Chapter 18: The Oceans and Their Margins J. Bruce H. Shyu May 24, 2010 Introduction: The World’s Oceans Seawater covers 70.8 percent of Earth’s surface, in three huge interconnected basins: The Pacific Ocean (太平洋). The Atlantic Ocean (大西洋). The Indian Ocean (印度洋). Figure 18.1 The Oceans’ Characteristics The greatest ocean depth yet measured (11,035 m) lies in the Mariana Trench (馬里亞納海溝). The average depth of the oceans, is about 3.8 km. The present volume of seawater is about 1.35 billion cubic kilometers. More than half this volume resides in the Pacific Ocean. Figure 18.2 Ocean Salinity (1) Salinity (鹽度) is the measure of the sea’s saltiness, expressed in parts per mil (‰ = parts per thousand). The salinity of seawater normally ranges between 33 and 37‰. The principal elements that contribute to this salinity are sodium and chlorine. Ocean Salinity (2) More than 99.9 percent of the ocean’s salinity reflects the presence of only eight ions: Chloride. Sodium. Sulfate. Magnesium. Calcium. Potassium. Bicarbonate. Bromine. 海水中的元素組成 鹽度 : 35gm/1000gm 9 種主要元素: 氯 (Cl) 鈉 (Na) 硫酸鹽 (SO4-2) 鎂 (Mg) 鈣 (Ca) 鉀 (K) 次碳酸鹽 (HCO3-) 溴 (Br) 鍶 (Sr) Ocean Salinity (3) Cations are released by chemical weathering processes on land. Each year streams carry 2.5 billion tons of dissolved substances to the sea. The principal anions found in seawater are believed to have come from the mantle. Chemical analyses of gases released during volcanic eruptions show that the most important volatiles are water vapor (steam), carbon dioxide (CO2), and the chloride (Cl-) and sulfate (SO42-) anions. Salinity of the oceans Figure 18.3A Temperature and Heat Capacity of the Ocean (1) Global summer sea-surface temperature is displayed with isotherms (等溫度線) that lie approximately parallel to the equator. The warmest waters during August (>28°C) occur in a discontinuous belt between about 30° N and 10° S latitude. In winter, the belt of warm water moves south until it is largely below the equator. Temperature of the oceans in August Figure 18.3B 1994年5月時全球海洋表面平均溫度 (oF) 西 太 平 洋 表 面 海 水 年 平 均 溫 度 Temperature and Heat Capacity of the Ocean (2) Waters become progressively cooler both north and south of this belt. Since the water has a high heat capacity (熱容量), both the total range and the seasonal changes in ocean temperatures are much less than what we find on land. Coastal inhabitants benefit from the mild climate resulting from this natural ocean thermostat. Vertical Stratification (1) Temperature and other physical properties of seawater vary with depth. When fresh river water meets salty ocean water at a coast, the fresh water, being less dense, flows over the denser saltwater, resulting in stratified water bodies. The oceans also are vertically stratified as a result of variation in the density of seawater. Vertical Stratification (2) Seawater become denser as: Its temperature decreases. Its salinity increases. Gravity pulls dense water downward until it reaches a level where the surrounding water has the same density. These density-driven movements lead both to stratification of the oceans and to circulation in the deep ocean. 印 度 洋 海 水 溫 度 剖 面 海洋環流 表面洋流：風吹流 中層流與底層流：溫鹽環流 由行星風系所驅動 由海水的密度差所驅動 運行速度快 運行速度緩慢 兩者共同形成全球的海水輸送帶 Ocean Circulation Surface ocean currents (表面洋流) are broad, slow drifts of surface water set in motion by the prevailing surface winds. A current of water is rarely more than 50 to 100 m deep. The direction taken by ocean currents is also influenced by the Coriolis effect (科氏力效應). 科氏力：由地球自轉速度，移動粒子的水平速度、所在 的緯度所決定，其方向與速度向量的方向成90度夾角 Current Systems Each major current is part of a large subcircular current system called a gyre (環流). The Earth has five major ocean gyres. Two are in the Pacific Ocean. Two are in the Atlantic Ocean. One is in the Indian Ocean. Figure 18.4 黑潮流徑 觀 冬季 緯 測 速度比率 50 cm/s 度 ： 用 衛 星 追 蹤 夏季 緯 浮 速度比率 球 50 cm/s 度 細線條是1988~2006年間衛星追蹤約850個浮球隨流漂移的軌跡（又稱海流麵條 圖），箭矢是從軌跡計算出的海流平均速度。（台灣海洋大學胡健驊教授） Major Water Masses Ocean waters also circulate on a large scale within the deep ocean, driven by differences in water density. The water of the oceans is organized into major water masses, each having a characteristic range of: Temperature. Salinity. 大西洋的溫鹽環流 Figure 18.5 NADW 北大西洋深層水 AAIW 亞南極中層水 AABW 南極底層水 The Global Ocean Conveyor System (1) Dense, cold, and/or salty surface waters that flow toward adjacent warmer, less-salty waters will sink until they reach the level of water masses of equal density. The resulting stratification of water masses is thus based on relative density. The Global Ocean Conveyor System (2) The sinking dense water in the North Atlantic propels a global thermohaline circulation system, so called because it involves both the temperature (thermo) and salinity (haline) characteristics of the ocean waters. The Global Ocean Conveyor System (3) The Atlantic thermohaline circulation acts like a great conveyor belt, transporting low-density surface water northward and denser deep-ocean water southward. Heat lost to the atmosphere by this warm surface water, together with heat from the warm Gulf Stream, maintains a relatively mild climate in northwestern Europe. Figure 18.6A 全球海水輸送帶 Figure 18.6B Ocean Tides (1) Tides (潮汐): Twice-daily rise and fall of ocean waters. Caused by the gravitational attraction between the Moon (and, to lesser degree, the sun) and the Earth. The Moon exerts a gravitational pull on the solid Earth. Tide-Raising Force (1) A water particle in the ocean on the side facing the Moon is attracted more strongly by the Moon’s gravitation than it would be if it were at Earth’s center, which lies at a greater distance. This creates a bulge on the ocean surface due to the excess inertial force (called the tide-raising force). Figure 18.7 Tide-Raising Force (2) At most places on the ocean margins, two high tides and two low tides are observed each day as a coast encounters both tidal bulges. Twice during each lunar month, Earth is directly aligned with the Sun and the Moon, whose gravitational effects are thereby reinforced, producing higher high tides and lower low tides. Figure 18.8 Tide-Raising Force (3) In the open sea tides are small (less than 1 m). Along most coasts the tidal range commonly is less than 2 m. In bays, straits, estuaries, and other narrow places along coasts, tidal fluctuations are amplified and may reach 16 m or more. Associated tidal currents (潮汐水流) are often rapid and may approach 25 km/h. The incoming tide locally can create a wall of water a meter or more high (called a tidal bore). Ocean Waves (1) Ocean waves receive their energy from winds that blow across the water surface. The water particles move in a loop-like, or oscillating manner. Because waveform is created by this loop-like motion of water parcels, the diameters of the loops at the water surface exactly equal wave height (波高). Figure 18.10 Ocean Waves (2) Downward from the surface, a progressive loss of energy occurs, resulting in a decrease in loop diameter. “L” is used to represent wavelength (波長), the distance between successive wave crests or troughs. At a depth equal to half the wavelength (L/2), the diameters of the loops have become so small that motion of the water is negligible. Wave Base The depth L/2 is therefore referred to as the wave base (浪基面或波底). Landward of depth L/2, as the water depth decreases, the orbits of the water parcels become flatter until the movement of water at the seafloor in the shallow water zone is limited to a back-and- forth motion. 由於摩擦力隨 深度增加，水 分子運動的能 量隨水深而不 斷減弱，到水 深為波長的一 半時即消失。 波底 ＝ ½波長水深 wave base = depth of ½ wave length Breaking Waves When the wave reaches depth L/2, its base encounters frictional resistance exerted by the seafloor. This causes the wave height to increase and the wave length to decrease. Eventually, the front becomes too steep to support the advancing wave and the wave collapses, or breaks. Figure 18.11 波浪的形狀從深海 進入淺海所發生的 變化 波長和波速不斷減小， 波高不斷增加。最後 波浪愈變愈陡，圓周 狀轉動的速度 終於 超過波浪本身前進的 速度，波浪於是崩潰， 形成破浪(breaking wave 或breaker)。 Surf Such “broken water” is called surf. The area between the line of breaking waves and the shore is known as the surf zone (破浪帶). Water piled against the shore returns seaward partly in localized narrow channels as rip currents (離岸流). The geologic work of waves is mainly accomplished by the direct action of surf. Wave Refraction (1) A wave approaching a coast generally does not encounter the bottom simultaneously all along its length. As any segment of the wave touches the seafloor: That part slows down. The wave length begins to decrease. The wave height increases. This process is called wave refraction (波浪折屈). Wave Refraction (2) Wave refraction affects various sectors of a coastline differently. Waves converge on headlands, which are vigorously eroded. Refraction of waves approaching a bay will make them diverge, diffusing their energy at the shore. In the course of time, irregular coasts become smoother and less indented. Figure 18.13 波浪折屈 (wave refraction) 波峰線平行海濱線，波向線則垂直波峰線，波浪能量和侵蝕將 集中於岬角(headland)。波向線又稱能流線(energy-flow lines)。 Coastal Erosion and Sediment Transport (1) Erosion below sea level: Ocean waves rarely erode to depths of more than 7 m. The lower limit of wave motion is half the wavelength of ocean waves. Abrasion in the surf zone: An important kind of erosion in the surf zone is the wearing down of rock by wave-transported rock particles. The activity is limited within a few meters, so the surf is like an erosional knife edge or saw cutting horizontally into the land. Coastal Erosion and Sediment Transport (2) Erosion above sea level: Waves pounding against a cliff compress the air trapped in fissures. Nearly all the energy expended by waves in coastal erosion is confined to a zone that lies between 10 m above and 10 m below mean sea level. Sediment Transport by Waves and Currents (1) Longshore currents (沿岸流): Longshore currents flow parallel to the shore. The direction of longshore currents may change seasonally. The longshore current moves the sediment along the coast. Figure 18.14 Sediment Transport by Waves and Currents (2) Beach drift: The swash (uprushing water) of each wave travels obliquely up the beach before gravity pulls the water back directly down the slope of the beach. This zigzag movement of water carries sand and pebbles first up, then down the beach slope in a process known as beach drift. Beach drift can reach a rate of more than 800 m/day. Figure 18.15 Sediment Transport by Waves and Currents (3) Beach placers: Gold, diamond, and several other heavy minerals have been concentrated in beach sands by surf and longshore currents. Offshore transport and sorting: Far from shore only fine grains can be moved. Sediments grade seaward from sand into mud. Figure 18.16 Coastal Deposits and Landforms The shore profile (海岸剖面) is a vertical section along a line perpendicular to the shore. The three important features of the shore profile are: Beaches (海灘). Wave-cut cliffs (波蝕崖或海蝕崖). Wave-cut benches (波蝕台地). Beaches (1) Beach is: The sandy surface above the water along a shore. A wave-washed sediment along a coast, including sediment in the surf zone (sediment is continually in motion). Sediment of a beach may derived from: Erosion of adjacent cliffs or cliffs elsewhere along the coast. Alluvium brought to the shore by rivers. Beaches (2) On low, open shores an exposed beach typically has several distinct elements: A rather gently sloping foreshore (前濱) (lowest tide to the average high-tide level). A berm (灘台) (bench formed of sediment deposited by waves). The backshore (後濱) (from the berm to the farthest point reached by surf). Figure 18.17 海灘標準平衡剖面及其各部名稱 何春蓀：普通地質學， p. 382 Rocky (Cliffed) Coasts The usual elements of a cliffed coast due to erosion are: A wave-cut cliff, which may have a well-developed wave-cut notch (海蝕凹壁) at its base. A wave-cut bench, a platform cut across bedrock by surf. A beach, the result of deposition. Other erosional features associated with cliffed coasts are sea caves (海蝕洞), sea arches (海蝕拱), and stacks (海蝕柱). Figure 18.18 A small sea notch in the western coast of Myanmar A sea arch with sea cliffs in the coast of northern Chile Factors Affecting the Shore Profile Through erosion and the creation, transport, and deposition of sediment, the form of a coast changes, often slowly but sometimes very rapidly. During storms, the increased energy in the surf erodes the exposed part of a beach and makes it narrower. In calm weather, the exposed beach is likely to receive more sediment than it loses and therefore becomes wider. Major Coastal Deposits and Landforms Marine deltas (三角洲) are a typical constructional coastal landform. The extent of marine deltas is a compromise between the rate at which a river delivers sediment at its mouth and the ability of currents and waves to erode sediment along the delta front. Figure 18.21 Spits and Related Features A spit (沙嘴) is an elongated ridge of sand or gravel that projects from land and ends in open water. It is merely a continuation of a beach. It is built of sediment moved by longshore drift and dropped at the mouth of a bay. The free end curves landward in response to currents created by refraction as waves enter the bay. A spit-like ridge of sand or gravel that connects an island to the mainland or to another island, called a tombolo (連島沙洲). A ridge of sand or gravel may be built across the mouth of a bay to form a bay barrier (灣口沙洲). Figure 18.22 海岸地形及其侵蝕與沈積的現象 Beach Ridges and Barrier Islands Beach ridges (灘脊) are low sandy bars parallel to the coast. They are old berms. A barrier islands (障蔽島或離岸沙洲) is a long narrow sandy island lying offshore and parallel to a coast. An elongate bay lying inshore from a barrier island or strip of land such as coral reef is called a lagoon (潟湖). The cross-section of a barrier island Figure 18.24B Organic Reefs and Atolls A fringing reef (裙礁) is either attached to or closely borders the adjacent land (no lagoon). A barrier reef (堡礁) is separated from the land by a lagoon that may be of considerable length and width. An atoll (環礁), a roughly circular coral reef enclosing a shallow lagoon, is formed when a tropical volcanic island with a fringing reef slowly subsides. Figure 18.26 Fringing Reef 裙礁 Barrier Reef 堡礁 Atoll 環礁 海洋火山隨著海洋地殼年齡老化而沈陷 中途島 東沙環礁 How Coasts Evolve (1) The configuration of coasts depends largely on: The structure and erodibility of coastal rocks. The active geologic processes at work. The length of time over which these processes have operated. The history of world sea-level fluctuations. How Coasts Evolve (2) Types of coasts: Most of the Pacific coast of North America is steep and rocky. The Atlantic and Gulf coasts traverse a broad coastal plain that slopes gently seaward and are festooned with barrier islands. Where rocks of different erodibilities are exposed along a coast, marine erosion is strongly controlled by rock type and structure. Coastlines of Croatia, which are highly controlled by the trends of local structures Figure 18.27 Geographic Influences on Coastal Processes Coasts lying at latitudes between about 45° and 60° are subjected to higher-than-average storm waves generated by strong westerly winds. Subtropical east-facing coasts are subjected to infrequent but often disastrous hurricanes or typhoons. Sea ice is an effective agent of coastal erosion in the polar regions. Changing Sea Level Sea level fluctuates: Daily as a result of tidal forces. Over much longer time scales as a result of: Changes in the volume of water in the oceans as continental glaciers advance and retreat. The motions of lithospheric plates that cause the volume of the ocean basins to change. Sea level fluctuations, on geologic time scales, contribute importantly to the evolution of the world’s coasts. Submergence: Relative Rise of Sea Level Nearly all coasts have experienced submergence, a rise of sea level that accompanies the most recent deglaciation. Most large estuaries, for example, are former river valleys that were drowned by the recent sea-level rise. Figure 18.28 Emergence: Relative Fall of Sea Level Many marine beaches, spits, and barriers exist from Virginia to Florida. The highest reaches an altitude of more than 30 m. These features indicate that the past sea level was higher. Sea-Level Cycles and Relative Movements of Land and Sea Many coastal and offshore features date to times when relative sea level was either higher or lower than now. The major rises and falls of sea level are global movements. By contrast, uplift and subsidence of the land, which cause emergence or submergence along a coast, involve only parts of landmasses. Movements of land and sea level may occur simultaneously, in either the same or opposite directions. Sea level fluctuation history of the past 140000 yrs Figure 18.29 Coastal Hazards (1) Storms cause infrequent bursts of rapid erosion. A strong earthquake, landslide, or volcanic eruption can generate a potentially dangerous tsunami (海 嘯) . Tsunamis can travel at a rate as high as 950 km/h. They have long wavelength up to 200 km. They can pile up rapidly to heights of 30 m. Tsunami travel times Figure 18.31 Coastal Hazards (2) Cliffed shorelines are susceptible to frequent landslides as erosion eats away the base of a seacliff. Sometimes landslides on cliffed shorelines give rise to giant waves that are even more destructive than the slides themselves. Very large tsunamis have also been produced by massive coastal landslides (Lituya Bay, Alaska, in 1958). Ocean Circulation and the Carbon Cycle Photosynthesizing marine organisms exchange dissolved CO2 for dissolved O2 in surface waters. A wide variety of organisms draw bicarbonate anions out of seawater to form calcium carbonate shells. Calcium carbonate accumulates on the seafloor if it is shallower than about 4 kilometers. Cold O2-rich water sinks into the deep ocean from the surface waters. Sediments at the Beginning and the End of an Ocean’s Life Cycle (1) Unusual depositional conditions are common when an ocean basin initially opens, and in its last stages of closure. If evaporation dominates the regional climate, salinity increases in small semi-isolated ocean basins. Evaporite deposits can form if the connection to the world’s oceans is broken by tectonic activity or by a drop in sea level. Sediments at the Beginning and the End of an Ocean’s Life Cycle (2) Geologists have estimated that the Mediterranean would evaporate completely in only 1000 years if the Straits of Gibraltar were blocked. Thick salt deposits beneath the Mediterranean seafloor tell us that it dried out as many as 40 times between 5 and 7 million years ago.