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Transcript
Geomorphology and Sea Level Figure 1 The shape of the ocean basins has much to do with the way the ocean moves, the way the wind blows, and the distributions of water temperature and salinity, among other physical phenomena. This chapter discusses the geomorphology of the ocean basins in the context of physical oceanography. This chapter also discusses the recent trends in sea level. The Ocean Basins The four ocean basins (Pacific, Atlantic, Arctic, Indian) and the major marginal seas comprise the global ocean. Features that are important to the physics of the ocean include: ! ! ! ! ! ! ! The placement of the continents is asymmetrical (larger area in northern hemisphere). The Southern Ocean is continuous zonally (along co-latitude lines). The large marginal seas, some of which are evaporative basins are found mostly in the Northern Hemisphere The South Pole is land and the North Pole is ocean River input to the Northern Hemisphere is higher than that to the Southern Hemisphere The Pacific, Atlantic and Indian are connected only at very high southern latitudes. The Arctic basin is only connected to the other basins by relatively shallow sills thus might be considered a marginal sea. 1 The global system of rises and rifts stands several thousand meters above the abyssal ocean floor. Such relief is sufficient to affect the flow of deep and bottom waters. This is most noticeable in the Atlantic basin where the Mid-Atlantic Ridge isolates the northward flow of Antarctic Bottom Water. Some of the gaps in the ridge system such as the Vema Channel and the Iceland-Faroe gap exemplify the effect of topography on water mass distribution. A secondary effect of the distribution of land and water is related to the advection of cold air masses from continents to the ocean. Clearly because the major continental land masses lie in the Northern Hemisphere the major air sea exchanges will occur in the northern Hemisphere. The exception is the cold air flowing off the Antarctic continent that modifies the Southern Ocean. The typical ocean basin has several characteristic that are shown schematically in Figure 2. Note the following: The broad abyssal plain interrupted by the mid ocean ridge system The presence of sea mounts Deep trenches adjacent to island arc systems The continental rise sloping up from the abyssal plain to the continental slope The shallow shelf break forming the boundary between the relatively steep slope and the gently sloping continental shelf. The slope of the shelf break is like approaching the Blue Ridge or Rockies while the shelf itself would resemble a coastal plain. The hypsometric curve (Fig. 3) shows the frequency distribution of depth in the ocean basins. The following should be noted about the depths of the ocean: Average depth of ocean is ~4000 m in contrast to mean land height of 1000 m. Most of the ocean is deep Shelves and slopes occupy a small percentage of the ocean Continental Margin Continental margins are composed of the Continental Rise, Continental Slope, Continental Shelf and Shelf Break (Fig. 4). Some regions have large shelf areas others have steep shelves Example of California continental margin (Fig. 5). Shelf Break As you will learn the shelf break depth is very important. This is because wind-driven process obviously originate at the surface and penetrate downward into the ocean. Over most of the ocean 2 the downward penetration never reaches the bottom. However as we approach the coast the surface processes on the ocean eventually reach the bottom. Thus the depth of the shelf break is important in defining how the waters over the continental shelf interact with the deeper waters of the adjacent ocean. Figure 6 shows a plot of the shelf break depth along the east coast of the U.S. Note how shallow the shelf break depth is off Florida compared to farther north. Off Virginia it is about 100 m and deepens to 300 m off Newfoundland. Figure 7 shows the width of various bathymetric features. Note that the widest feature is the abyssal plain. Continental shelf and rise are also typically wide. Seamounts, trenches and canyons are typically very narrow. These dimensions have important implications for the productivity of the region as they may determine the transport and exchange of nutrients between different regions of the ocean. Continental shelves are among the most productive regions of the ocean. On the continental shelves, as we approach the coast we usually see one of two types of coasts: steep or shallow (Fig. 8). The shallow coast slowly shoals to the beach. Often barrier islands lie offshore creating shallow lagoons. These lagoons, because they are shallow, respond quickly to atmospheric variability. The steep coast, as we see off the west coast features steep shore, narrow shelf, and often, rocky headland. Here the currents of the open ocean often reach to within several kilometers of the coast. Estuaries such as the Chesapeake Bay (Fig. 9) can get extremely complicated just in their bathymetry let alone the dynamics and biology. Here we see the complex system of channels, deeps, and shoals that make up the Chesapeake. Note how the deep channels are not connected. Submarine Canyons Submarine canyons (Fig. 10) are important because they cut through the slope and often the shelf providing a route for shelf and deeper slope waters to interact. A more obvious indicator of their importance is the way fisherman congregate around them because of the concentrations of fish. Canyons incise much of the U. S. coastline (Fig. 11) but some areas are noticeably lacking in them (Gulf of Mexico, Southeast U.S., part of Oregon). When studying shelf areas you must be aware of the proximity of canyons as they will affect the flow field significantly. Many studies seek to avoid them. The canyon system draining Baffin Bay (Fig. 12) is important in the flow of North Atlantic Deep Water (NADW) that is sinking in this area. A multitude of canyons concentrate in the Nova Scotia Shelf (Fig. 13). Also note the large channels (Northeast and Laurentian) connecting the deeper ocean to basins (Gulf of Maine and Gulf of St. Lawrence, respectively). Also note the basin system on this shelf partially isolated from the deep ocean by shelf edge banks. 3 The Hudson canyon (Fig. 14) reaches the coast at the entrance to the New York-New Jersey Harbor estuary. Represents an important source of nutrients to the shelf but also may become anoxic in the stratified summer. The Monterey and Indus canyons(Fig. 15) are other examples of canyons that extend across the shelf close to the coast or river mouth. The Congo canyon (Fig. 16) is the extreme. It extends into the river mouth. The Magdalena in Colombia is similar. The Mississippi River, because of human control, has extended its delta completely across the shelf and spills over the slope (Fig. 17). Thus Mississippi River waters do not enter the shelf area directly and may be returned onshore by the slope-to-shore flow. Mississippi River waters can interact directly with currents in the Gulf of Mexico. Deep Ocean Basin - Submarine Trenches In addition to the extensive abyssal plains, a relevant feature in the deep basins are the trenches and the seamounts (Fig. 18). Trenches (Fig. 19) are the deepest parts of the ocean and represent areas of crust subduction. The Mariana Trench ~11 km is the deepest point on Earth, Ocean currents in trenches are very weak but it would be interesting to see what kind of vertical flows develop around these features. Seamounts are very ubiquitous throughout the world’s oceans (Fig. 20). They tend to disturb the typical flow pattern in a given region (Fig. 21). Such disturbances may enhance the productivity of the area by retaining nutrients or early life stages of different organisms, thus providing a strong base for a food chain. Importance of bathymetric features The importance of bathymetric features in coastal waters is illustrated in three figures. The first (U.S. east coast, Fig. 22) shows surface color that is nearly equivalent to biological production. Note the flow of low chlorophyll water into the Gulf of Maine through Northeast Channel. Note the high values over shoals, and headlands. The second (Fig. 23) illustrates pigment concentration in the Mediterranean and the Arabian Sea. Note the highest productivity around coastal areas. The third is a sea surface temperature (SST) image for the U.S. west coast and the coast of Chile (Fig. 24). Note the activity off some of the major headlands. Sills, analogously to seamounts but over shallower water, also produce important disturbances to the flow. Sills are rises in the bottom that isolate one deep water area from another. The sills at the Strait of Gibraltar are classic examples. Figure 25 shows how dense waters formed in the Mediterranean are isolated and flow over the sill as the basin fills. What other large ocean basins have sills? Look at Figure 1. One last look at sills. Fjords are by definition formed by glaciers and often the entrance to the fjord is blocked by the terminal moraine of the glacier. Here is an example of such a sill and the effect. First figure (Fig. 26a) shows the inlet with bathymetry. The bottom rises from 80 to 10 meters over the sill. The water accelerates as it flows over the sill and causes disturbances at the pycnocline. This process enhances mixing between bottom and surface waters and likely provides food to upper trophic levels. 4 Sea Level The final part of this lecture deals with sea level variations. Obviously sea level changes will not have an immediate effect on the open ocean. However you must be aware that sea level has changed in the past and is now. This is because interpretation of historical data from coastal areas may require this knowledge. The fact that sea level is now rising may be of little significance to people who live on shorelines that have relief in regions close to the coast. But for those who live in relatively flat regions, like the western Pacific, there are reasons to be very concerned. Within several generations their territories may disappear. Approximately 15,000 years ago the sea level was over 100 m lower (Fig. 27). The coastal line during these low stands of sea level did not have a continental shelf (Fig. 28). But what is sea level doing now? Where is it going? Sea level is rising at a rate of about 2 mm per year or about 20 cm per century (Douglas, 1991). What is the significance of this? There is probably little significance to ocean dynamics at less than century time scales. The noticeable short term effect is on coastlines and thus indirectly on ocean circulation. For a shallow slope coastline having a slope of 1:500, such a rise in sea level means a horizontal transgression speed of 1 meter per year. Such transgression will have significant effects on the coast by causing morphological changes that will no doubt affect coastal ocean flow. The sea level changes produced by glaciation/deglaciation can be understood quantitatively through the principle of Isostacy. This principle can also be used to explain ocean currents. It assumes that the pressure at a certain depth underneath the earth’s crust is the same everywhere. Thus, if we add mass to a continent in the form of ice, the continent will sink. Quantitatively, this is explored with the relationship also know as the hydrostatic equation that relates pressure to the density of the material, the acceleration due to gravity and the thickness of the material. This can be illustrated with a couple of examples in class. Summary We have discussed the shape of the ocean and changes in sea level. While this will not be discussed any further you must keep these ideas in mind as you think about the deep or shallow ocean. Size and shape are important; they often are the most significant factors in ocean dynamics. Sea level has changed and is presently rising. The effects on the coastal ocean may be significant, especially on shallow slope coasts. 5