Copyright ( ) 2008, Elsevier Inc. rights reserved. All Chapter 2. Ocean dimensions, shapes and bottom materials 2.1 Dimensions The oceans are basins in the surface of the solid earth containing salt water. The purpose of this chapter is to introduce some of the nomenclature and to direct attention to features of the basins which have a close connection with the circulation and are of importance to the physical oceanographer. More detailed descriptions of the geology and geophysics of the ocean basins are given in Seibold and Berger (1982), The Sea Floor: an Introduction to Marine Geology, and in Kennett (1982), Marine Geology. The shape, depth and geographic location of an ocean affect the general characteristics of its ocean's circulation. A case in point is the dramatic difference between the older Pacific Ocean and the younger Atlantic Ocean. The edges of the Pacific are ringed with trenches, volcanoes and earthquakes that signal the gradual descent of the ocean bottom crustal plates under the surrounding continental plates. In contrast, the Atlantic is the site of dynamic sea-floor spreading as material added in the center of the mid-atlantic ridge pushes the plates apart, enlarging the Atlantic Ocean by a few centimeters each year. In addition the Atlantic has a very marked S shape while the Pacific has a much more oval shape. In order to appreciate the shapes of the oceans and seas it is useful to examine them on a globe, since map projections on flat paper introduce distortions when large portions of the earth are to be represented. From the oceanographic point of view it is convenient to distinguish the various regions in terms of their oceanographic characteristics, particularly their circulations. Fig. 2.1 Map of the world ocean from the National Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/global/global.html) The major ocean areas are defined as the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, the Arctic Ocean and the Southern Ocean (Fig. 2.1). The first four are clearly divided from each other by land masses but the divisions between the Southern Ocean and the others to its north are determined only by the characteristics of the 28
ocean waters and by their circulations, as will be described in Chapter 9. The geographical peculiarities of each ocean are described in Section 2.4. Marginal seas are fairly large basins of salt water that are connected to the open ocean by one or more fairly narrow channels. Those that are connected by very few channels are sometimes called Mediterranean seas, after the prototype, the (European) Mediterranean Sea. As discussed later, the Mediterranean provides an example of a negative water balance in a sea, with less inflow (river runoff and precipitation) and greater evaporation, while an excellent example of a positive water balance marginal sea, with net precipitation, is found in the Black Sea which connects with the Mediterranean. Both of these seas will be discussed further in Chapter 5. Other examples of marginal seas that are separated from the open ocean by multiple straits or island chains are the Caribbean Sea, the Sea of Japan, the Bering Sea, the North Sea, the Baltic Sea, etc. The term sea is also used for a portion of an ocean that is not divided off by land but has local distinguishing oceanographic characteristics. Examples are the Norwegian Sea, the Labrador Sea and the Tasman Sea. Looking at a globe again, it is evident that more of the earth s surface is covered by sea than by land, about 71% in contrast to the 29% for land surface. Furthermore, the proportion of water to land in the southern hemisphere is much greater (4:1) than in the northern hemisphere (1.5:1). In area, the Pacific Ocean is about as large as the Atlantic and Indian Oceans combined. If one includes the neighboring sectors of the Southern Ocean with the three main oceans north of it, then the Pacific Ocean occupies about 46% of the total world ocean area, the Atlantic Ocean about 23%, the Indian Ocean about 20%, and the rest combined about 11%. The average depth of the oceans is close to 4,000 m while the marginal seas are generally about 1,200 m deep or less. Relative to sea level, the oceans are much deeper than the land is high. While only 11% of the land surface of the earth is more than 2,000 m above sea level, 84% of the sea bottom is more than 2,000 m deep. However, the maxima are similar: the height of Mt. Everest is about 8,840 m while the maximum depth in the oceans is 11,034 m in the Mariana Trench in the western North Pacific. Figure 2.2 shows the distributions of land elevations and of sea depths relative to sea level in 1,000-m intervals as the percentage of the total area of the earth s surface. The land data are from Kossina (1921) and the ocean data from Menard and Smith (1966). Fig. 2.2 Areas of earth s surface above and below sea level as percentage of total area of earth (in 1,000 m intervals). Although the average depth of the oceans of 4 km, is a considerable distance, it is small compared with the horizontal dimensions of the oceans, which are of the order of 5,000 to 15,000 km. An idea of the relative dimensions of the Pacific Ocean may be obtained by stating that they are much the same as a sheet of paper. Relative to the major dimensions of the earth the oceans are a thin skin, but there is a great deal of detail 29
and structure in this layer between the sea surface and the bottom of the ocean. 2.2 Sea-floor dimensions 2.2.1 Scales Very often we wish to present some of the characteristics of the ocean by drawing a vertical cross-section of a part of the oceans. A drawing to true scale would have the relative dimensions of the edge of a sheet of paper and would be either too thin to show details or too long to be convenient. Therefore we usually distort our cross-section by making the vertical scale much larger than the horizontal one. For instance, we might use a scale of 1 cm on the paper to represent 100 km horizontally in the sea while depths might be on a scale of 1 cm to represent 100 m, i.e. 0.1 km. In this case the vertical dimensions on our drawing would be magnified by 1,000 times compared with the horizontal ones (a vertical exaggeration of 1,000:1). This gives us room to show the detail but exaggerates the slope of the sea bottom or of contours of constant water properties (isopleths) drawn on the crosssection, such as in Fig. 2.3. Such slopes are in reality far less than they appear on the cross-section drawings. For instance, a line of constant temperature (isotherm) with a real slope of 1 in 10,000 would appear as a slope of 1 in 10 on the plot. The continents form the major lateral boundaries to the oceans, and the detailed features of the shoreline and of the sea bottom are important in their effects on circulation. Starting from the land, the main divisions recognized are the shore, the continental shelf, the continental slope and rise, and the deep-sea bottom, part of which is the abyssal plain (shown schematically in Fig. 2.3). Fig. 2.3 Schematic section through ocean floor to show principal features. Bottom topography often plays an important role in the distribution of water masses and the location of currents. For instance, bottom water coming from the Weddell Sea (Antarctica) is unable to fill the eastern part of the Atlantic basin directly due to the height of the Walvis Ridge (South Atlantic Ocean). Instead the bottom water travels to the north along the western boundary of the South Atlantic, finds a deep passage in the mid-atlantic ridge and then flows south to fill the eastern side of the ridge. At shallower depths the sills (shallowest part of a channel) defining the marginal seas strongly influence both the mid-level currents and the distribution of water masses associated with the sea. Near the shore, of course, it is bottom topography that dictates the breaking of surface gravity waves, while bottom topography also directly influences the local tidal expressions. Tidal mixing is also strongly influenced by bottom relief. Coastal upwelling is a direct consequence of the shape of the coast and its related bottom topography. Alongshore currents are often determined by the coastal bottom topography while the instabilities in this system are also often a function of the horizontal scales of the bottom topography. 2.2.2 Shore The shore is defined here as a part of the landmass, close to the sea, which has been modified by the action of the sea. There is ample evidence to indicate that sea level in the past has varied over a range of about 120 30
m as land glaciers were smaller or larger than they are now. The beach is the seaward limit of the shore and extends roughly from the highest to the lowest tide levels. Sandy beaches are often in a state of dynamic equilibrium. In other words, they may be composed of sand all the time but it may not always be the same sand. This sand may be continually moving along the shore under the influence of waves and nearshore currents which are established to balance the shoreward moving of water with wave action and tidal currents. Evidence for this process can be seen in the way in which sand accumulates against new structures built on the shore, or by the way in which it is removed from a beach when a breakwater is built in such a way as to cut off the supply of sand beyond it. On some beaches, the sand may be removed by currents associated with high waves at one season of the year and replaced by different currents associated with lower waves at another season. These currents are influenced by seasonal and interannual wind variations. In addition to their effects on sand transport it is important to have knowledge of these near-shore currents particularly in areas where people are engaged in water sports. Locally intense and narrow currents can carry swimmers very rapidly out to sea. The evolution of coastal regions is influenced by changes in sea level that have occurred over geologic time. The sedimentary record shows a series of marine intrusions and retreats corresponding to layers that reflect periods when the surface was above and below sea level. The ability of the coast to resist the erosionary forces of the ocean depends directly on the type of material the coast is made of. Sands are easily redistributed by the ocean currents while granitic coasts are slow to erode. Often sea level changes are combined with the hydrologic forces of an estuary, which dramatically changes the dynamical relationship between the ocean and the solid surface. Coasts are relatively difficult to classify due to the many types of variability. One early method was to classify a coast as rising or falling in a net, long-term sense (Johnson, 1919). The causes of uplift or subsidence were not clearly understood at that time. More recently the emergence of our understanding of plate tectonics has led to new classifications of coasts, which can be considered to be either active or passive. Located at the front of a moving plate, active coasts are fundamentally different from the passive coasts located at the trailing edge of a plate. However, coastal changes due to extremely slow plate movements are mostly obscured by shorter-term variations due to changes in mean sea level (decadal to millennial time scales) and due to waves and tides (much shorter time scales). Coasts can also be referred to as erosional or depositional, depending on whether materials are being removed or added. Three basic factors contribute to longterm sea level change: (1) the total amount of water in the ocean, (2) changes in the containment volume of the world s ocean, and (3) changes in the temperature/salinity characteristics of the ocean that alter its density and hence cause the water to expand or contract. Changes in total amount of water are due primarily to changes in the volume of landfast ice, contained in ice sheets and glaciers. (Since sea ice floats in water, changes in sea ice volume, such as that in the Arctic or Antarctic, do not affect sea level.) Changes in containment volume are due to tectonics, and include the slow rebound of continents, continuing into the present, after the melt of landfast ice after the last deglaciation, and rebound due to continuing melt of glacial ice. Changes in heat content cause seawater to expand (heating) or contract (cooling). 31
Sea level rose 20 cm from 1870 to 2003, including 3 cm in just the last 10 years (1993-2003). Because good global observations are available for that last ten years, it is possible to ascribe 1.6 cm to thermal expansion, 0.4 cm to Greenland and Antarctic ice sheet melt, and 0.8 cm to other glacial melt, with a residual of 0.3 cm. Sea level is projected to rise 30 + 10 cm in the next 100 years, mainly due to warming of the oceans, which absorb most of the anthropogenic heat increase in the earth's climate system. (See the summary in the 4 th assessment report of the Intergovernmental Panel on Climate Change, Bindoff et al., 2007.) Erosional coasts are attacked by both waves and currents, which carry fine material that abrades the coast. Some coastal elements are dissolved into the sea water while most of the coastal material is carried out by local currents that are forced by tides and by the onshore buildup of water from wind-driven surface waves. This latter build-up is generally compensated by a strong narrow current, often referred to as a rip current, that carries water and sediment out to sea from the coast (Chapter 15.3). Marine erosion is greatest on highenergy coasts which are frequently exposed to large waves. These include the coasts of Maine, British Columbia and the southern tip of South Africa. Low energy coasts are generally not exposed to strong wave fields; areas such as the Gulf of Mexico are therefore stable until dramatic events such as a hurricane visit the region with strong and destructive waves. Tidal levels modify the elevation that the waves influence so a coast with little tidal variation erodes more quickly than one with a significant tidal range. Coasts protected by offshore island arcs erode more slowly than those directly exposed to the wind wave environment. Erosive forces carve characteristic features on coastlines such as sea cliffs and sea caves. A sea cliff is formed when the ocean undercuts the land surface and then the over-hanging portion falls into the sea. Sea caves are subsurface features, formed at points where the coastal composition material is weak. These can lead to the creation of a blowhole if the weakness extends to the surface. Initially the effect of marine erosion on newly-exposed coastlines is to create irregularities where weaker material is eroded more quickly than the harder components. Eventually, however, continued erosion smoothes the entire coast, with the loose, eroded material deposited into relatively calm bays located between the headlands that are exposed to the strong waves focused upon them by the local topography. The rip currents referred to above are fed by alongshore currents that move water from the incoming waves to feed the offshore rip currents. The directions of these alongshore currents are generally determined by the propagation direction of the incoming waves which generally are not precisely aligned to the coast. Frequently the load of sediment carried by the alongshore currents is joined by river sediment discharge, which can form a river delta which fans out the sediment from the river. The most familiar feature of a depositional coast is a beach, which is a zone of unconsolidated particles that covers part or all of a shore. Beaches result when sediment, usually sand, is transported to places suitable for continued deposition. Again these are often the quiet bays between headlands and other areas of low surf activity. Often a beach is in equilibrium, with new sand being deposited to replace sand that is being scoured away. The landward limit of the beach might be vegetation, permanent sand dunes or human construction. The seaward limit of a beach, where the sediment movement on and offshore ceases, is at a depth of about 10 m at low tide. 32
Beaches are made of material that can range from boulders, cobbles, pebbles, and gravel to very fine silt. Some beaches consist of shell debris or fragments of coral. In general, the flatter the beach the finer the material from which it is made. This relationship between sand particle size and beach slope depends on the wave energy incident on beach, the particle shape and the porosity of the material. 2.2.3 Continental shelf The continental shelf extends seaward from the shore with an average gradient of 1 in 500. Its outer limit (the shelf-break ) is set where the gradient increases to about 1 in 20 on average to form the continental slope down to the deep sea bottom. The shelf has an average width of 65 km. In places it is much narrower than this, while in others, as in the northeastern Bering Sea or the Arctic shelf off Siberia, it is as much as ten times this width. The bottom material is dominantly sand, with rock or mud being less common. The shelf break is usually clearly evident in a vertical cross-section of the sea bottom from the shore outward. The average depth at the shelf break is about 130 m. Most of the world s fisheries are located on the continental shelves for a multitude of reasons including proximity of estuaries, depth of penetration of sunlight compared with bottom depth and upwelling of nutrient-rich waters onto some shelves, particularly those off western coasts. 2.2.4 Continental slope and rise The continental slope averages about 4,000 m vertically from the shelf to the deep sea bottom, but in places extends as much as 9,000 m vertically in a relatively short horizontal distance. In general, the continental slope is considerably steeper than the slopes from lowland to highland on land. The material of the slope is predominantly mud, with some rock outcrops. Very typical features of the shelf and slope are the submarine canyons, which are of worldwide occurrence. They are valleys in the slope, either V-shaped or with vertical sides, and are usually found off coasts with rivers. Some, usually in hard granitic rock, were originally carved as rivers and then submerged - examples are around the Mediterranean and southern Baja California. Others, commonly in softer sedimentary rock, are formed by turbidity currents, described in the next paragraph. The lower part of the slope, where it grades into the deep-sea bottom, is referred to as the continental rise. Turbidity currents are common on continental slopes. These are not water currents like those we have been referring to in terms of the circulation. Instead these are episodic events that transport large volumes of sediment and bottom material down the slope into the deep ocean (Fig. 2.4). In these events material builds up on the slope until it is no longer stable and the force of gravity wins out with the material all crashing down the slope at once, at speeds up to 100 km/hr. These events have often snapped underwater cables that they cross. The precise conditions that dictate when a turbidity current occurs vary with the slope of the valley and the nature of the material in the valley. Turbidity currents carve many of the submarine canyons found on the slopes. Some giant rivers like the Congo carry such a dense load of suspended material that they form continuous density flows of turbid water down their canyons. 33
Fig. 2.4 Turbidity current schematic 2.2.5 Plate tectonics and deep-sea topography From the bottom of the continental slope, the bathymetric gradient decreases down the continental rise to the deep-sea bottom, the last and most extensive area. Depths of 3,000 to 6,000 m are found over 74% of the ocean basins with 1% being deeper. Perhaps the most characteristic aspect of the deep-sea bottom is the variety of its topography. Before any significant deep ocean soundings were available the sea bottom was regarded as uniformly smooth. When detailed sounding started in connection with cable laying, it became clear that this was not the case and there was a swing to regarding the sea bottom as predominantly rugged. Neither view is exclusively correct, for we know now that there are mountains, valleys and plains on the ocean bottom just as on land. Movement of the earth s tectonic plates shapes the sea floor. Sea floor spreading creates new sea floor as the earth's plates spread apart. This creates the midocean ridge system. The ocean plates spread apart at a rate of about 2 cm/year (Atlantic) to 16 cm/year (Pacific), causing extrusion of magma into the surface at the centers of the ridges. Over geologic time the orientation of the Earth s magnetic field has reversed causing the ferromagnetic components in the molten new surface material to reverse. Thus, spreading at the mid-ocean ridge was proven by observing the reversals in the magnetic orientations in the surface material (Figure 2.5). The recurrence interval for magnetic reversals is of the order of 500,000 to 1,000,000 years. Fig. 2.5 Schematic of ridge with series of magnetic reversals 34
Fig. 2.6 Schematic reconstruction of continents prior to and after sea-floor spreading The earlier geographic layout of the Earth can be reconstructed from the magnetic pattern left by sea-floor spreading. Here in Fig. 2.6 we see a suggested pattern for fitting the continents together in the way they might have been millions of year ago. Extrapolating this activity forward in time it is clear that the redistribution of land and ocean basins will continue to take place The 14,000 km long Mid-Ocean Ridge is a tectonic spreading center. The ridge is the most extensive feature of the earth s topography. Starting in the Arctic Ocean, it extends through Iceland, down the middle of the Atlantic, wrapping around the tip of Africa and then through the Indian and Pacific Oceans, ending in the Gulf of California. In the Atlantic the ridge separates the bottom waters, as can be seen from their very different properties east and west of the ridge. However, there are narrow gaps in this ridge, called fracture zones, created by lateral jogs in the spreading center. The fracture zones are roughly vertical planes, perpendicular to the ridge, on either side of which the crust has moved in opposite directions perpendicular to the ridge. This leaves a gap in the ridge through which water below the ridge top may leak from one side of the ridge to the other. One example is the Romanche Fracture Zone through the Mid- Atlantic Ridge close to the equator. Individual mountains (seamounts) are widely distributed in the oceans. Some project above the surface to form islands, while the tops of others are below the surface. Virtually all are volcanic in origin. Some of these seamounts have reached the surface where they have been worn flat before 29
sinking again below the surface to form a feature know as a guyot. In some of the large basins the sea floor is very smooth, possibly more so than the plains areas on land. Stretches of the abyssal plain in the western North Atlantic have been measured to be smooth within 2 m over distances of 100 km. The deepest parts of the oceans are in the trenches (Fig. 2.7). Trenches are the active locations where the oceanic plates are sinking (subducting) beneath other plates causing earthquakes and volcanoes. In many cases oceanic plates sink beneath the lighter granitic continental plates. Along the landward side of trenches are volcanoes caused by the introduction of water into the hot interior of the earth. The majority of the deep trenches are in the Pacific, including the Aleutian, Kurile, Tonga, Philippine and Mariana Trenches, with a few in other oceans such as the Puerto Rico and the South Sandwich Trenches in the Atlantic and the Sunda Trench in the Indian Ocean. In dimensions these trenches are narrow relative to their length and have depths to 11,000 m. They are often shaped like an arc of a circle in plan form with an island arc on one side. Examples of island arcs are the Aleutian Islands (Pacific), the Lesser Antilles (Atlantic) and the Sunda Arc (Indian). The landward side of a trench extends as much as 10,000 m from the trench bottom to the sea surface while the other side is only half as high, terminating at the ocean depth of about 5,000 m. Fig. 2.7 Perspective map of a trench Many seamounts and islands were created by volcanic hot spots that are relatively stationary in contrast to the tectonic plates. As the plates move across them, chains of seamounts are formed. Examples include the Hawaiian Islands/Emperor Seamounts chain, Polynesian island chains, the Walvis Ridge, and the Ninetyeast Ridge in the Indian Ocean. 2.2.6 Methods for mapping bottom topography Our present knowledge of the shape of the ocean floor results from an accumulation of sounding measurements, most of which have been made within the last 60 years, and very recently using the gravity field measured by satellites. The early measurements were made by lowering a weight on a measured line until the weight touched bottom, as discussed in chapter 1. This method was slow; in deep water it was uncertain because it was difficult to tell when the weight touched the bottom, and to be certain that the line was vertical. Since 1920 most depth measurements have been made with echo sounders, which measure the time taken for a pulse of sound to travel from the ship to the bottom and be reflected back to the ship. One half of this time is multiplied by the average speed of sound in the seawater under the ship to give the depth. With present-day 29
equipment, the time can be measured very accurately and the main uncertainty over a flat bottom is in the value used for the speed of sound. This varies with water temperature and salinity (see Section 3.7) and if these are not measured at the time of sounding an average value must be used. This introduces a possibility of error. Over trenches or places where rapid changes with depth occur there may also be some uncertainty about whether the echo comes from directly under the ship (to give the true depth) or from one side (to give too small a value). Great detail has been added to our knowledge of the sea floor topography by satellite measurements. These satellites measure the earth's gravity field, which depends on the local mass of material. These measurements allow mapping of many hitherto unknown features such as fracture zones and seamounts in regions remote from intensive echo sounder measurements (Fig. 2.1), and provide much more information about these features even where they had been mapped. Echo sounder measurements are still needed to verify the gravity-based measurements since the material on the ocean bottom is not uniform. For instance a bottom shaped by extensive sediment cover might not be detected from the gravity field. 2.2.7 Sills One other term used frequently in reference to bottom topography is sill. This refers to a ridge, above the average bottom level in a region, which separates one basin from another or, in the case of a fjord (Chapter?), separates the landward basin from the sea outside. The sill depth is the depth from the sea surface to the deepest part of the ridge, i.e. the maximum depth at which direct flow across the sill is possible. 2.3 Bottom Material On the continental shelf and slope, most of the bottom material comes directly from the land, either brought down by rivers or blown by the wind. The material of the deep-sea bottom is often more fine-grained than that on the shelf or slope. Much of it is pelagic in character, i.e. it has been formed in the open ocean itself. The two major deep ocean sediments are red clay and the biogenic oozes. The former has less than 30% of biogenic material and is mainly mineral in content. It consists of fine material from the land (which may have traveled great distances through the air before finally settling into the ocean), volcanic material, and meteoric remains. The oozes are over 30% biogenic and originate from the remains of living organisms (plankton). The calcareous oozes have a high percentage of calcium carbonate from the shells of animal plankton, while the siliceous oozes have a high proportion of silica from the shells of silicasecreting planktonic plants and animals. The siliceous oozes are found mainly in the Southern Ocean and in the equatorial Pacific. The relative distribution of calcareous and siliceous oozes is clearly related to the nutrient content of the surface waters, with calcareous oozes common in low nutrient regions and siliceous oozes in high nutrient regions. Except when turbidity currents deposit their loads on the ocean bed, the average rate of deposition of the sediments is from 0.1 to 10 mm per 1,000 years, and much information on the past history of the oceans is stored up in them. Samples of bottom material are obtained with a corer which is a steel pipe 2 to 30 m long, which is lowered vertically and forced to penetrate into the sediments by the heavy weight at its upper end. The core of sediment retained in the pipe may represent material deposited over a period from 1,000 to 10 million years per meter of length. Sometimes the material is layered, indicating stages of sedimentation of 30
different materials. In some places, layers of volcanic ash can be related to historical records of eruptions; in others, organisms characteristic of cold or of warm waters are found in different layers and suggest changes in temperature of the overlying water during the period represented by the core. In some places gradations from coarse to fine sediments in the upward direction suggest the occurrence of turbidity currents bringing material to the region with the coarser material settling out first and the finer later. Large sediment depositions from rivers create a sloping, smooth ocean bottom for thousands of miles from the mouths of the rivers. This is called a deep-sea sediment fan. The largest, the Bengal Fan, is in the northeastern Indian Ocean and is created by the outflow from many rivers including the Ganges and Brahmaputra. Other examples of fans are at the mouths of the Yangtze, Amazon and Columbia Rivers. The physical oceanographer looks at the sediments for the information that they give on the movement of the water at the ocean floor. Photographs of the deep-sea bottom have been obtained in recent years and some of them show ripples such as one sees on a sand beach after the tide has gone out. Such ripples are only found on the beach where the water speed is high, such as in the backwash from waves. We conclude from the ripples on the deep-ocean bottom that currents of similar speed occur there. This discovery helped to dispel the earlier notion that all deep-sea currents are very slow. Sediments can affect the properties of sea water in contact with them. For instance, silicate and carbonate are dissolved from sediments into the overlying seawater. Organic carbon mainly from fecal pellets is biologically decomposed (remineralized) into inorganic carbon dioxide in the sediments, with oxygen consumed in the process. The carbon dioxide-rich, oxygen-poor pore waters in the sediments are released back into the sea water, affecting its composition.. Organic nitrogen and phosphorus are also remineralized in the sediments, providing an important source of inorganic nutrients for sea water. In regions where all oxygen is consumed, methane forms from bacterial action. This methane is often stored in solid form, called a methane hydrate. Vast quantities (about 10 19 grams) of methane hydrate have accumulated in marine sediments over the earth's history. They can spontaneously turn from solid to gaseous form, causing submarine landslides and releasing methane into the water, affecting its chemistry. 2.4 Ocean Basins The Pacific Ocean (Fig. 2.8) is the world s largest ocean basin. To the north there is a physical boundary broken only by the Bering Strait, which is quite shallow (about 50 m) and 82 km wide. There is a small net northward flow from the Pacific to the Arctic through this strait. At the equator, the Pacific is very wide so that tropical phenomena that propagate east west take much longer to cross the Pacific than across the other oceans. The Pacific is rimmed in the west and north with trenches and ridges. The associated volcanoes have led to this being called the ring of fire". The East- Pacific Rise, a major topographic feature of the South-Pacific, is a spreading ridge that separates the deep waters of the southeast from the rest of the Pacific. Fracture zones allow some communication. Where the ridge encounters the major eastward current of the Southern Ocean, the Antarctic Circumpolar Current (Chapter 8), the current is deflected and becomes unstable. 31
Fig. 2.8 Map of the Pacific Ocean with topography from NGDC The Pacific has more islands than any other ocean. Most of them are located in the western tropical regions. The Hawaiian Islands and their extension northwestward into the Emperor Seamounts were created by motion of the Pacific oceanic plate across the hot spot that is now located just east of the big island of Hawaii. The Pacific Ocean has numerous marginal seas, mostly along its western side. Some for the North Pacific are the Bering, Okhotsk, Japan, Yellow, East China and South China Seas in the west and the Gulf of California in the east. In the South Pacific are found the Solomon, Coral and Tasman Seas. In the southern South Pacific is found the Ross Sea, which contributes to the bottom waters of the world ocean. The Atlantic Ocean has an S shape (Fig 2.9). The mid-atlantic ridge, a spreading ridge down the center of the ocean, dominates its topography. Deep trenches are found just east of the Lesser Antilles in the eastern Caribbean and east of the South Sandwich Islands. The Atlantic is open both at the north and the south connecting to the Arctic and Southern Oceans. The northern North Atlantic is the formation location of much of the world's deep water. One of the Atlantic's marginal seas, the Mediterranean, is evaporative and contributes high salinity, warm water to the mid-depth ocean. At the 29
southern boundary, the Weddell Sea is the formation location for most of the bottom water found in the oceans. Other marginal seas connecting to the Atlantic are the Norwegian, Greenland and Iceland Seas (sometimes known collectively as the Nordic Seas), the North Sea, the Baltic Sea, the Black Sea, and the Caribbean The Irminger Sea is the region east of southern Greenland, the Labrador Sea is the region between Labrador and Greenland and the Sargasso Sea is the open ocean region surrounding Bermuda. Fresh outflow from the Amazon and Congo Rivers form marked low-salinity tongues at the sea surface. eastern boundary of the Indian Ocean is porous and connected to the Pacific Ocean through the Indonesian archipelago. Marginal seas for the Indian Ocean are the Andaman Sea, the Red Sea and the Persian Gulf. The open ocean region west of India is called the Arabian Sea and the region east of India is called the Bay of Bengal. Fig. 2.10 Map of the Indian Ocean with topography from Smith and Sandwell (1997) Fig. 2.9 Map of the Atlantic Ocean (xx map is inaccurate, replace xx) The Indian Ocean (Fig. 2.10) is closed off by land just north of the tropics. The topography of the Indian Ocean is very rough because of the ridges left behind as the Indian plate moved northward into the Asian continent, creating the Himalayas. The only trench is the Sunda trench where the Indian plate subducts beneath Indonesia. The The differential heating of land and ocean in the tropics results in the creation of the monsoon weather system. Monsoons occur in many places, but the most dramatic and best described monsoon is in the northern Indian Ocean. The Northeast Monsoon from October to May sends cool, dry winds from the continental land masses in the northeast over the Indian subcontinent to the ocean. Starting in June and lasting until September, the system shifts to the southwest monsoon, which brings warm, wet rains from the western tropical ocean to the Indian subcontinent. While these monsoon conditions are best known in India they also dominate the climate in the western tropical and South Pacific. Another interesting factor in the geography of this region is that fact that most 29
of the Indian rivers outflow on the eastern side of India. This causes the surface of the Bay of Bengal east of India to be quite fresh. The enormous amount of silt carried from the eroding Himalayan Mountains into the Bay of Bengal creates the Bengal Fan that slopes smoothly downward for thousands of kilometers. West of India the Arabian Sea, Red Sea and Persian Gulf are very salty due to the dry climate and subsequent high evaporation. Much like the Mediterranean, the saline Red Sea water is sufficiently dense to sink to mid-depth in the Indian Ocean and affects water properties over a large part of the Arabian Sea. The Arctic Ocean (Fig. 2.11) is sometimes not regarded as an ocean, but rather as a mediterranean sea connected to the Atlantic Ocean. It is characterized by very broad continental shelves surrounding a deeper region, which is split down the center by the Lomonosov Ridge. Areas around the Arctic are named seas: the Beaufort, Chukchi, East Sibersian, Laptev, Kara and Barents. The Arctic is connected to the North Pacific through the shallow Bering Strait. It is connected to the Nordic Seas (Norwegian, Greenland and Iceland Seas) through passages on either side of Svalbard. The Nordic Seas are separated from the Atlantic Ocean by the submarine ridge between Greenland, Iceland and the United Kingdom, with a maximum sill depth of about 480 m between Greenland and Iceland (Denmark Strait). Dense water formed in the Nordic Seas spills into the Atlantic over this ridge. The central area of the Arctic is perennially covered with sea ice. 30
Fig. 2.11 The Arctic Ocean (xxmap has obvious mistakes, replacexxx) The Southern Ocean (Fig. 2.12) is not geographically distinct from the Atlantic, Indian and Pacific Oceans, but is often considered separately since it is the only region outside the Arctic where there is a path for eastward flow all the way around the globe. This occurs at the latitude of Drake Passage between South America and Antarctica. This allows the three major oceans to be connected. The South Sandwich Islands and trench east of Drake Passage partially block the open circumpolar flow. The ocean around Antarctica includes permanent ice shelves as well as seasonal sea ice. Unlike the Arctic there is no perennial long-term pack ice; except for some limited ice shelves and all of the first year ice melts and forms each year. The densest bottom waters of the world ocean are formed in the Southern Ocean, primarily in the Weddell and Ross Seas and also in other areas distributed along Queen Adelie Land. 29
Fig. 2.12 The Southern Ocean around Antarctica 30