CHEMICALOCEANOGRAPHY

Characteristics of Seawater

Characterictics

Due to the water cycle the oceans gets pure water(from nature - via the water cycle) and naturalmaterials from the land and ocean dissolve in it

Solids dissolved in seawater come from:– Chemical weathering of rocks on land, carried to sea by

rivers.– Earth’s interior (minerals and gases released at

hydrothermal vents, underwater volcanoes, and crackson the sea floor.)

Salt Composition in Seawater Only 6 ions compose

over 98 % of the solidsin seawater– Sodium (Na+) and

Chloride (Cl-) account for85% of all solids. (NaCl)-salt, other minor salts inseawater include (MgSO4,KCl, CaCl2)

Salinity

Definition: total amount of saltdissolved in seawater

= # of grams of salt found in 1,000grams of seawater

Ex: 35 grams, then 35/1000 would be“35 parts per thousand”

Salinity in Our Oceans Average ocean salinity is

35 ppt Varies from 32 ppt to 37

ppt– Rainfall– Evaporation– River runoff– Ice formation– Ice Melting– Ex: Black sea 16 ppt due

to river runoff

How does pure water changesalinity?

Precipitation - rain and snow– Carried in by rivers– Direct

Evaporation: water removal Melting of Icebergs-Glaciers Freezing

– Leaves ions in the unfrozen water– Increase salinity– Ice = almost pure water

Composition of Sea Water

96.5 % oxygen and hydrogen 3% Chlorine, Sodium, Magnesium,

Sulfur, Calcium, Potassium Minor Elements: Carbon, Silicon,

Nitrogen Every naturally occurring element has

been found in seawater

Chemical Equilibrium of the OceanIn 1865, Johann Georg Forchhammer found that the

ratio of major salts in seawater from various locationswas a constant, this constant known asForchhammer Principle or the Principle of ConstantProportions.

“Regardless of how salinity may vary, the RATIOSbetween the amounts of major ions in pelagic watersare nearly constant” - The proportion and amount ofdissolved salts per unit volume of ocean is nearlyconstant.

Example: Chloride ion will always make 55.03% ofsalt present

Therefore, the oceans appear in chemicalequilibrium. Ions/elements are added to the ocean atthe same rate ions/elements are removed, known asRESIDENCE TIME.

Implications of ConstantProportions

Marine organisms do not have to dealwith changes in the ratios of variousions, pelagic organisms do not need toexpend energy towards homeostasis.– Just adjust to different salinity– Easy to control internal salt and water

balance Oceans are thoroughly mixed Changes in salinity are controlled by

additions and removal of pure water

Density, Salinity, andTemperature

Density is determined by salinity andtemperature– Higher salinity, higher the density– Lower the temperature, higher the density

Salinity and temperature help formcurrents– Thermohaline circulation

Thermohaline Circulation(Ocean Conveyor Belt)

Helps bring nutrient rich deep water tosurface

Accounts for deep slow currents inocean basins

Circulate and mix water over the globe Mixing takes 500-800 yrs in Atlantic

– Twice as long in Pacific

Dissolved Gases ________________________

Oxygen O2, Carbon Dioxide CO2, Nitrogen N2 Gases dissolve better in cold water “Gas exchange” between ocean and atmosphere driven

diffusion based on the concentration gradient of thesegases in the atmosphere and ocean, gases will flow fromareas of high concentration to low concentration.

Photosynthesis adds oxygen, respiration from organismsadds carbon dioxide

Low salinity holds more dissolved gases than high salinity Deep water under high pressure holds more dissolved

gases Carbon dioxide is the most important dissolved gas in the

oceans, driving photosynthesis and creating chemicalreactions with other elements like calcium to be used bymarine organisms.

The oceans are a “sink” for the absorption of carbondioxide from our atmosphere.

CO2 O2 N2Air-water interface

Transparency Seawater is transparent which allows sunlight

penetration Depending on the amount of suspended and

dissolved material in water, light can penetrate tovarious depths, the more suspended and dissolvedmaterial in water, the lower the transparency and lightis not able to penetrate very deep.

Sunlight is composed of all the colors of the rainbow,based on their wavelength, therefore, not all colorspenetrate the water equally well, short wavelengthsare able to penetrate deeper than long wavelengths.

Therefore the most transparent light is blue, since ithas a short wavelength.

Pressure I atm = 14.7 pounds per square inch For every 10 m or 33 feet, 1 atm of

pressure is added Gas filled structure, like swim bladders

collapse with increasing depth OR may expand largely when being

brought to the surface too quickly