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Ocean Chemistry
What is chemistry and why is it important to the study of marine biology?
Much of chemistry falls into 3 categories: inorganic, organic and biochemistry, which are all useful sciences to a
Marine Biologist.
Inorganic chemistry is often defined as the study of compounds that do not contain carbon; however, some nonliving matter, diamonds for example, contains carbon and therefore falls outside the rule. Marine biologists who
study the minerals that seep from hydrothermal vents would use inorganic chemistry.
Broadly speaking, organic chemistry has been defined as the study of compounds that occur naturally from plants
and animals and inorganic chemistry as the study of compounds created by non-living elements such as those
found in minerals. Organic chemistry is also defined as the chemistry of compounds that contain both carbon and
hydrogen—two elements found in all living things. The majority of marine science related chemistry would fall
under organic chemistry.
Methane is a saturated hydrocarbon that consists of a single
carbon atom bonded to four hydrogen atoms. The molecule
assumes a tetrahedral geometry, and is nonpolar.
Biochemistry, the chemistry of biological processes, applies organic chemistry to the study of living things. A
marine biochemist might study biologically active compounds found in the ocean that might be useful in
medicine, or the genetics of marine species. This discipline has a wide variety of applications to marine science.
What is ocean chemistry?
The chemical properties of the ocean are important to understand because the marine environment supports the
greatest abundance of life on earth. This life is largely made up of the same chemicals that comprise the ocean—
water and salts.
Water
A water molecule is made up of two hydrogen atoms joined to one oxygen atom by weak
hydrogen bonds, H2O. The hydrogen atoms are slightly positively charged and the oxygen
atom is slightly negatively charged which is what attracts the atoms to each other and forms
the weak hydrogen bond.
Water is present in the marine environment as a liquid, a solid, and a
gas regulated by temperature. Heat causes the water molecules to move. The greater the
heat, the faster they move until the movement causes the hydrogen bonds to break
converting liquid water to gas. Water turns to vapor at 100° C. Cold slows down the
movement of water molecules and their density increases. As water gets colder the
hydrogen bonds override the motion of the molecules and water begins to crystallize
forming water's solid state—ice. Ice is formed at 0° C. Ice is, however, less dense than
liquid water because it expands as it freezes causing the molecules to grow farther apart.
The decrease in density causes ice to float. Density differences between different masses
of seawater are one of the major driving forces of deep-sea circulation.
Similarly, temperature regulates the surface tension and viscosity of
water. In spite of its weak hydrogen bonds, water has a strong surface
tension that will support small animals or objects.
The viscosity of the water dictates the force needed for objects to move
through the water; it increases with cold and decreases with heat.
One of water's most important properties is its ability to dissolve other
substances, such as salts and minerals. The dissolved solids in ocean
water come from particles on land dissolved by weather such as rain and
snow, and from the earth's mantle underwater where minerals are released by hydrothermal vents.
Dissolved minerals also make their way into the ocean from active
volcanoes.
The acidity or alkalinity of a substance is measured as its pH, the
negative logarithm of the concentration of hydrogen ions in an
aqueous solution. The pH scale ranges from 0 to 14. A low pH
value has greater acidity (acids), a higher pH is more alkaline
(bases). When an acid is added to water, it gives up hydrogen to the
water. When a base is added to water, it gives up OH (hydroxide).
Pure water has a neutral pH of 7. Seawater has a slightly higher pH
because of its dissolved salts—usually seawater has a pH of 8. For
comparison, lemon juice has a pH of 4, milk 6, and soap 10.
Salts
Salts are the most common chemical dissolved by ocean water. Salts are
up of electrically charged particles known as ions. The ions are either
positively or negatively charged, and most salts are comprised of ions with
opposite charges.
Unlike the weakly charged hydrogen and oxygen atoms that make up water,
opposite charges of salt ions form strong bonds that create crystals. When
is introduced, the slight charges of the hydrogen and oxygen atoms are
attracted to the strong charges of the salt ions and water clusters around the
until the bonds weaken, and the crystals dissolve.
made
the
water
crystals
The salt in seawater is largely made up of sodium (Na+) and chloride (Cl-)—called sodium chloride which is the
same chemical in table salt. Sodium chloride makes up about 85% of the solids in seawater. The remaining 15%
include: sulfate, magnesium, calcium, potassium, bicarbonate, bromide, borate, strontium, fluoride and others.
The average salinity of seawater is about 35 parts per thousand which stays relatively constant throughout the
ocean varying between 33 and 37 parts per thousand depending on how much freshwater is present. Most marine
life depends on this consistency as their bodies cannot adapt to significant changes in the salinity of their
environment. Few marine species are able to live in both fresh and salt water, and those that do have special
mechanisms to cope with fluctuations in salinity.
In the late 19th century, scientists discovered that the relative amounts of ions in different samples of seawater
remained constant. For example, a sample of seawater taken from the Atlantic will contain 55.03% chloride and
30.59% sodium as a percentage of its total salinity. A similar sample taken from the Pacific will contain the same
proportion of these ions. This consistency demonstrates the interconnectedness of all the oceans and the sea life
they contain.
Where do these minerals and salts come from?
Some come from erosion on land, dissolution of rocks and runoff into the ocean, some come from volcanic
activity both on land and in the ocean, and some from hydrothermal vent activity in the deep sea which plays an
important role in regulating seawater chemistry.
How does marine life cope with its saline environment?
Most marine vertebrates are able to regulate the salt
and water content in their bodies through a chemical
process called "osmoregulation." They pass water
through osmosis, where water diffuses from high salt
concentration to low salt concentration between
permeable membranes, and excrete excess salt through
their skin and gills. They take in water through
drinking to maintain the water balance.
Gases
Gases are also dissolved by seawater. Carbon dioxide (CO2) is dissolved in ocean water and used by
phytoplankton to produce plant matter. Oxygen and nitrogen dissolved at the surface from the atmosphere are
also present in seawater. Conversely, the ocean also releases these gases into the atmosphere. Like solids,
temperature also regulates the dissolution of gases; however, the rate of dissolution is reversed for gases: cold
water holds gases better than warm water. This is an important distinction because marine life depends on
oxygen and carbon dioxide for metabolic processes such as respiration.
Although it is often where the warmest ocean water is found, the ocean's
surface, primarily the top 20 m where photosynthesis takes place and
where most marine life lives, contains the highest concentration of
oxygen. The ocean's oxygen minimum layer, or the depth at which
oxygen becomes depleted, is usually around 500 m. As the ocean gets
deeper, however, seawater becomes oxygenated again from cold oxygen
rich water that has sunk (remember cold water increases in density and
therefore sinks) from the surface. The oxygen does not get used in the
deep as readily as it does closer to the surface, and oxygen levels are
therefore maintained to sustain life in the deep sea.
In our industrial society, fossil fuels are being burned at record levels. The oceans are a vital component of the
atmosphere and the air we breathe, given their role in carbon dioxide removal. Carbon dioxide is needed by
phytoplankton to produce plant material during photosynthesis which absorbs carbon dioxide and produces
oxygen. But it won't be long before the amount of carbon dioxide in the atmosphere exceeds the ability of the
ocean to absorb and process it. CO2 absorption on land is also reaching critical mass because of deforested rain
forests and other forests that play an important role in CO2 uptake and oxygen production.
Carbon dioxide is a greenhouse gas in the atmosphere contributing to global warming, which is beginning to
show an impact on the planet. Sea levels and surface temperatures are rising, ice is melting, and weather patterns
are changing. Carbon dioxide resulting from human pollution is absorbed in the upper 10% of the ocean, which
is the ocean zone with the greatest biological activity. Recent research on the impact of CO2 levels indicated that
the acidity of CO2 could decrease the ocean's surface pH levels dramatically. In addition, rising surface
temperatures cause corals to expel the symbiotic algae needed for their survival resulting in coral bleaching.
These are problems that could easily imbalance the ocean's chemistry and disrupt important biological processes
such as food webs and wide-scale marine life productivity.