Page 442 - Environment: The Science Behind the Stories

ide (CO ). As we pump excess CO into the atmosphere
Chloride, 2 2
–
Cl (1.9%) by burning fossil fuels, more CO diffuses into the oceans.
2
We shall soon see (pp. 446–447) how this affects ocean pH
Sodium, (p. 46), turning the water more acidic and posing problems
+
Na (1.1%) for marine life.
Sulfate,
SO 4 2– (0.3%) Solar energy structures ocean water
Magnesium,
Mg 2+ (0.1%) from surface to bottom
Calcium, Sunlight warms the ocean’s surface but does not penetrate
Ocean water Ca 2+ (0.04%) deeply, so ocean water is warmest at the surface and becomes
Potassium,
+
K (0.04%) colder with depth. Surface waters in tropical regions receive
Bicarbonate, more solar radiation and therefore are warmer than surface
HCO 3 – (0.01%) waters in temperate or polar regions. Warmer water is less
dense than cooler water, but water also becomes denser as it
gets saltier. This occurs because as salt dissolves in water, it
increases solution mass (the mass of water and its dissolved
3.5% salts) more than it increases solution volume. These relation-
Figure 16.4 Ocean water consists of 3.5% salt, by mass. ships give rise to different layers of water: Heavier (colder and
Most of this salt is NaCl in solution, so sodium and chloride ions saltier) water sinks, whereas lighter (warmer and less salty)
are abundant. A number of other ions and trace elements are also water remains nearer the surface. Waters of the surface zone
present. are heated by sunlight and stirred by wind such that they are

If you had a beaker containing one kilogram (1000 grams) of similar density down to a depth of about 150 m (490 ft).
of seawater, how many grams of salt are in the beaker? Of Below this zone lies the pycnocline, a region in which density
this salt, how many grams are from negatively-charged ions? increases rapidly with depth. The pycnocline contains about
18% of ocean water by volume, compared to the surface
zone’s 2%. The remaining 80% lies in the deep zone beneath
(Figure 16.4). Ocean water is salty primarily because ocean the pycnocline. The dense water in this deep zone is sluggish
basins are the final repositories for water that runs off the land. and unaffected by winds, storms, sunlight, and temperature
Runoff on land collects salts from minerals in weathered rocks fluctuations.
and carries them, along with sediments, to the ocean. Wind Despite the daily heating and cooling of surface waters,
also blows salts from the land out to sea. Whereas the water in ocean temperatures are much more stable than temperatures on
the ocean evaporates, the salts do not, and they accumulate in land. Midlatitude oceans experience yearly temperature varia-
ocean basins. If we were able to evaporate all the water from tion of only around 10°C (18°F), and tropical and polar oceans
the oceans, their basins would be left covered with a layer of are still more stable. The reason for this stability is that water
dried salt 63 m (207 ft) thick. has a high heat capacity, a measure of the heat required to
The salinity of ocean water generally ranges from 33,000 increase temperature by a given amount. It takes more energy
to 37,000 parts per million, varying from place to place to increase the temperature of water than it does to increase the
because of differences in evaporation, precipitation, and temperature of air. High heat capacity enables ocean water to
freshwater runoff (freshwater has less than 500 parts per mil- absorb a tremendous amount of heat from the air. In fact, just
lion salinity) from land and glaciers. Coastal waters are often the top 2.6 m (8.5 ft) of the oceans holds as much heat as the
less saline because of the influx of freshwater runoff. Salinity entire atmosphere! By absorbing heat and releasing it to the
near the equator is low because this region has a great deal of atmosphere, the oceans help regulate Earth’s climate (Chapter
precipitation, which is relatively salt free. In contrast, surface 18). They also influence climate by moving heat from place to CHAPTER 16 • M AR in E A nd Co A s TA l s ys TEM s A nd R E sou R CE s
salinity is high at latitudes roughly 30–35 degrees north and place via the ocean’s surface circulation, a system of currents
south, where evaporation exceeds precipitation. that move in the pycnocline and the surface zone.
Besides dissolved salts, nutrients such as nitrogen and
phosphorus occur in seawater in trace amounts (well under 1 Surface water flows horizontally in currents
part per million) and play essential roles in nutrient cycling
(p. 135) in marine ecosystems. Another aspect of ocean Earth’s ocean is composed of vast, riverlike flows driven by
chemistry is dissolved gas content. Roughly 36% of the gas density differences, heating and cooling, gravity, and wind.
dissolved in seawater is oxygen, which is produced by pho- Surface currents flow horizontally within the upper 400 m
tosynthetic plants, bacteria, and phytoplankton (p. 93) and (1300 ft) of water for great distances and in long-lasting
enters by diffusion from the atmosphere. Oxygen concentra- patterns across the globe (Figure 16.5). Warm-water cur-
tions are highest in the upper layer of the ocean, reaching 13 rents carry water heated by the sun from equatorial regions,
ml/L of water. Marine animals depend on dissolved oxygen, while cold-water currents carry water cooled in high-latitude
and if oxygen is depleted, a hypoxic “dead zone” may ensue, regions or from deep below. Some surface currents are very
killing animals or forcing them to leave (pp. 123, 428–429). slow. Others, like the Gulf Stream, are rapid and powerful.
Another gas that is soluble in ocean water is carbon diox- From the Gulf of Mexico, the Gulf Stream flows up the U.S. 441

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