Chemical Oceanography Flashcards
Chemistry of the ocean?
• Mostly Cl, Na and Mg
• Determined by atmospheric, glacial and river input
o Hydrothermal inputs and outputs from basement rock
o Outputs into bottom dwelling sediment and eventually the basement rock
• Inputs and outputs balanced
River inputs:
o Transport organic carbon, chemical by-products of rock weather and particulate matter
• Whether matter is exported as particulate matter or a dissolved species depends on bedrock present and type of erosion (climate/latitude)
o Ocean dissolved inputs largely dominated by large tropical rivers (weathering dominated)
o Erosion
• Chemical erosion on rock types such as limestone
• Hydrolysis
• Dissolution
• Mechanical erosion for harder rocks in high energy rivers
Three types of river (independent of bedrock)
Types of river?
Precipitation dominated:
• Composition mostly controlled by rainfall (diluted seawater)
• These rivers are dilute seawater plus organics and dissolved bedrock
• Rivers in low relief areas. Can be far away from the sea
• Tropical rivers in Africa and South Africa
• High weathering
• Tend to go across area of crystalline rock
Weathering Dominated • Largely tropical/subtropical rivers with moderate rainfall (Congo, Orinoco, Niger) • Also Mississippi • Abundant dissolved species • In “equilibrium” with their basins • High weathering • Medium precipitation Evaporation dominated • Composition of concentrated rainwater + dissolved species • Arid regions • VERY high dissolved species concentrations • Ions concentrated by evaporation • Colorado River 98
Composition of the three river types?
- Distinguished by N, Cam TDS, Cl and HCO3
- River water discrimination diagram
- Rainfall dominant – high weight ratio of Na and low total dissolved salts
- Weathering dominant – Low weight ratio of Na and medium dissolved salts
- Evaporation dominant – High weight ratio of Na and high total dissolved salts
Estuarine Processes
• Where the river meets the Sea
• Mixing between freshwater and seawater
• Change from fresh to brackish to salt water
o Extreme change in salinity
• Rivers have more Si, Fe, Al, P, N, DOC
• Oceans have more Na, Ca, Mg, K , Cl, SO4
• The change in salinity can cause precipitation
• Slowing of flow can give time for reaction
o Thus across simple mixing may not occur
Salinity?
• Mass of inorganic substances per mass of water – total dissolved salts
• Measured by conductivity (pure water very poor conductor of electricity)
o Expressed as ppt or percentage
o Oceans have average 35%
Conservative mixing line?
• Addition to solution results from desorption or dissolution
• Subtraction results from ‘sorption’, ‘flocculation’, ‘precipitation’, biological activity
• Important because:
o Limits extent some solutes enter ocean
o High concentration of some species found in flocculants
Atmospheric inputs:
• Aerosols:
o Fine particle of liquid or solid in air
o Sea to air flux contributes seawater composition to rainwater
o Gases CO2
• Wet deposition
o Over land atmospheric water dissolves gases and particles
• Dry deposition
o Particles in air deposit out without influence of rain
• Dust fluxes
o From deserts
o Provide nutrients
o Sahara
• Phytoplankton in the worlds oceans (summer)
o Nutrient rich north
o Sahara Desert dust in equator
o River inputs in South America
o Upwelling on West Coast of South America
Hydrothermal systems
- Chemical mass balance showed an additional sink/source required
- Mass balance satisfied with discovery of hydrothermal systems
- Outgassing and circulation of seawater through crust
- High and low T/P systems
- Provides metals to oceans
Importance of sources?
- Rivers – supply to surface and margins
- Atmosphere – supplies to the surface
- Hydrothermal systems – supplies to deep water and mid ocean
- Rivers dominate in coastal regions
- In open oceans, rivers dominate for all but some scavenged metals
Le Chatelier’s Principle:
“If a system at equilibrium is disturbed, the equilibrium moves in the direction that tend to reduce this disturbance”
• If you add extra reactant more product is made
• If a reaction is exothermic and you heat it the equilibrium shifts to decrease the temp
• If a reaction is endothermic and you heat it the equilibrium shifts in favour of taking in more heat
• Pressure changes affect reactions involving gases
o Decrease in volume will shift equilibrium to the side with fewer moles
• Evaporation in tropical areas increases concentration of dissolved ions, pushing equation to the let
o Results in calcite deposition
pH
- pH is a concentration measure
- pH = -log(H+)
- It is important because it controls other reactions
- The most important pH system in the oceans is that due to dissolved CO2
- Essentially a way of stating the H+ concentration of a solution
- Scale of 0 to 14 (acidic-neutral-basic)
Marine Carbonate System?
Gaseous carbon dioxide dissolves to become aqueous (CO2(g) ⇋ CO2(aq))
Then reacts with water to produce carbonic acid (H2CO3) (CO2(aq) + H2O ⇋ H2CO3(aq) )
Carbonic acid then dissociates H2CO3(aq) ⇋ HCO3-(aq) + H+(aq) or reacts with rocks in the lithosphere to produce the bicarbonate ion (HCO3-)
The bicarbonate ion can dissociate further to produce hydrogen and carbonate ions CO32-(aq) + H+(aq) ⇋ HCO3-(aq)
The carbonate ion can also react with calcium ions to form calcium carbonate CO32-(aq) + Ca2+(aq) ⇋ CaCO3(s)
Both bicarbonate and carbonate are the main forms of Dissolved Inorganic Carbon (DIC) in the oceans
The carbonate system has importance in both controlling the global temperature, as CaCO3 serves as a major sink of carbon dioxide, and in regulating the pH of the oceans
The system itself allows for some control over the pH of natural waters through the production or removal of hydrogen ions as carbonic acid dissociates or associates
Alkalinity?
- Acid neutralising capacity of water
- Carbonate and bicarbonate are basic ion – capable of absorbing H+
- If excess basic ions over the H+ = water has a buffering capacity
- Easy to measure, difficult to calculate
Composition and structure of seawater?
• Polar water molecule due to differences in electronegativity between hydrogen and oxygen
• Covalent bonding between oxygen and hydrogen
• Dipole – positive and negative region = allows for hydrogen bonds
o Causes unusually high boiling point and freezing point
• Does not follow the trend of other dihydride molecules in the same column
• Will dissolved salts into ionic solution due to polar nature
o Interaction between ions and water molecules by polarity
o Decreases hydrogen bonding
Thermostatic Properties of water?
• Possess the largest heat capacity of any common substance
o Heat capacity: “amount of heat required to raise the temperature of 1 gram of any substance by 1 degree centigrade”
• A substance with large heat capacity can absorb (or release) large quantities of heat with only small changes in temperature
• Large heat capacity due to hydrogen bonding
• Latent heat: heat energy absorbed or emitted during changes of state
o Water has large latent heat
• Water regulates the temperature of the Earth by absorbing heat during evaporation (latent heat of evaporation) at low latitudes and releasing it in colder, high-latitude regions, during condensation
Water density:
- Density of pure water increases until 4 degrees then decreases – why ice floats in the oceans
- Frozen water forms a lattice structure (ice domains) causing a decrease in density
- Dissolved salt (seawater) reduces the temperature of maximum density for water as well as its freezing point
