Ch7 Ocean ChemistryMr. SternscheinMarine Science

Water is a polar molecule (chapter 6) Water will attract other polar substances or ions. What is an ion? Solutions are composed of:

Solvent- the more abundant substance, usually a liquid Solute- the less abundant substance, often a solid

Salt, NaCl is held together by ionic bonds (electrostatic forces)

Salt dissociates in water to Na+ and Cl- Solutes can diffuse through water until homogenous. A saturated solution can dissolve no more solute.

Water, as a Solvent

Salinity is a measure of dissolved inorganic solids in water.

Seawater is 96.5% water, 3.5% dissolved substances.

Sodium and chlorine ions, from salt, are the major constituents.

Colligative properties:Freezing point depression Decreased heat capacitySlower evaporation Higher osmotic pressure

Components of Seawater

Salinity is measured in parts per 1000 (g./kg.) Dissolved substances = 3.5% or 35 ppt. The ratio of dissolved substances in the ocean

is constant. (Forchhammer’s principle) Chlorinity- a method of measuring salinity,

uses this principle. Salinometers measure salinity using

conductivity with great accuracy.

Measuring Salinity

Dissolved solids originated from the erosion of crustal rocks

Rivers running off continental crust

Wave action on shorelines Excess volatiles: components

of ocean water that are not accounted for by the weathering of surface rocks.

Outgassing of volcanoes Hydrothermal vents Decay of once-living

organisms

Sources of minerals and elements

The ocean is in chemical equilibrium. The proportion and amounts of dissolved

solids remain constant. Ions are being added to and removed

from the ocean at the same rate. Residence time is the average length of

time an element spends in the ocean.

The ocean is in a steady state

Residence Time

Conservative constituents of seawater are those constituents that occur in constant proportions.

Conservative elements have long residence times and are the most abundant dissolved material in the ocean.

Non-conservative constituents have short residence times, and are usually associated with seasonal, biological or short geological cycles.

Constituent time

Gases from the atmosphere dissolve at the ocean’s surface.

Nitrogen (conservative) Oxygen (non-conservative) Carbon Dioxide (non-conservative) Different proportions than in the atmosphere

due to rate of solubility in the ocean

Dissolved gases

Unlike solids, gases dissolve best in cold water.

Cold polar water will have more dissolved gases than the warm tropics

Dissolved Oxygen (DO) in the tropics may be so low that animals cannot survive

Tropical waters may be stressed more due to pollutants that consume oxygen (examples: sewage and agricultural runoff)

Dissolved gases continued

Temperature and Dissolution

Nitrogen in the most abundant gas in seawater (48%) Upper layers are usually saturated Nitrogen is essential for:

living things to make amino acids amino acids then help to build proteins plant life

Cannot be taken directly from the atmosphere Must first be bound, or fixed, into usable forms (by bacteria) Dissolved oxygen is critical to marine life. (36%) Vital for organisms that extract oxygen with their gills Sources of oxygen:

photosynthetic activity diffusion from the atmosphere

Nitrogen & Oxygen Gas

Carbon dioxide (CO2) is low because of demand by photosynthetic organisms (15%)

Concentration is 60x more in ocean than the atmosphere

Can combine with calcium to make calcium carbonate (CaCO3)

CaCO3 is used to build shells and skeletons. Organisms die, sink and become limestone

rock

The ocean is a vast carbon reservoir

Oxygen levels are more abundant near the surface. why? Photosynthesis occurs there

Decreases with depth as marine organisms consume it.

CO2 levels are low near the surface (again, photosynthesis)

Increase with depth as organisms produce it.

Gas Concentrations Vary with Depth

Carbon dioxide vs. Oxygen

When water dissociates, it breaks into OH- and H3O+ Solutions that have excess H3O+ (hydronium ions)

are acids. Solutions that have excess OH- (hydroxide ions) are

bases. Solutions that have equal amounts are neutral

(water) pH indicates whether a solution is acidic or basic

Acids have pH less than 7 Bases have pH less greater than 7 Basic solutions are also called alkaline

Acid and base refresher

CO2 and water form Carbonic acid The more CO2 in the atmosphere, the more

there is in the ocean. This is causing the oceans pH to drop and

become more acidic. Some marine life cannot adapt and will

perish.

The ocean is slowly becoming more acidic