Marine ChemistryChemical oceanography

Objectives• Definitions.• Properties of sea water• Physical Properties of sea water

• Temperature• Salinity• Density• Water circulations• Ocean

• Temperature distribution in ocean• Salinity distribution in ocean• Sea water composition

• Major Ions Conservative elements• Trace elements Dissolved gases• Micronutrients Organic matter

Definition• Marine chemistry/ Chemical Oceanography is the study

of Sea/ocean water chemistry

• The ocean is unique (Why?) • It contains nearly every element in the periodic table.

Behavior of Chemical

elements within Ocean

Describe cycles of elements

Biogeochemical Cycle

Cycle of Biologically

active elementsC, N, P, & trace

Fe

Behavior of isotopes and their role as

tracers for past and present

Marine Chemistry studies

• Some Isotopes' applications

• 1-The incidence of 18O (the heavy isotope of oxygen) can be used as an indicator of polar ice sheet extent. and

• 2- Boron isotopes are key indicators of the pH and CO2 content of oceans in the geologic past.

Properties Of Sea water

• Water is the universal solvent because of its ability to dissolve at least a little of every substance.

• Water is a particularly good solvent for substances held together by different bonds.

Den

sity

Dec

reas

ed

• Activity #1• What happened when we add salts to water?• Think about the following items?

• 1- Density 2- Freezing point 3- Boiling point 4- Conductivity

Density is increased.

The freezing point is depressed

The boiling point is elevated

The conductivity is increased

• The density is increased:• Any substance dissolved in a liquid has the effect of

increasing the density of that liquid. • The greater the amount of solute, the greater the effect• As a result of the salt content in seawater, the density

of seawater increases continuously with decreasing temperature.

• There is no density maximum like for freshwater.

• The colder the water temperature, the (heavier- lighter-moderate) the water density. (Choose between brackets)

•

• The freezing point is depressed:

• Salts lower the temperature at which water freezes

• (This is why salt is spread on frozen roads).

• That is because dissolved salts inhibit the tendency of water molecules to form direct bonds with other water molecules.

• The freezing point of salty water is (higher than- lower than-equal to) that of pure water. (Choose between brackets

• The boiling point is elevated:

• The salts have the effect of making the water molecules cluster and become more “ordered” thus harder to pull apart and evaporate.

• Activity #2• Ice melts and mixes with seawater of salinity 35 g/kg.

Will this have the effect of raising or lowering the freezing point of seawater?

• How would this effect the formation of more sea ice when temperatures fell once more?

• This will add fresh water, dilute seawater, make it less saline and raise the freezing point thus facilitate the formation of further sea ice.

• The conductivity is increased: • The transport of electrons causing and electric current

to flow is enhanced by the strong electrolyte nature of salts.

• If an electromagnetic field is applied to a solution of strong electrolytes, the ions will migrate, producing an electric current (e.g. conductivity is increased).

Temperature, Salinity, Density and Ocean Circulation

Temperature

Salinity

Control DensityThat

GovernsVertical

MovementOf Ocean water

Temperature and salinity are together two of the most important characters of sea water because …..(Complete).

by winds in the atmosphere and by currents in the ocean.

Heat is transferred from low to high latitudes

There is a net input of heat to the earth's surface into the tropical regions, where we find the warmest waters

Infrared radiation heat loss to space

Is more constant with latitude

Incoming energy from the sun at the earth's surface is about 4 times at the equator> than at the poles

At the sea surface

temperature is fixed due to exchange with atmospheric heat

Temperature Distribution in the Ocean

Red Sea has

The highest surface salinity

The highest S‰ are found in sub-tropical regions centered at 20º to 3º North and South

Evaporation > Rainfall

At the sea surface, salinity is controlled by

Evaporation Precipitation

Salinity Distribution in the Ocean

Sea water Composition

Major ions

Dissolved gases

Trace elements

Diagram showing concentrations of various salt ions in seawater: Cl− 55%, Na+30.6%, SO2−4 7.7%, Mg2+ 3.7%, Ca2+ 1.2%,K+ 1.1%, Other 0.7%. Note that the diagram is only correct in units of wt/wt, not wt/vol or vol/vol.

Major Ions• Definition: • Those elements whose concentration is greater than 1 ppm. • Why 1ppm? • Because Salinity is reported to ± 0.001 or 1 ppm. • What are their effect? • They contribute significantly to the salinity. • How many?• Eleven (11) major ions.• What are names and ranking?• Sodium> Magnesium> Calcium> Potassium> Strontium>

chloride> Sulfate> Bicarbonate> Bromide> Borate> Flouride

• Activity #2:

• There are 11 major ions contribute significantly to salinity although of this Na+ and Cl- are the most dominant. (Explain)

• The major ions are conservative. (why?)

The major ions are conservative. This means that they have constant ratios, to one another and to salinity, in almost all ocean water.

Sea salts have constant composition. They almost always consist of 55% sodium ion, 31% chloride, 8% sulfate, 4% magnesium ion, 1% calcium ion, and 1% potassium ion.

The main exception is where freshwater is mixing with seawater. River water has a different composition than seawater, it contains more calcium ion. 

At a salinity of S = 35.000 seawater, The major ions concentration is >1mg/kg seawater.

Na+ and Cl- account for >86% of the salt content.

The order of dominance of the other cations isMg2+ > Ca2+ > K+ > Sr2+.

The anion Cl- is approximately equal to the sum of the cations.

The other anions are barely significant in the charge balance of seawater.

What are the synonym of other elements that their concentration are < than 1mg/kg seawater?

Sea water composition Via river waterProperties Sea water River water

Amount/ Number Huge number of dissolved elements

Low number of dissolved elements

Sea water has 300 times more elements than river water

Mixture Heterogeneous nearly Homogenous Different from sea water.

Source TerrestrialBiogenicCosmogenicHydrogenic

chemical weathering of rocks.recycled sea salts via rainfall

Contribution Na+ and Cl- account for >86% of the salt content

A minute fraction of river Cl- content (0.01%) from rock weathering.Most from recycled salts.

Gases and Gas ExchangeThe ocean is a sink for anthropogenic CO2. The major transfer modes of CO2 to the ocean from the atmosphere is by gas exchange.

Oxygen is a chemical tracer for photosynthesis. The gas exchange flux of O2 is an important parameter for calculating net biological production.

Some gases can act as tracers for ocean circulation.

• Trace Elements in Seawater:• Definition:• Those elements that do not contribute to the salinity. • Concentrations:• All elements are present in concentrations less than

1mg kg-1. Many are present at very low concentrations (>10-21 M).

• like 1 drop of ink in 100,000 liters of swimming pool.• Restrictions• This presents analytical challenges to measure and• avoid contamination.

Importance

Many are nutrients and required to sustain life (e.g. P, N, Fe)

Others are toxic (e.g. Cu, Hg)

Some are tracers for re-dox conditions (Cr, I, Mn,).Some are tracers for pollution (Pb).

Some form economic deposits such as manganese nodules (e.g. Cu, Co, Ni, Cd)

• Macronutrients• Definition:• Macronutrients are those elements that are believed to be limiting to plant

growth in the surface ocean.• Concentration• They have concentrations in the range of micromoles kg-1. • Components• Phosphate, nitrate and silicate are the important macronutrients. • Distribution• Po4

-4 and NO3-3 have maximum concentrations shallower than silicate

(Why?).

• 1- Because maximum degradation of organic debris happens shallow in the thermocline.

• 2- SiO2 increases with depth because of the dissolution of siliceous tests of diatoms, which dissolve deeper in the water column and on the sea floor.

Dissolve Organic Matter (DOM)• Any material with a carbon atoms joined to each other, and often

to H, O, N and P.• We study organic matter in seawater because:• 1. OM is the principle chemical form in which solar energy is

made, stored, and used on Earth (source of fossil fuels).

• 2. About 80% of the total particulate carbon flux through the thermocline is in the form of organic matter.

• 3. About 20% of the total carbon buried in marine sediments is organic.

• 4. Over geological time scales OM burial in marine sediments is a major source of atmospheric O2.

Summary