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oceanic conveyor

Heating from the Sun drives the cycling of water through the Earth system. On a global scale, over millennia and longer time spans, climate cooling causes water to accumulate in polar ice sheets and lower-latitude glaciers. Conversely, during periods of climate warming, the ice sheets and glaciers will release water back into the ocean—potentially much more quickly than it accumulated, as exhibited at the end of the Younger Dryas (Fig. 7.6). Water also flows through land, air, and sea reservoirs in response to daily and seasonal weather processes (Fig. 1.5). Since nearly 98% of the water on Earth is in the ocean, internal dynamics of the sea are fundamental to understanding the Earth system.

Water is special because of its unique properties for transferring heat and moderating environmental changes as well as for sustaining life in the Earth system (Table 7.1). Unlike other molecules, water exists in all three phases (liquid, solid,

TABLE 7.1 Properties of Water in the Earth System

Properties

Characteristics

Ecosystem impacts

Boiling point

Freezing point

Latent heat of vaporization

Latent heat of fusion Heat capacity

Density

Solvent power

Sound transmission Light transmission

Surface tension

High (100°C) for molecular size High (0°C) for molecular size High (540 calories/gram)

High (80 calories /gram) High (1 calorie/

Dissolves more substances in greater amounts than any other liquid 1500 meters/

second Proportional to water clarity

High

Water exists as a liquid at Earth surface temperatures and pressures Water exists as a liquid at Earth surface temperatures and pressures Liquid-gas interaction moderates temperatures of oceans and other large water bodies by transferring heat to the atmosphere through evaporation Solid-liquid thermal interactions inhibit large-scale freezing of the oceans and other water bodies Moderate daily and seasonal temperature changes, stabilize body temperatures of organisms Causes ice to float and inhibits large-scale freezing of the oceans and other water bodies Maintains large variety of substances in solution which enhances chemical reactions because of the polar nature of bonding between the oxygen and hydrogen atoms

Travels farther and faster than in air (334 meters/ second)

Varies with scattering of different wavelengths, with blue-green wavelengths deepest transmission and red shallowest attenuation Critical to maintaining position of organisms in aquatic habitats and gas) at standard atmospheric pressures and temperatures on the Earth's surface. These three phases of matter are distinguished by the degree of bonding between adjacent molecules—generally with solids having the strongest and closest bonding between molecules while gases have the weakest bonding with molecules furthest apart.

An important feature of water is its unusually high latent heats, where phase transitions occur without continuous temperature changes. For example, while being heated, liquid water will increase in temperature continuously, whereas ice remains at the freezing point (0°C for freshwater) until after it has completely melted. Actually, an extra 80 calories of heat are required to melt each gram of ice before it becomes a liquid and can continue warming.

Similarly, liquid water will warm until the vaporization point (100°C for freshwater) and stay at this temperature until 540 calories of heat have been absorbed by each gram of liquid water. Afterward, the water vapor can continue warming (if the gas is confined) until the water molecules finally disassociate into hydrogen and oxygen atoms. There also is a latent heat of sublimation, involving a direct transition between the solid and gas phases, which is why ice cubes get smaller over time in your freezer. Together, these latent heats for water represent the energy that is either absorbed when molecular bonds are broken or liberated when molecular bonds are formed.

On a global scale, the latent heat of vaporization causes the ocean to cool as liquid water evaporates (because the evaporating water absorbs heat) and the atmosphere to warm as water vapor condenses (because the condensing water liberates heat). As an analogy, consider why your skin cools after getting out of the shower. Similarly, the latent heat of fusion causes heating and cooling of the atmosphere as ice forms and melts seasonally. Together, these latent heats limit the temperature variability in the Earth system as water shifts among its solid, liquid, and gas phases.

Without the salt, the ocean would act like freshwater, with its maximum density 3.98°C above its freezing point (Fig. 7.7a). This unusual density behavior of freshwater (Table 7.1) occurs because the lattice of an ice crystal contains fewer molecules than liquid water (Fig. 7.7b). Seawater, however, has an average content of 34.7 grams of dissolved salts per thousand grams of water (Table 7.2)—about 96.5% water and 3.5% salt, most of which is common table salt (sodium chloride). Dissolved elements alter the basic properties of freshwater such that seawater, with salinities above 24.7 parts per thousand (%c), has its maximum density at the freezing point (Fig. 7.7a). Fortunately, in both freshwater and seawater, ice floats because it is less dense than the underlying water (Table 7.1). Otherwise, aquatic systems across the planet would have frozen from the bottom upward and the evolution of life on Earth would have been vastly different.

Together, the thermohaline (temperature and salinity) properties of seawater influence the vertical distribution of water masses in the ocean. Unlike currents at the sea surface, which are driven by winds (Chapter 8: Breathing Planet), the deep water masses are driven by their relative densities—where cold saline waters sink

TABLE 7.2 Major Ionic Constitutents in Seawater with a Salinity of 35 Parts per

Thousand (%o)a

Element

Chemical symbol

Concentration (%o)

Percentage

Chloride

Cl-

0 0

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