Page 529 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 16 Oceans, Coastal Systems, and Wind Processes 493
160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160°
Greenland Sea
80°
Beaufort Sea
Baffin Bay
Labrador Sea
Hudson Bay
North Sea
Caspian Sea Aral Sea
Persian Gulf
Arabian Sea
0
60°
Bering Sea
Bering Sea
Tropic of Cancer
40°
60°
▲Figure 16.2 Principal seas of the world.
Properties of Seawater
Sea of Okhotsk
40°
Gulf of Alaska
Gulf of California
Black Sea
Red Sea
40°
Gulf of Mexico
East China Sea
Sea of Japan (East Sea)
20°
0°
20°
Caribbean Sea
South China Sea
Gulf of Carpentaria
20°
Philippine Sea
Gulf of Guinea
Equator 0° Coral
Sea 20° Tasman
Sea
Java Sea
Tropic of Capricorn
Scotia Sea
Wedell Sea
1500
3000 KILOMETRES
Antarctic Circle
ROBINSON PROJECTION
60°
As mentioned in Chapter 14, water dissolves at least 57 of the 92 elements found in nature and is known as the “universal solvent.” In fact, most natural elements and the compounds they form are found in the world’s oceans and seas as dissolved solids, or solutes. Thus, seawater is a solution, and the concentration of dissolved solids in that solution is known as salinity, commonly expressed as dissolved solids per volume. Water, you recall, moves continuously through the hydrologic cycle, driven by energy from the Sun, but the dissolved solids remain in the ocean. The water you drink today may have water molecules in it that not long ago were in the Pacific Ocean, in the Yangtze River, in groundwater in Sweden, or airborne in the clouds over Peru.
Chemical Composition The uniform chemical compo- sition of seawater was first demonstrated in 1874 by sci- entists sampling seawater as they sailed around the world aboard the British HMS Challenger. The ocean continues to be a remarkably homogeneous mixture today—the ratio of individual salts does not change, despite minor fluctuations in overall salinity.
The chemical composition of seawater is affected by the atmosphere, minerals, bottom sediments, and living organisms. For example, the flows of mineral-rich water from hydrothermal (hot water) vents in the ocean floor (“black smokers,” as seen in Figure 12.10) alter ocean chemistry in that area. However, the continuous mix- ing among the interconnected ocean basins keeps the overall chemical composition mostly uniform. Until
recently, experts thought the chemistry of seawater to have been fairly constant over the past 500 million years. However, samples of ancient seawater gathered from fluid inclusions in marine formations, such as limestone and evaporite deposits, suggest that slight chemical variations in seawater have occurred over time. The variations are consistent with changes in sea- floor spreading rates, volcanic activity, and sea level.
Seven elements account for more than 99% of the dissolved solids in seawater. In solution they take their ionic form (shown here in parentheses): chlorine (as chlo- ride ions, Cl–), sodium (as Na+), magnesium (as Mg2+), sul- fur (as sulfate ions, SO42–), calcium (as Ca2+), potassium (as K+), and bromine (as bromide ions, Br–). Seawater also contains dissolved gases (such as carbon dioxide, nitro- gen, and oxygen), suspended and dissolved organic mat- ter, and a multitude of trace elements.
Commercially, only sodium chloride (common table salt), magnesium, and bromine are extracted in any sig- nificant amount from the ocean. Mining of minerals from the seafloor is technically feasible, although it re- mains uneconomical.
Average Salinity Scientists express the worldwide aver- age salinity of seawater in several ways:
• 3.5% (parts per hundred)
• 35 000 ppm (parts per million)
• 35000mg·L−1
• 35g·kg−1
• 35‰ (parts per thousand); this is the most common
notation
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