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Transcript
Introduction to Oceanography Earth as a System A system is an interacting set of components that behave in an orderly way according to the laws of nature. One example of a system is the human body, which consists of various subsystems including the circulatory, digestive and respiratory systems plus energy and matter input/output. In a healthy person, these subsystems function internally and interact with one another in regular and predictable ways. Based on extensive observations and understandings of a system, scientists can predict how the system and its components are likely to respond to changing conditions. This predictive ability is important, for example, in dealing with the complexities of global climate change and its potential impacts on Earth’s subsystems. The Earth system consists of four major interacting subsystems: hydrosphere, atmosphere, geosphere and biosphere. The photo of Earth above resembling, “a blue marble,” shows all the major subsystems of the Earth system. (See our website for this photo in color.) The ocean, the most prominent physical feature, appears blue; the atmosphere is made visible by swirling storm clouds off the coast of California. Land (part of the geosphere) is mostly green because of vegetative cover (biosphere). The dominant color of Earth is blue because the ocean covers more than two-thirds of Earth’s surface; in fact, often Earth is referred to as the “blue planet” or “water planet.” Hydrosphere The hydrosphere includes water in all three phases (i.e., ice, liquid water and water vapor) that continually cycles from one reservoir to another within Earth’s system. Water is unique among the chemical components of the Earth system in that water is the only naturally occurring substance that co-exists in all three phases at normal temperatures and pressures found near Earth’s surface. The ocean, by far, is the largest reservoir of water in the hydrosphere, covering approximately 71% of the planet’s surface and has an average depth of about 3.8 km. 97% of the hydrosphere is ocean salt water. The next largest reservoir in the hydrosphere is glacial ice, most of which covers much of Antarctica and Greenland. Ice and snow make up 2% of water in the hydrosphere. Much smaller quantities of water occur on the land surface (lakes, ponds, rivers), in the subsurface (soil moisture, groundwater), the atmosphere (water vapor, clouds, precipitation), and biosphere (plants, animals). 02.25.2012 1 Ocean Basins and Plate Tectonics The lithosphere is the rigid outer part of the planet, consisting of the crust and upper mantle. Many forces work together to shape and reshape Earth’s lithosphere, creating and destroying both continents and ocean basins. Forces are primarily from the movement of tectonic plates and geological processes that occur mostly at plate boundaries. These processes produce the features on the ocean floor. Features on the ocean bottom provide clues as to the origin of the ocean basins. Ocean depth varies markedly from one place to another. Cross-sectional profile of continental margin and ocean bottom Continental Margins Based on measurements of water depth, ocean scientists delineate three distinct zones along the edges of continents - from the coastline seaward. The zone closest to the beach features a very gentle slope extending out to the water depth that averages about 130 m. This gently sloping zone is the continental shelf. Seaward from the continental shelf to a depth of about 3000 m, the water depth increases much more rapidly. This more steeply sloping zone is called the continental slope. Then a relatively narrow zone is transitional from the steep slope of the previous zone to the more-or-less flat ocean basin. This transitional zone is called the continental rise. The continental shelf, slope and rise together comprise the continental margin. The bedrock of the continental margin is the same as the continental crust, predominately granite. From a geological perspective, the continents do not end at the beach, but at the continental rise. At its outer edge, a continental margin merges with the deep-sea floor or descends into an oceanic trench. The continental shelf is nearly flat, sloping less than 1°. As a general rule, the shelf is narrowest where the continental margin is tectonically active (e.g., subduction zones) and is widest where the continental margin is passive (i.e., no plate boundary nearby). Hence, the North American shelf is much wider along the tectonically passive East Coast (several hundreds of kilometers) than the tectonically active West Coast (a few kilometers). About 7.5% of the total area of the ocean overlies the continental shelves. In a passive continental margin, such as along the margin of the Atlantic Ocean, sediment accumulates over the ocean floor forming vast, flat abyssal plains seaward of the continental rise. Along tectonically active continental margins, the slope descends directly into deep ocean trenches. 02.25.2012 2 Submarine Canyons and Submarine Fans Large numbers of deep, steep-sided submarine canyons slice into the continental slope and some run up onto the continental shelf; many of these canyons cut into solid rock. A variety of mechanisms are responsible for creating submarine canyons including erosion by turbidity currents or ancient rivers. A turbidity current is a gravity driven flow of water heavily laden with suspended sediment (silt, sand, and gravel) making the water flow denser than normal seawater. Turbidity currents can create underwater avalanches of sediment. Eventually the pile of sediments builds to an unstable height and suddenly slides downhill, scouring the ocean bottom in the process. These sediments accumulate at the base of the continental rise as a series of overlapping submarine fans. Turbidity currents have been clocked at speeds as great as 100 km/hr. Bathymetry of the coastal ocean showing New York’s Hudson Submarine Canyon; this submarine canyon appears to be the natural extension of the Hudson River. Ocean Basins Fringed by continental margins, ocean basins encompass the remaining portion of the oceanic area. Earth has one ocean with many ocean basins, such as the Pacific, Atlantic, Indian, Southern and Arctic. Ocean basins have a varied topography featuring deep trenches, seamounts, and submarine mountain ranges. Indeed, undersea terrain is just as diverse as terrestrial terrain and exhibits an even greater relief. Over large areas water depth is less than 200 m; in other areas the water is as deep as 11,000 m. The average ocean depth is about 3800 m. Mid-Ocean Ridge and Spreading of Ocean Basin When adjacent plates move apart, they create a divergent plate boundary. Diverging lithospheric plates produce rifts (fractures) in the crust through which magma wells up from below. Magma cools and solidifies rapidly when it comes in contact with much colder seawater. Through this process, new oceanic lithosphere is generated at divergent plate boundaries. 02.25.2012 3 Properties of ocean water Hydrogen Bonding Compared to other naturally occurring substances, water’s thermal properties are unique. For example, based on water’s molecular weight as well as the freezing and boiling temperatures of chemically related substances, fresh water should freeze at about -90 °C and boil at about -70 °C. Actually, fresh water’s freezing point is 0 °C and its boiling point is 100 °C at average sea level air pressure. Water’s unusual properties arise from the physical structure of the water molecule (H2O) and the bonding that occurs between water molecules. Without this intermolecular force of attraction, known as hydrogen bonding, water would exist only as a gas within the range of surface temperature and pressure on Earth. Furthermore, the planet would have no water cycle, no ocean, no ice caps, and probably no life as we know it. Water as Ice, Liquid and Vapor Matter exists in three basic phases: solid, liquid, and gas. Transformations from one phase to another always involve a large amount of energy to alter the molecular configuration. Water readily changes phases on Earth, contributing to the dynamic nature of the Earth system. In changing phase from ice to liquid to vapor, the energy state (i.e., molecular activity) of water increases. Hence, water molecules absorb heat energy from the environment. In changing phase from vapor to liquid to ice, water molecules release heat energy to the environment. Melting, evaporation, and sublimation are phase changes that absorb heat. Phase changes that release heat to the surroundings are freezing, condensation, and deposition. Sea Salts Seawater is a salt solution of nearly uniform composition. A solution is a homogenous mixture of two or more substances. Salinity is a measure of the amount of salt dissolved in seawater. On average, seawater is 96.5% water and 3.5% dissolved salts. If all the ocean water evaporated and the precipitated salts were spread evenly over the surface of the Earth, the salts would form a layer about 45.5 m, the height of a fifteen-story building. What is the origin of the salts dissolved in seawater? The principal source is weathering and erosion of rock and sediment on land and transport by rivers and streams to the ocean. As the largest reservoir in the global water cycle, the ocean receives most of its water from rivers. Processes operating at the atmosphere/ocean interface add or remove water molecules from seawater and largely explain variations in sea-surface salinity. Most dissolved substances are left behind when seawater freezes or evaporates, increasing the surface salinity locally. On the other hand, precipitation, runoff from rivers and melting ice add fresh water and decrease local surface salinity. Photo shows hand harvesting of sea salt in Portugal 02.25.2012 4 Water Density and Temperature Density is defined as mass divided by volume. When placed in water an object that is less dense than water will float to the surface whereas an object that is denser that water will sink. Freshwater density varies primarily with temperature whereas seawater density varies chiefly with temperature and salinity. In a liquid state, cooler fresh water is denser than warmer fresh water. In a liquid state, when comparing ocean water of the same salinity, cooler salt water is denser that warmer salt water. In a liquid state, when comparing ocean water of the same temperature, water with a higher salinity will be denser than water with a lower salinity. Marine Sediments Sediment is generated at the interfaces between the lithosphere, atmosphere, hydrosphere and biosphere. Physical and chemical weathering processes break down exposed bedrock forming rock fragments (sediments) that are transported by rivers, glaciers, wind and gravity to the ocean. Weathering also releases soluble constituents, such as calcium and sodium that dissolve in water and are transported in solution to the ocean. Image of two deltas being formed by Mississippi River Sediments that accumulate on the sea floor differ in source, composition, size and the rate at which the sediments accumulate on the sea floor. Of the many ways of classifying marine sediments, the two most popular are by particle size and source of the sediment. Size and Sorting of Sediment Marine sediments are classified by size into three broad categories: mud (clay and silt), sand, and gravel. Accumulations of sediment (sediment deposits) on the seafloor also vary in the range of grain size, known as sorting. A well-sorted sediment deposit has a narrow range of grain sizes whereas a poorly sorted deposit has a broad range of grain sizes. In general, the greater the distance that sediment is transported by running water, ocean currents, or wind, the better sorted the sediments become. For example marine sediments that originated on land tend to be larger and more poorly sorted in the continental margin than on the ocean floor. Sources of Marine Sediment Ocean scientists classify marine sediments based on their source as lithogenous (from rock), biogenous (from organisms or their remains), hydrogenous (precipitated from seawater), and cosmogenous (from outer space). Lithogenous sediment accounts for about ¾ of all marine sediments and owes its origin mostly to weathering and erosion of pre-existing rock. Biogenous sediment includes excretions, secretions and remains of organisms. Examples include shells, fragments of coral or skeletal parts. Biogenous sediments account for 25% to 50% of all particles suspended in seawater. 02.25.2012 5 Marine Sedimentary Deposits Different types of marine sediments (i.e., lithogenous, biogenous, hydrogenous, and cosmogenous) occur in varying proportions and thicknesses on the ocean bottom as marine sedimentary deposits. Continental Margin Deposits Most marine sediment deposits in the continental margin are lithogenous and occur in a wide range of sediment sizes. Most riverborne lithogenous sediments that reach the coast do not travel very far seaward of the shoreline. About 95% of the largest sediments transported to the ocean by rivers are trapped and deposited in bays, wetlands, estuaries, beaches or deltas. Only about 5% of river-borne sediment brought to the shoreline reaches the continental shelf or slope and very little sediment of terrestrial origin is transported beyond the continental margin into the deep-ocean basins. Deposits of marine sediment are thickest on the continental margins (and near islands) where accumulation rates are relatively high compared to deep-ocean basins. Ocean floor sediments generally are thickest where the ocean crust is oldest so that deposits become thicker with increasing distance from the mid-ocean ridges (divergent or spreading plate boundaries.) A notable exception is seaward of the mouths of major sediment-transporting rivers such as the Mississippi (United States), Ganges (Bangladesh), and Yangtse (China). Massive submarine avalanches and turbidity currents can transport sediments hundreds or thousands of kilometers out onto the continental rise and to the sea floor beyond. Deep-Ocean Deposits Fine-grained sediments that gradually accumulate particle-by-particle on the deep-ocean floor are mostly biogenous and their accumulation rates are considerably slower than continental-margin deposits. On average, a 1 mm-thick layer forms in about 1000 years. (A 1-inch thick layer forms in about 25,000 to 250,000 years.) Marine Sedimentary Rock As part of the rock cycle, over the millions of years that constitute geologic time, sediment that is deposited on the ocean floor is gradually converted to solid marine sedimentary rock through lithification. Lithification usually involves both compaction and cementing of sediments. Sediments are compacted by the increasing weight of sediments accumulating above that squeeze deeper sediments closer together. Siliceous and calcareous fluids migrating through the tiny openings between individual sediment grains precipitates minerals that fill the pore spaces, cementing grains to one another. The product of lithification is sedimentary rock such as shale, sandstone, or limestone depending on the composition and particle size of the constituent sediments. With deeper burial and further increases in temperature, pressure, and access to chemically active fluids, sedimentary rock may be converted to metamorphic rock such as slate, schist or gneiss. Marine Sedimentary strata found in the interior of North America is evidence of an extensive shallow sea, the Western Interior Seaway during the mid-Cretaceous, about 100 million years before the present 02.25.2012 6 The atmosphere and the ocean What role does the ocean play in the long-term average state of the atmosphere? The ocean plays a key role in the global radiation budget, the transport of heat between Earth’s surface and atmosphere; the flow of heat from the tropics to the higher latitudes, and the development of storm systems. Weather and Climate Weather and climate are closely related concepts. Weather is the state of the atmosphere at some place and time described in terms of such variables as temperature, precipitations, cloud cover, and wind speed. A place and time must be specified when describing weather because the atmosphere is dynamic; that is, its state is always changing from one place to another and with time. From personal experience, we know that tomorrow’s weather may differ considerably from today’s weather. Climate is defined as weather at a particular place averaged over a specified interval of time. By international convention, average values of weather elements, such as temperature, precipitation, are computed over a 30-year period. Other useful climate parameters include average seasonal snowfall, length of growing season, and frequency of thunderstorms. Ultimately, climate governs the supply of fresh water, the geographical distribution of plants and animals and the types of crops that can be cultivated. Heating and Cooling Earth’s Surface As the Earth orbits the sun, its atmosphere and the surface are absorbing energy radiated by the sun. This energy, when absorbed, is called heat energy. Absorption of solar radiation heats the Earth-atmosphere system. At the same time the entire Earth-atmosphere system is emitting infrared radiation to space, which has a cooling effect on the Earth-atmosphere system. Over the long term, radiational cooling of the planet essentially balances radiational heating of the planet so that Earth remains in radiative equilibrium with surrounding space. Solar Radiation Budget Solar radiation intercepted by Earth travels through the atmosphere Average Albedo of some common and interacts with its component gases and aerosols, minute solid surface types and liquid particles which are suspended in the atmosphere. These Surface Albedo (% reflected) interactions consist of scattering, reflection and absorption. Solar Forest 7-18% radiation that is not absorbed or scattered or reflected back to space Tundra 15-35% reaches Earth’s surface where additional interactions occur. With Desert 25-30% scattering, a particle disperses radiation in all directions: up, down, Urban Area 14-18% and sideways. Within the atmosphere, both gas molecules and Crops 15-25% aerosols scatter solar radiation. Reflection is a special case of Water 10-100% scattering in which a large surface area redirects the radiation in a Fresh snow 75-95% backward direction. The fraction of incoming radiation that is Old snow 40-60% reflected by a surface is known as the albedo of that surface. Cirrus Clouds 40-50% Surfaces having a high albedo, such as glaciers, reflect a relatively Soil 5-30% large fraction of incoming solar radiation and appear light in color. Surfaces having a low albedo, such as the ocean, reflect relatively little incoming solar radiation and appear dark in color. 02.25.2012 7 Scattering and reflection within the atmosphere alter the direction of solar radiation without converting the radiation to heat. Absorption, however, is the process whereby some of the radiation that strikes an object is converted to heat energy. Oxygen, ozone, water vapor, and some aerosols absorb radiation in the atmosphere. Solar radiation not scattered to space or absorbed by atmospheric gases or aerosols reaches Earth’s surface where it is either reflected or absorbed. The portion that is not reflected is absorbed (i.e., converted to heat). High-albedo surfaces, such as snow-covered ground or pack ice, reflect a considerable amount of incoming solar radiation whereas low-albedo surfaces, such as an asphalt parking lot or a forest, reflect much less incoming solar radiation. (See the table on page 7 for the albedo of some common surfaces.) Measurements by sensors onboard Earth orbiting satellites indicate that the Earth-atmosphere system reflects or scatters back to space on average about 31% of the incoming solar radiation intercepted by the planet. This is Earth’s planetary albedo. The atmosphere (i.e., gases, aerosols, clouds) absorbs only about 20% of the total solar radiation intercepted by the Earth-atmosphere system. In other words, the atmosphere is relatively transparent to solar radiation. The remaining 49% of solar radiation is absorbed by Earth’s surface, mostly by the ocean. Earth’s surface is the principal recipient of solar heating, and heat is transferred from Earth’s surface to the atmosphere, which eventually radiates this energy to space. Hence, Earth’s surface is the main source of heat for the atmosphere; that is, the atmosphere is heated from below. This is evident from the average vertical temperature profile of the troposphere, the lowest layer of the atmosphere; where the atmosphere interfaces with the ocean, lithosphere, and biosphere and where most weather takes place. Solar Radiation and the Ocean Whereas the atmosphere is relatively transparent to (and absorbs little) solar radiation, the ocean absorbs most solar radiation within relatively shallow depths. The photic zone is the sunlit surface layer of the ocean, down to the depth where light is just sufficient for photosynthesis. With some notable exceptions, marine life depends directly or indirectly on sunlight and organic productively in the ocean’s photic zone. Even the diverse community of animals living at great depths on the ocean floor depends on organic particles produced within the photic zone that settle to the sea floor; exceptions are organisms living near hydrothermal vents. Close up view of giant tubeworms living near hydrothermal vent 02.25.2012 8 Infrared Radiation and the Greenhouse Effect If solar radiation were continually absorbed by the Earth-atmosphere system without any compensating flow of heat out of the system, Earth’s surface temperature would rise steadily. Eventually, life would be extinguished and the ocean would boil away. Actually, global air temperature changes very little from one year to the next. Global radiative equilibrium keeps the planet’s temperature in check to some extent; that is, emission of heat to space in the form of infrared radiation balances solar radiational heating of the Earthatmosphere system. Although solar radiation is supplied only to the illuminated half of the planet, infrared radiation is emitted to space ceaselessly, day and night, by the entire Earth-atmosphere system. This explains why nights are usually colder than days and why air temperatures typically drop throughout the night. While the clear atmosphere is relatively transparent to solar radiation, certain gases in the atmosphere impede the escape of infrared radiation to space thereby elevating the temperature of the lower atmosphere. This important climate control, the so-called greenhouse effect, refers to the heating of Earth’s surface and lower atmosphere caused by strong absorption and emission of infrared radiation (IR) by certain gaseous components of the atmosphere, known as greenhouse gases. Water vapor is the principal greenhouse gas. Other greenhouse gases include carbon dioxide (CO2), ozone (O3), and methane (CH4). Solar radiation and infrared radiation have different properties and interact differently with the atmosphere. As noted earlier, the atmosphere absorbs only about 20% of the solar radiation intercepted by the planet. However, the atmosphere absorbs a greater percentage of the infrared radiation emitted by Earth’s surface, and the atmosphere, in turn, radiates some IR to space and some back to Earth’s surface. Hence, Earth’s surface is heated by absorption of both solar radiation and atmosphere-emitted infrared radiation. Arizona Dessert in Southwestern U.S. Florida Everglades just off the Gulf Coast Warming caused by atmospheric water vapor is evident even at the local or regional scale. Consider an example. Locations in the Desert Southwest and along the Gulf Coast are at about the same latitude and on a clear day receive essentially the same input of solar radiation. In both places, summer afternoon high temperatures commonly read 32 °C. At night, however, air temperatures often differ markedly. Air is relatively dry (low humidity) in the Southwest so that infrared radiation readily escapes to space and air temperatures near Earth’s surface may drop well under 15 °C by dawn. People who camp in the desert are well aware of the dramatic fluctuations in temperature between day and night. Infrared radiation does not escape to space as readily through the Gulf Coast atmosphere where the air is more humid. Water vapor strongly absorbs IR and emits IR back towards Earth’s surface so that early morning low temperatures may dip no lower than the 20s Celsius. The smaller day-to-night temperature contrast along the Gulf Coast is due to more water vapor and a stronger greenhouse effect. 02.25.2012 9 Latent Heating and the Exchange of Heat Energy from the Ocean to the Atmosphere Latent heating refers to the transfer of heat energy from one place to another as a consequence of phase changes of water. When water changes phase, heat energy is either absorbed from the environment (i.e., melting, evaporation, sublimation) or released to the environment (i.e., freezing, condensation, deposition). As part of the global water cycle, latent heat that is used to vaporize water at the Earth’s surface is transferred to the atmosphere when clouds form. Significantly for Earth’s climate, ocean water covers a large portion of Earth’s surface and is the principal source of water vapor that eventually returns to Earth’s surface as precipitation. In general, only well inland does most precipitations originate as evaporation from the continents. Also, the ocean is a major source of salt crystals that spur condensation and cloud development in the atmosphere. As Earth’s surface absorbs radiation (both solar and infrared), some of the heat energy is used to vaporize water from oceans, glaciers, lakes, rivers, soil and vegetation. The latent heat required is supplied at the Earth’s surface, and heat is subsequently released to the atmosphere during cloud development. During cloud formation, water changes phase - water vapor condenses into liquid water or deposits as ice crystals - and latent heat is released to the atmosphere. Through latent heating, then, heat is transferred from Earth’s surface to the troposphere. In fact, latent heat transfer is more important than either radiational cooling or sensible heat transfer (conduction and convection) in the cooling of Earth’s surface. Acquisition and subsequent release of latent heat in storm systems play an important role in the poleward transport of heat. At low latitudes, near the equator, water that evaporates form the warm ocean surface may be drawn into a developing storm system. As the storm travels into higher latitudes, some of that water vapor condenses into clouds, thereby releasing latent heat to the troposphere. 02.25.2012 10 Map of major wind surface currents on Earth Heat Transport by Ocean Circulation The ocean contributes to poleward heat transport via wind-driven surface currents and the deeper thermohaline circulation. Surface water that is warmer than the overlying air is a heat source for the atmosphere; that is, heat is conducted from sea to air. Surface water that is cooler than the overlying air is a heat sink for the atmosphere; that is, heat is conducted from the air to the sea. Warm surface currents, such as the Gulf Stream, flow from the tropics into middle latitudes, supplying heat to the cooler middle latitude troposphere. At the same time, cold surface currents, such as the California Current, flow from high to low latitudes, absorbing heat from the relatively warm troposphere and greater solar radiation in the tropics. The ocean’s thermohaline circulation is the density-driven movement of water masses, traversing the lengths of ocean basins. The density of seawater increases with decreasing temperature and increasing salinity. More dense water tends to sink while less dense water rises. The thermohaline circulation transports heat energy, salt and dissolved gases over great distances and to great depths in the World Ocean and plays and important role in Earth’s climate system. 02.25.2012 11 Moving Water Energy and matter (e.g., heat, water) are continually exchanged between the ocean and atmosphere, and theses processes drive the ocean circulation. Evaporation, precipitation, runoff from the continents plus heating and cooling bring about changes in the temperature and salinity of surface waters. Density changes that accompany variations in temperature and salinity can cause water to sink or rise in the ocean. Kinetic energy is transferred from near-surface winds to the ocean’s surface layer, driving the currents that dominate the motion of the ocean’s surface, the upper hundred meters or so. Most seawater (90%) is in the deep ocean, isolated from the atmosphere and its winds. Deep ocean waters are cold and dark and come to the surface primarily at high latitudes where they interact with the atmosphere. Differences in water density drive the sluggish circulation of deep water. Typically these waters flow at speeds of less than 1 km per day – about 240 times slower than the Gulf Stream. Wind Driven Surface Currents Wind driven surface currents in the surface layer are maintained by kinetic energy transferred from the horizontal winds to ocean surface waters. Once the wind sets surface waters in motion as a current, the Coriolis Effect and the shape of the ocean basin modify the speed and direction of these surface currents. The Coriolis Effect is a force due to Earth’s rotation that causes deflection of moving objects to the right in the Northern Hemisphere and the left in the Southern Hemisphere. Density Driven Thermohaline The upper portion of the ocean is put into motion by wind forcing at the surface. Below the upper 100 meters, the deep ocean is shielded from the direct action of the wind and ocean currents are driven primarily by density differences in the water masses. These density contrasts are caused by variations in water temperature and salinity with cold salty water being the densest combination. The deep-ocean circulation driven by variations in density is called the thermohaline circulation, the name coming from thermo meaning heat and haline referring to salinity. Another name for this circulation system is the meridional overturning circulation (MOC) because this is the primary mode of water motion (sinking at high latitudes and upwelling at low latitudes). Upwelling refers to the upward circulation of cold, nutrient-rich bottom water toward the ocean surface. Downwelling refers to the downward motion of surface waters. Schematic of upwelling 02.25.2012 12 Waves Kinetic energy (energy of motion) of air in motion (the wind) is transferred to the ocean surface causing surface waves. Wind-driven ocean waves are oscillations, regular fluctuations, of the sea surface that move horizontally along the ocean/atmosphere interface away from where the waves were generated. Energy is propagated with the wave – not the water. Wave height, depends on wind speed, duration of the wind in the same direction, and the distance over which the wind blows. Ocean Tides Astronomical tides are the regular rise and fall of the sea surface caused by the gravitational attraction between the rotating Earth and the moon and the sun. The tide generating force is produced by the combination of (1) the gravitational attraction between the Earth and the moon and sun, and (2) the rotations of the Earthmoon system and the Earth-sun system. Forces combine to deform Earth’s ocean surface into a roughly egg –shape with two bulges. One ocean bulge faces towards the moon and the other is on the opposing side of the planet, facing away from the moon. A similar interaction between the Earth and sun produces two other ocean bulges that line up towards and away from the sun. The ocean tidal bulges produced by the moon remain in the same alignment relative to the moon, as well as the ocean tidal bulges produced by the sun remain in the same alignment relative to the sun. Its Earth’s daily 24 hour rotation underneath these tidal bulges that allows us to experience low and high tides. Schematic of gravitational forces causing tides According to the law of universal gravitation, the greater the mass, the greater the force of attraction whereas the greater the distance, the smaller the force of attraction. Although the sun is 107 times more massive than the moon, the moon is much closer to Earth and for that reason exerts a greater gravitational pull on Earth. Tides typically are measured at coastal locations as local changes are sea level as a function of time. The difference in height between water levels at high and low times is called the tidal range and the time between successive high tides is the tidal period. Newton’s first law of motion, an object in constant straight line motion remains in that motion unless acted upon by an unbalanced force. In a rotating system, such as the Earth-moon system, the net force confines an object to a curved path (rather than a straight) path. Consider an analogy. Suppose that you are a passenger in an auto that rounds a curve at high rate of speed. You feel a force that pushes you outward from the turning auto. Actually, you are experiencing the tendency for our body to continue moving in a straight path while the auto follows a curved path. In the same way, the rotation of the Earth-moon system causes the ocean to bulge outward on the side of the planet opposite the moon. 02.25.2012 13 Marine Ecosystems and Life in the ocean The ocean is home to a huge number of different types of species or organisms-perhaps as many as 10 million-and many more are yet to be discovered and studied. The wide variety of marine habitats makes possible this great biodiversity. Some marine ecosystems, such as coral reefs, are structurally complex and provide niches for many types of organisms that have evolved different adaptations to their environment. The discovery of hydrothermal vents and their associated thousands of species of marine organism dates only to 1979. Many of the fundamental requirements for life in the ocean are the same as those needed by terrestrial organisms. Essential for life on Earth are a source of energy (e.g., sunlight), liquid water, the appropriate mix of chemical constituents (e.g., nutrients) and the right combination of environmental conditions (e.g., range of temperature). One major difference between life in the ocean and life on land is the much greater space and variety of marine habitats. All living organisms require energy, which they obtain either as producers (via photosynthesis or chemosysthesis) or as consumers eating other organisms. Photosynthetic organisms in the ocean, such as plants, algae and some bacteria, use light energy (solar radiation) to convert simple inorganic compounds (carbon dioxide and water) into complex energy-rich organic substances, which provide energy to consumers that eat these photosynthetic organisms. Decomposers are consumers that feed on dead organic matter, either on the ocean bottom or within the water. Decomposers are essential components of all ecosystems because they break down organic matter and recycle nutrients back into the ecosystem. Food webs, such as above, show the sequences of feeding relationships among organisms (e.g., producers, consumers, decomposers) through which energy and materials move within an ecosystem. Within food chains and food webs, energy transfer occurs in one direction, from the lower feeding levels (producers) to the higher feeding levels (consumers). Each feeding position occupied by an organism within a food web is called a trophic level. In the food web above, the organisms in the lowest trophic level are dinoflagellates and diatoms, while the organisms on the highest trophic level are the large sharks. Food webs are pathways not only for energy but also for toxins, that is, poisons that persist in the environment because they do not break down physically, chemically, or biologically. Marine life is commonly defined in terms of distance from shore and water depth. The open ocean constitutes the pelagic zone, home to passive floaters, weak swimmers and strong swimmers. The environment of the sea floor at all depths is called the benthic zone, home to organisms that live either on the ocean bottom or within sediment deposits. 02.25.2012 14 Review Questions 1) Describe the subsystem on Earth called the Hydrosphere. 2) Identify the second largest reservoir in the hydrosphere and its location. 3) Describe the general relationship between continental shelf width and tectonic activity in the continental margin. 4) Describe the unique properties of the water molecules mentioned in the reading. 5) Identify and describe the three phase changes of water during which heat energy is released to the environment. 6) Identify and describe the three phase changes of water during which heat energy is absorbed from the environment. 7) Describe what processes decrease sea-surface salinity of ocean water. 8) Describe what processes increase sea-surface salinity of ocean water. 9) Compare the salinity of surface ocean waters in regions where the precipitation is low but evaporation rates are high to the salinity of surface ocean waters in regions where the precipitation is high but evaporation rates are low. 10) At a constant temperature, how does the density of seawater change with increasing salinity? With constant salinity, how does the density of seawater change with falling temperatures? 11) What are the four major groups of marine sediments? Briefly describe the source of each sediment type. 12) Compare continental-margin deposits to deep-ocean deposits. 13) Describe the difference between weather and climate. 14) Describe what happens to solar radiation during scattering, reflection and absorption. 15) Give an example of how the albedo of a specific surface type will impact the solar radiation budget of the Earthatmosphere system. 16) What is the major source of heat energy for the troposphere, the lower atmosphere? 17) Using one of the maps of the wind surface currents on Earth, compare the rotation direction of surface currents in the Northern Hemisphere to the rotation direction of surface currents in the Southern Hemisphere. 18) Describe the thermohaline circulation system and what is causing this circulation system. 19) What major forces cause the tides? 20) Describe what does a food web shows. Guide to reading textbook: Overview of Reading (due Wednesday, Feb. 29) 1) Read over the Review Questions, usually at the end of the reading. 2) Skimming. Skim the chapter quickly, noticing headlines, pictures and graphs. This takes just a few minutes. You are not trying to understand the chapter yet, but just getting the general topic being covered Tackling Vocabulary (due Friday, March 2) 3) Read the article in its entirety. As you are reading the article, circle any words that you do not know. 4) Go back and find the words that you have circled and read the entire sentence that contains this word. 5) Do you recognize any parts of this word, what parts of this word do you recognize? In the margins jot down what each word could mean using clues from the reading. If you still don’t have any idea…look up the term in a dictionary. Understanding the Reading (due Monday, March 5) 6) Go back and read the whole chapter, section by section, for comprehension. Next to each paragraph or on a separate piece of paper, write one or two sentences that summarize the ideas in that paragraph, draw images that are described and write down any questions you have after reading that paragraph. Practice your Understanding (due Wednesday, March 7) 7) After you understand the chapter and each paragraph, answer the Review Questions. Remaining Questions (due Wednesday, March 7) 8) Follow up with Teacher. Utilize your teachers extra help days to ask for clarification. Think of it as Mrs. Ciarametaro’s and Ms. Schofield’s office hours . 02.25.2012 15