Marine Geology Phần 6 ppt

travel time over an extended period of 5 to 10 years could definitely indicate

that the oceans are indeed warming.

ABYSSAL STORMS

The dark abyss at the bottom of the ocean was thought to be quiet and almost

totally at rest, with sediments slowly raining down and accumulating at a rate

of about 1 inch in 20 centuries. Recent discoveries reveal signs that infrequent

undersea storms often shift and rearrange the sedimentary material that has

rested for long periods on the bottom. Occasionally, the surging bottom cur￾rents scoop up the top layer of mud, erasing animal tracks and creating ripple

marks in the sediments, much like those produced by wind and river currents.

On the western side of the ocean basins, undersea storms skirt the foot

of the continental rise, transporting huge loads of sediment and dramatically

modifying the seafloor.The storms scour the ocean bottom in some areas and

deposit large volumes of silt and clay in others.The energetic currents travel

at about 1 mile per hour. However, because of the considerably higher den￾sity of seawater, they sweep the ocean floor just as effectively as a gale with

winds up to 45 miles per hour erodes shallow areas near shore.

The abyssal storms seem to follow certain well-traveled paths, indicated

by long furrows of sediment on the ocean floor (Fig. 117).The scouring of

the seabed and deposition of thick layers of fine sediment results in much

more complex marine geology than that developed simply from a constant

rain of sediments.The periodic transport of sediment creates layered sequences

that look similar to those created by strong windstorms in shallow seas, with

overlapping beds of sediment graded into different grain sizes.

Sedimentary material deposited onto the ocean floor consists of detri￾tus, which is terrestrial sediment and decaying vegetation, along with shells

and skeletons of dead microscopic organisms that flourish in the sunlit waters

of the top 300 feet of the ocean. The ocean depth influences the rate of

marine-life sedimentation. The farther the shells descend, the greater the

chance of dissolving in the cold, high-pressure waters of the abyss before

reaching the bottom. Preservation also depends on rapid burial and protection

from the corrosive action of the deep-sea water.

Rivers carry detritus to the edge of the continent and out onto the con￾tinental shelf where marine currents pick up the material.When the detritus

reaches the edge of the shelf, it falls to the base of the continental rise under

the pull of gravity.Approximately 25 billion tons of continental material reach

the mouths of rivers and streams annually. Most of this detritus is deposited

near the river outlets and onto continental shelves. Only a few billion tons fall

into the deep sea. In addition to the river-borne sediment, strong desert winds

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in subtropical regions sweep out to sea a significant amount of terrestrial

material.The windblown sediment also contains significant amounts of iron,

an important nutrient that supports prolific blooms of plankton. In iron￾deficient parts of the ocean,“deserts” exist where “jungles” should have been

even though plenty of other nutrients are available.

The biologic material in the sea contributes about 3 billion tons of sed￾iment to the ocean floor each year.The biologic productivity, controlled in

large part by the ocean currents, governs the rates of accumulation. Nutri￾ent-rich water upwells from the ocean depths to the sunlit zone, where

microorganisms ingest the nutrients.Areas of high productivity and high rates

of accumulation normally occur near major oceanic fronts, such as the region

around Antarctica. Other areas are along the edges of major currents, such as

the Gulf Stream that circulates clockwise around the North Atlantic basin

and the Kuroshio or Japan current that circles clockwise around the North

Pacific basin.

The greatest volume of silt and mud and the strongest bottom currents

are in the high latitudes of the western side of the North and South Atlantic.

These areas have the highest potential for generating abyssal storms that form

and shape the seafloor.They also have the largest drifts of sediment on Earth,

covering an area more than 600 miles long, 100 miles wide, and more than 1

mile thick.Abyssal currents at depths of 2 to 3 miles play a major role in shap￾ing the entire continental rise off North and South America. Elsewhere in the

Figure 117 A wide, flat

furrow on the seabed of

the Atlantic Ocean.

(Photo by N. P. Edgar,

courtesy USGS)

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Abyssal Currents

world, bottom currents shape the distribution of fine-grained material along

the edges of Africa,Antarctica,Australia, New Zealand, and India.

Instruments lowered to the ocean floor measure water dynamics and

their effects on sediment mobilization (Fig. 118). During abyssal storms, the

velocity of bottom currents increases from about 1

/10 to more than 1 mile per

hour. The storms in the Atlantic seem to derive their energy from surface

eddies that emerge from the Gulf Stream.While the storm is in progress, the

suspended sediment load increases tenfold, and the current is able to carry

about 1 ton of sediment per minute for long distances.The moving clouds of

suspended sediment appear as coherent patches of turbid water with a resi￾dence time of about 20 minutes.The storm itself might last from several days

to a few weeks, at the end of which the current velocity slows to normal and

the sediment drops out of suspension.

Not all drifts are directly attributable to abyssal storms. Material carried

by deep currents has modified vast areas of the ocean as well.The storm’s main

effect is to stir sediment that bottom currents pick up and carry downstream

for long distances.The circulation of the deep ocean does not show a strong

seasonal pattern. Therefore, the onset of abyssal storms is unpredictable and

likely to strike an area every 2 to 3 months.

TIDAL CURRENTS

Tides result from the pull of gravity of the Moon and Sun on the ocean.The

Moon revolves around Earth in an elliptical orbit and exerts a stronger pull

when on the near side of its orbit around the planet than on the far side.The

difference between the gravitational attraction on both sides is about 13 percent,

which elongates the center of gravity of the Earth-Moon system.The pull of

gravity creates two tidal bulges on Earth.As the planet revolves, the oceans flow

into the two tidal bulges,one facing toward the Moon and the other facing away

from it.Between the tidal bulges,the ocean is shallower,giving it an overall egg￾shaped appearance.The middle of the ocean rises only about 2.5 feet at maxi￾mum high tide. However, due to a sloshing-over effect and the configuration of

the coastline, the tides on the coasts often rise several times higher.

The daily rotation of Earth causes each point on the surface to go into

and out of the two tidal bulges once a day.Thus, as Earth spins into and out

of each tidal bulge, the tides appear to rise and fall twice daily.The Moon also

orbits Earth in the same direction it rotates, only faster. By the time a point

on the surface has rotated halfway around, the tidal bulges have moved for￾ward with the Moon, and the point must travel farther each day to catch up

with the bulge.Therefore, the actual period between high tides is 12 hours,

25 minutes.

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Marine Geology