Chapter 18 The Ocean And Their Margins

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					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.

				
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