Download Ocean Chemistry

Ocean Chemistry
Chemistry of Seawater
Water is a Powerful Solvent
• Water is polar (positive end and negative end)
▫ Positive of one water molecule is attracted to the
negative of another water molecule
▫ Opposites attract
• Polarity is what allows water to be a great
solvent (a liquid that can dissolve)
▫ When water comes into contact with compounds
that are held together by this same type of
attraction, water causes the compound to separate
Water as a Solvent
Seawater
• 97% of water found on Earth is seawater
• By weight, seawater is 96.5% water and 3.5%
dissolved substances (mostly salts)
• Ocean contains 5.5 trillion tons of salt
▫ If all water evaporated and left the salt behind, the
entire planet would be covered with a 150 ft layer
of salt
Salinity
• Salinity: the total quantity of dissolved inorganic
solids in water
▫ Inorganic = no carbon
• Ocean’s average salinity is 3.5%, but can range
from 3.3-3.7%
▫ Depends on evaporation, precipitation, runoff
• The most common dissolved solid in seawater is
sodium chloride (NaCl)
Salinity – How does it affect the
properties of water?
• Heat capacity of water decreases as the salinity
increases
▫ Less heat is necessary to raise the temp of
saltwater than freshwater
• Hydrogen bonds between water molecules are
interrupted
▫ Increased salinity = lower freezing point
▫ Sea ice forms at lower temps than freshwater ice
• Dissolved salts attract water molecules
▫ Seawater evaporates more slowly
Ions Found in Saltwater
• Most abundant ions are:
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Chloride – 1.9%
Sodium – 1.1%
Sulfate - 0.3%
Magnesium - 0.1%
Calcium - 0.04%
Potassium - 0.04%
Bicarbonate - 0.01%
• Actually, most elements are found in seawater
but in extremely small amounts (less than
0.0001%)
Salinity Comes from Earth’s Crust
• Some come from the ability of rain,
groundwater, or crashing surface to dissolve
crustal rock – called weathering
• Some come from the upper mantle – enter the
ocean through outgassing by underwater
volcanoes and hydrothermal vents
• Some solutes are formed by the combination of
elements from outgassing and weathering
▫ Example: Sodium chloride
 Sodium comes from weathering of crustal rocks
 Chloride comes from mantle (vents and volcanoes)
Weathering
Outgassing
Dissolved Gases in Seawater
• Most gases in the air dissolve in seawater at the
ocean’s surface
• Marine plants and animals need these gases for
survival
• Most abundant gases in seawater are nitrogen,
oxygen, and carbon dioxide
• Cold water can hold more dissolved gases than
warm water
Dissolved Nitrogen
• 48% of dissolved gases in seawater
• Upper layers of the ocean are usually saturated
with nitrogen, meaning that additional nitrogen
will not dissolve
• Required by organisms to build proteins and
other biochemicals
• Problem:
▫ Nitrogen gas can’t be used directly by organisms
▫ It must be fixed into a useable form by nitrogen
fixing bacteria
▫ Nitrates and nitrites can be used by organisms
Dissolved Oxygen
• 36% of dissolved gases in seawater
• Sources:
▫ Photosynthesis of aquatic plants
▫ Diffusion of oxygen from the atmosphere
• Required by animals for respiration
▫ Oxygen is extracted by gills in fish, crustaceans,
squid, etc
▫ Some get oxygen by diffusion through tissues
(worms, sponges, cnidarians)
Dissolved Carbon Dioxide
• 15% of dissolved gases in seawater
• Water can hold a large amount of carbon dioxide
but is usually not saturated
▫ Quickly used by aquatic plants for photosynthesis
• Moves quickly from the atmosphere to ocean,
but more slowly from ocean to atmosphere
• Some carbon dioxide molecules combine with
calcium to create calcium carbonate
▫ Used by many marine organisms to build shells
and skeletons
Gases and Depth
• Concentration of gases varies with depth
• Changes are a result of photosynthesis – Why?
▫ Where are plants found?
 Photic zone
▫ Where will we find the most oxygen?
 Photic zone – plants make it
 Bottom – very little respiration to use it up
▫ Where will we find the least oxygen?
 Middle layer – many animals use it for respiration
▫ Where will we find the most carbon dioxide?
 Aphotic zone – no plants to use it up
▫ Where will we find the least carbon dioxide?
 Photic zone – plants take it in
Gases and the pH Scale
• Water can separate to form hydrogen ions (H+)
and hydroxide (OH-)
• In pure water, the ions are equal in number
▫ More hydrogen ions = acidic
▫ More hydroxide ions = basic or alkaline
• Seawater is slightly alkaline at an avg pH of 8.0
▫ Caused by the amount of carbon dioxide present
• In areas of rapid photosynthesis, the pH is
slightly higher at 8.5 because there is less carbon
dioxide
• Deep, cold water has a slight lower pH of 7.5
because there is more carbon dioxide