Chemistry of Seawater

The Salty Sea

• The salt in the ocean exists in the form of charged particles, called ions.

• Sodium and Chloride ions make up 85% of all the salt in the sea.

• The rest is made up of Sulfate, magnesium, calcium, potassium & several others present in smaller quantities.

Principle of Constant Proportions

• Alexander Marcet (1770-1822)• The Swiss chemist and doctor carried

out some of the earliest research in marine chemistry.

• 1819 he discovered that all the main chemical ions (sodium, chloride, & magnesium ions) in seawater are present in exactly the same proportions throughout the world’s oceans.

Sources of Salt

• Some were dissolved out of rocks on land by the action of rainwater and carried to the sea in rivers.

• Others enter through Hydrothermal vents, in dust blown off the land, or came from volcanic ash.

Sinks

• Processes that remove salts from seawater

• Salt spray onto land• Precipitations of various ions

onto the seafloor as mineral deposits

Salinity

• The amount of salt in a fixed mass of seawater.

• It is determined by measuring a seawater sample’s electrical conductivity and averages about ½ oz of salt/lb of seawater.

• The salinity depends on what processes or factors that are operating at that location to either add or remove water.

Factors that effect Salinity

• Add water, lower salinity:– High rainfall– River input– Melting of sea ice

• Remove water, increase salinity:– High evaporative losses– Sea-ice formation

Salinity

• At depth, salinity is near constant throughout the ocean

• Between the surface and deep water is in a region called Halocline, where salinity gradually increases or decreases with depth.

Thermocline

• Below the surface:– 3300 ft. down - Region of

steep decline in temperature (called the thermocline)

– Temperature drops more as you go deeper, but at a slower rate, until the sea floor (constant 36 degrees around the globe)

Salinity’s Expression:

• Salinity is usually expressed as parts per thousand (ppt) or 0/00.

• Parts per thousand literally means “x” amount of solutes per thousand parts of water. – (Remember that solutes are the substances being

dissolved by water…the universal solvent).

Where do you Find the Salt?

• Surface salinity is highest in the subtropics, where a lot of water is lost or removed as a result of evaporation.

• Salinity is also high in enclosed or partially-enclosed basins (such as the Mediterranean.)

• Salinity is lowest in colder regions or where there are large inflows of river water.

Gas Exchange

• Main gases are dissolved oxygen, carbon dioxide, and nitrogen.– Levels of oxygen and carbon dioxide vary

depending on how many organisms are respiring or performing photosynthesis

Oxygen

• Highest levels are found near the surface– 1. oxygen in the atmosphere is diffusing into the

water– 2. Phytoplankton live in high numbers here,

producing oxygen from photosynthesis

• Levels drop as you go deeper, but then rise again once you get past 3300 ft.

Why Levels of O2 Drop & than Rise:

• High levels of bacteria and other animals break down or eat the organic matter at this level, and use up a lot of the available oxygen in the process. As you continue to move deeper, O2 levels begin to rise again.

– ** Oxygen levels in the upper ocean depend on the balance between its being produced by photosynthesizing organisms, such as seaweed, and being used by animals, such as fish.

Carbon Dioxide

• Highest levels are at depth, and lowest levels are at the surface– 1. phytoplankton live in high numbers at the

surface and use large amounts of carbon dioxide for photosynthesis

– 2. Carbon dioxide diffuses out of the water and into the atmosphere

Carbon Dioxide

• ** Carbon Sink - Many aquatic organisms make shells out of carbonate, a compound of carbon and oxygen. When they die, their shells may fall to the ocean floor, and become sediments and rocks over time.

Carbon in the Ocean

• The ocean contains the world’s largest store of CO2.• Biological and chemical processes turn some of this

CO2 into the calcium carbonate shells and skeletons of organisms, other organic matter, & carbonate sediments.

• However, the CO2 concentration is beginning to acidify the oceans, threatening shell and skeleton formation in marine organisms

Nutrients & Nutrient Cycles

• There are many nutrients and substances found in small amounts in oceans that are necessary for aquatic organisms to grow.

• Phytoplankton, for example, are microscopic floating life-forms that obtain energy by photosynthesis.

Nutrients & Nutrient Cycles

• At the base of the food chain: Phytoplankton-microscopic floating life-forms that obtain energy by photosynthesis.– Need nitrates, iron, & phosphates in

order to grow– No nutrients = no growth– Too much nutrients = blooms (rapid

growth phase)

Nutrients & Nutrient Cycles

• Although some nutrients return to bodies of water from sources outside the aquatic world, most of the nutrients come from a continuous cycle within the bodies of water itself.

– As organisms die, they sink to the floor, where their tissues decompose and release or return nutrients.

– Upwelling of seawater from the ocean floor recharges the surface waters with new nutrients, where they are taken up by the phytoplankton, refueling the chain.

• (We will learn more about upwelling in the near future.)

Ocean’s Carbon Cycle

• Single-celled marine plants (phytoplankton and other marine microalgae) take in carbon dioxide (CO2) and convert it into biomass.

• By converting carbon dioxide into more complex carbon compounds, the phytoplankton effectively make atmospheric carbon available to other marine organisms

Ocean’s Carbon Cycle

• Bacteria eat dissolved organic C compounds secreted by the phytoplanktonphytoplankton are eaten by protozoa protozoa & phytoplankton are eaten by zooplankton eaten by fish passing the carbon through the food chain and into animals like seals & polar bears

• When any of these organisms die without then being consumed, or when they defecate, the carbon locked away in their bodies gradually settles to the sea floor.

Ocean’s Carbon Cycle

• However most of the carbon captured from the atmosphere by phytoplankton never reaches a polar bear.

• Some will be lost back to the water and atmosphere as the different plankton species respire.

• And a vast amount will be retained in the microscopic community of phytoplankton, bacteria, and viruses living near the sea surface

Nitrogen Cycle

• Nitrogen cycles just like water and carbon• The nitrogen cycle is more complex because

of the many chemical forms of N such as: Organic-N; NO3; NH4; and the gases N2, N2O, NO + NO2 (=NOx).

• N has to be taken from the atmosphere and converted into a usable form, either through lightning or “nitrogen fixing” bacteria.

Nitrogen Cycle

The role of bacteria:Convert harmful ammonia into non-toxic

nutrients.○Nitrosomonas – convert ammonia (NH4)

into nitrite (NO2).

○Nitrobacteria – convert nitrite (NO2) into nitrate (NO3).

○These processes together are called nitrification.

Nitrogen Cycle

Food

DecomposersMineralization by

HeterotrophicBacteria

Fish Wastes

Plants

Nitrates (NO3)

Nitrites (NO2)

Ammonia (toxic)(NH4/NH3)

Decaying plant Fragments and Uneaten food

Water Changes

Nitrogen Cycle

What happens to the nitrate?Absorbed by algaeConverted to nitrogen gas

• Basic Ocean Nitrogen Cycle

• Describe how the land and ocean nitrogen cycles are similar

Explain how the Carbon Cycle and the Nitrogen Cycle are connected.

• What do you think would happen to the environment if all of the nitrogen fixing bacteria in the ocean died?

Temperature

• Temperature varies depending on location and depth.

Temperature in the Tropics & Subtropics

• In the Tropics and Subtropics, solar heating keeps the ocean surface warm throughout the year.

Temperature in Mid-latitudes

• In mid-latitudes there is much more seasonal change in surface temp.

Temperature in Polar Oceans

• In high latitudes and polar oceans, the water is constantly cold, sometimes below 32 °F.

Temperature below the surface

• Below the surface, the temp declines steeply to about 46-50° F at a depth of 3,300ft. – This region of steep decline is called Thermocline

• As you move even deeper, temperature continues to drop, but it drops more gradually to a uniform 36° F (2 ° C) on the sea floor. This temperature remains constant throughout the deep oceans around the globe.

Density & Buoyancy

• Density is the mass of a substance per unit volume (usually measured in grams per milliliters, g/ml).

• Buoyancy is the upward force that a fluid exerts on an object less dense than itself.

• Density:– Depends on temperature and salinity.

Density

• Any decrease in temperature or increase in salinity makes seawater more dense.

• There is an exception to this rule, however. When the temperature drops below 4 °C (39 °F), water becomes a little less dense – (remember that it freezes at 0 °C and floats on

water.)

Ocean Currents• In any part of the ocean or other body of water, the

density of the water increases with depth, because dense water always sinks if there is less dense water below it.

•Processes that change the density of seawater cause it to either rise or sink, and drive large-scale circulation in the oceans between the surface and deep water.

Ocean Currents

• It also causes circulation of oceans from middle latitudes toward the poles, and vice versa. – Example: The oceans each contain distinct, named

water masses that increase in density from the surface downward. The denser, cooler masses sink and move slowly toward the equator. The cold, high-density deep and bottom waters comprise 80% of the total volume of the ocean!

Q: How do Ships Float?

A: A greater force is pushing up on the ship than the weight force pushing down.

• This supportive force is called buoyant force.

Density & Buoyancy• If the buoyant force is equal

to the object’s weight, it will float.

• If the buoyant force is less than the object’s weight, it will sink.

Density & Buoyancy • Bouyant force was explained by

Archimedes, a Greek mathematician around 3rd century B.C., and it became known as Archimedes’ Principle.

• Archimedes’ Principle states that an objects weight will cause the object to sink while at the same time displacing the fluid.

Buoyancy & Density

• If the weight of the water displaced becomes equal to weight of the object, it floats.

• If the weight of the water displaced becomes less than the weight of the object, it sinks.

• Archimedes’ Principle is important b/c: Properties of fluids ultimately determine the design of ships, airplanes, cars, and hydraulic machines.

Nutrient Upwelling & Turnovers

• An upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water

Pressure

• Scientists measure pressure in units called bars.• At sea level, the atmosphere exerts a pressure of

about 1 bar.• Underwater, pressure increases by 1 bar every

33ft.• Divers must breath pressurized air or other gas

mixtures.

Boyle’s Law

• If the temperature of a gas does not change, Its volume decreases as pressure increases and vice versa

•Fish have a swim bladder, gas filled space for buoyancy. Swim bladder expands or explodes if brought to the surface to quickly.

Pressure sicknesses

• Nitrogen Narcosis– Nitrogen gas will dissolve better under higher

pressure and therefore will be forced into body tissues

– “rapture of the deep” – divers feel intoxicated by excess nitrogen

– Feeling subsides as diver returns to surface

Pressure sicknesses continued

• Decompression sickness “The Bends”– As diver ascends to the surface, bubbles

form in the blood and body tissues.– Small bubbles are of no danger (slow

ascension)– Medium bubbles block smaller vessels,

causing tingling and slight bruising.– Larger bubbles block blood flow to vital

organs or cause nerve damage to joints (the bends).

Overcoming Pressure

• If diver gets decompression sickness, they must be put into a recompression chamber to relieve symptoms.

• Underwater habitats – aquanauts live for several days at the depth and pressure at which they are working in specialized housing.