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a. do
b. do not Most of the sediments delivered to the ocean by rivers are deposited in the coastal zone
where sedimentation rates are considerably greater than they are in the open ocean. Next we
consider some of the processes that take place within the coastal zone and on the continental
shelf that affect the distribution of sediments of terrestrial origin. Sediment Samples from our North Atlantic Voyage Recall the voyage we took fi'om the Mid-Atlantic Ridge toward Cape Hatteras, NC, in Inves-
tigation HA. Along the way. we measured the thickness of ocean floor sediments and took
samples. In the deep waters of this portion of the open ocean. marine sediment consists of
mostly red clays and calcareous ooze. Red clay is fine-grained lithogenous sediment that is
carried out to sea in suspension and by wind. Clays are present in most marine sediments,
though their proportion may be minor. Caleareous ooze is composed primarily of the calci-
um carbonate shells of foraminifera, coccolithophores, and pteropods derived from primary
production in surface ocean waters. As our oceanographic vessel pulls to within about 200 km (124 mi.) of Cape Hatteras. the
bottom rapidly shoals from the deep ocean up onto the continental margin. We continue tak-
ing bottom sediment samples and acoustic or seismic readings that give us the thickness of
the sediment deposits. in the deep ocean, crustal rock is mostly basalt that crystallized from
lava at the spreading center {Mid-Atlantic Ridge], but as we cross onto the continental mar-
gin. the crust changes to the more granitic composition of the continents. Our vessel travels
over the continental rise. the relatively steep continental slope that crests at the shelf break.
and finally into the ZOO-m (65641.) deep waters overlying the nearly flat continental shelf. 18. From our sediment samples we find that the bottom sediment has changed from red clays
and biogenous oozes to mud, sands. and gmvels derived from the land and delivered to
the ocean mostly by a. wind
b. glaciers
c. rivers [9. In general. as we approach the Cape Hatteras shoreline. we would find (as expected)
ocean bottom sediments becoming . Some of the sediments on the continental
margin date to the waxing and waning of glacial ice during the Pleistocene Ice Age (2.6
million to about H.700 years ago) and include ice-rafied rock fragments. a. finer
b. coarser Compared to sediments on the deep ocean floor, acoustic and seismic readings indicate much
thicker accumulations of lithogenous sediments in the continental margin, some locations
ranging in thickness from several kilometers to as much as 8 to 9 km. Rates of sedimentation
in waters of the continental margin are 20 to 100 times greater than in the deep ocean. In Investigation HA. we estimated the age of the Atlantic Ocean basin to be at least [50 mil-
lion years. lf the sediment deposition rate in the continental margin is of the order of l m per
1000 years, then the sediment thickness would be about 150 km, or almost 20 times the mea-
sured maximum sediment thickness of 8 to 9 km. Sediment does not accumulate to such a
great extent in the continental margin because many processes disperse lithogenous sediment
seaward over the continental shelf and beyond. These dispersal processes include ocean cur-
rents, waves. winds. tides. turbidity currents. and submarine avalanches. Consider some sediment dispersal processes operating in the continental margin. Figure 2 is
a NASA MODIS image from Aqua acquired in 2009. This view was the result of several fac-
tors. Tropical Storm Ida had passed to the east of the view producing sediment resuspension
from wind and wave action, seen in the wave-iike. lighter blue shadings. Also. brown sedi-
ment plumes can be seen entering the Gulf from the Mississippi River at the right to the
Houston-Galveston area near the upper center.

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