is from rocks
3. Phosphorous is very particle reactive, so it is not
present in very high concentrations
4. In the oceans, phosphorous is primarily regenerated
at depth, and the major source is upwelling of deep,
nutrient rich water, but it is also regenerated quickly,
so it spends a lot of time being recycled in the surface
waters
iii. Nitrogen
1. Geologists consider it to be non-limiting (biologically)
because there is always more available from the
atmosphere, via nitrogen fixation.
2. Biologists argue that on shorter time scales, nitrogen
is very limiting in the oceans (more than phosphorous)
3. Very complicated biogeochemical cycling, with many
sources and sinks, and several forms of nitrogen
(nitrate, nitrite, ammonium, urea, amino acids,
nitrogen gas, etc)
4. Nitrogen is not regenerated as quickly as
phosphorous in the surface ocean, but there is a lot of
recycling relative to other elements (such as silicon)
iv. Silicon
1. Only used by diatoms and radiolarians in the modern
ocean
2. Limits production of diatoms in some regions
3. ALWAYS undersaturated in the ocean (always
dissolves), so there is no equivalent of the CCD
4. Regeneration is limited in the surface ocean, so
silicon is exported from the surface to depth much
more efficiently than either C, N, or P.
5. Primary sources of Si are upwelled deep waters and
river water (dissolved terrestrial rocks)
v. Iron
1. Very scarce in the oceans (it is a “trace metal”)
2. In large regions of the ocean, it may limit biological
productivity
3. Primary sources are atmospheric, terrestrial (rivers),
and benthic (sediments), so it is most limiting in deep
waters with no upwelling far from land
VIII.
Box Models
- If we want to look at biogeochemical cycles, we often use box
models
- A box model is simply a schematic representation of a system that
is assumed to be in steady state, meaning that it is in balance (the
ins and outs equal each other)
- Box models get their name because each part of the model (the
oceans, for example) is described as a box, which has some
volume, some concentration (whatever you’re modeling), and
inflows/outflows.