CHEMICAL COMPOSITIONOF

SEAWATER

GEOL 1033

(Lessons 22-23, 25, and parts of Lesson 24)

(ppt file 103-20a)

Chemical Oceanography

• Chemical oceanographers are interested in– Composition of seawater

– Distribution of chemical elements (How they vary)

– Causes of these distributions

– Chemical reactions in the oceans

– Chemical cycles, e. g., carbon cycle, nitrogen cycle, etc.

– Use of chemistry to solve problems in oceanography

– Involved in biochemical and geochemical investigations

– Etc.

• Need:– Water sampling devices

– Storage for water samples

– Techniques to detect & measure elements, etc.

– Measurements of certain physical properties of seawater, too

– Etc.

A Water sampling Device• Must be large enough (amount varies with what measuring)

• Locatable as to depth

• No contamination – stainless steel, teflon, etc.

• Nansen bottle – Vertical & open at both ends until triggered with a messenger

– Thermometers for temperature and depth determinations

Rosette

• A rosette permits collecting many water samples

• Nansen-like bottles are electronically triggered

• Records other information at same time:– Temperature– Depth– Salinity– O2 probe– Suspended particulate matter

(laser nephelometer determines light scattering and converts measurements to concentrations)

Storage

• Water libraries– Water samples are stored in large containers

– May be cooled

– Available to members of the scientific community

WATER DISTRIBUTION ON EARTH'S SURFACE

• ~98% of Earth's surface and near-surface water is in the oceans as liquid or sea ice (1.37 x 109 km3)

• ~97-98% of this water is saline.• Remember: Frozen seawater excludes salts, leaves a cold

brine as a residue, and forms freshwater when it melts.

• Average salinity is about 34.482 o/oo (=34.482 g/kg)

Water Molecule – A Polar Molecule

Hydrogen Bonds in Liquid Water• Forms

"structured water"

What is the Source of the Water in the Oceans?

• Volcanic outgassing from the mantle early in Earth history? – 72% of volcanic gases is water vapour today!

• Cosmogenic?– Many small comets strike Earth’s upper atmosphere per year

– Would have been much more frequent in the distant past about 4 billion years ago

• Why has the volume remained nearly constant for about 4 billion years? – Plate tectonic recycling between Earth's surface and its

mantle may have established a dynamic equilibrium.

Generalized Periodic Table of the Elements

What are the major elements in sea salt?

Seawater Salt = Mostly Sodium Chloride

Salt in Aqueous Solution

WHAT ARE THE OTHER MAJOR ELEMENTS IN SEA SALT?

ELEMENT ABUNDANCE IN THE CRUST

• Abundance of elements in the crust by percentage weight:– Oxygen O 47

– Silicon Si 28

– Aluminum Al 8

– Iron Fe 5

– Calcium Ca 3.6

– Sodium Na 3

– Potassium K 2.6

– Magnesium Mg 2

– all others 1.5

Constituents of Seawater

• 96.5% water (96.5% = 965 ppt)

• Chloride• Sodium• Sulfate• Magnesium• Calcium• Potassium

• Bicarbonate

Constituents of Seawater

• Parts per thousand = ppt

Constituents of Seawater

• Major ions = ppt

• Minor components = ppm

• Trace elements = ppb

What is the Source of the Salts in Seawater?• Weathering and erosion on land provides many ions, e.g., sodium,

calcium, potassium, etc.• Hydrologic cycle carries these ions to the sea

• Volcanic outgassing from the mantle provides many ions, e. g., chloride.

River Water vs. Seawater Constituents• Many ions are in river water, but in much lower

amounts & in different proportions

trace

tracetrace

Residence Time

• Residence times help to explain why some ions are more abundant than others

END OF FILE

• Unit = mL/L• At 1 atm at sea level• STP = standard temperature & pressure

ELEMENT ABUNDANCE IN THE CRUST and WHOLE EARTH• Abundance of elements in the crust by percentage weight:

– Oxygen O 47

– Silicon Si 28

– Aluminum Al 8

– Iron Fe 5

– Calcium Ca 3.6

– Sodium Na 3

– Potassium K 2.6

– Magnesium Mg 2

– all others 1.5

• Whole Earth order of abundance:– Iron Fe 35

– Oxygen O 30

– Silicon Si 15

– Magnesium Mg 13

– Nickel Ni 2.4

– Sulfur S 2

– Calcium Ca 1

– Aluminum Al 1

– all others <1

CHARACTERISTICS OF THE WORLD OCEAN

• Oceans cover 71% of Earth's surface (This is equal to about 361 100 000 km2 or 3.611 x 108 km2)

• Oceans represent about 98% of Earth's surface and near-surface water (1.37 x 109 km3)

• Average depth of the oceans is about 3.8 km (~12,450').• Average temperature of the oceans is about 4 deg. C.

• Average salinity is about 34.482 o/oo (=34.482 g/kg)

HEAT CAPACITY OF COMMON MATERIALS

CROSS SECTIONS OF EARTH• Core (2 layers)

• Mantle (3 layers)• Crust (2 types)

Outer core is liquid

CROSS SECTIONS OF EARTHUpper mantle/crustal layers:

– lithosphere

– asthenosphere

– upper mesosphere

OVERVIEW OF PLATE TECTONIC PROCESSES

Thermal Convection