Sedimentia
Image of the Ganges River delta and the Bay
of Bengal acquired by the Moderate Resolution Imaging Spectroradiometer
(MODIS). This image shows the massive amount of sediments delivered
to the Bay of Bengal by the Ganges River, sediments that are derived
from erosion of the Himalayan mountain range to the north. Click
on this image to see a large high-resolution version that includes
the Himalayan range. Mt. Everest, the highest point in the world, is
located in the upper right corner of the high-resolution image.
No, it doesn't mean we're crazy about sediments. On the contrary,
most of the time sediments make life more difficult for the analysis
of remotely-sensed ocean color data. Some aspects of this subject
have been covered in Turbidity - Through A
Water Column, Darkly, a previous Science Focus! article.
That article primarily dealt with the reasons that sediments can be
a problem for accurate analysis of chlorophyll concentrations,
particularly in coastal regions.
Yet sediments, despite all the disparaging comments that might
be directed at them by remote-sensing scientists, are an important
aspect of the marine realm, and they can be geophysically
significant for several different reasons. One reason is that
sediment particles can be composed of materials that are highly
reactive in the marine environment. Some rivers deliver sediments
that are rich in organic matter into estuaries and the open
ocean, providing a food source for bacteria and zooplankton.
Other sediments (more on this below) are reactive in seawater and
are important players in the marine carbon and carbonate cycles.
Sediments also carry important materials on them: both clay
particles and organic particulates can adsorb dissolved metal
ions on their surfaces. This process of adsorption may quickly
transport metals that could be harmful to the sea floor, where
they may be buried, or digested by opportunistic bacteria --
but that's not always a good thing.
Furthermore, sediments are the source of one of mankind's favorite
recreational sites -- the beach! Sediments move around, of
course, and while they can move and pile up to make beaches, they
can also move and directly affect the sea floor environment.
This Science Focus! article will feature brief stops around the
world where sediments have been observed by SeaWiFS and MODIS (and
MERIS), and will finish with a phenomenon that may be an important facet
of the marine carbon cycle.
The first stop is the northern Adriatic Sea in Europe. In the northern
Adriatic, the Po River discharges a large amount of sediments derived from
erosion in the Alps, and this sediment underlies the historic city of Venice.
It also forms the famed Lido Beach. The Po River "plume" was actually observed
by the Coastal Zone Color Scanner and astronauts on the Space Shuttle on the
same day, as described and illustrated in Classic CZCS Scenes Chapter
10: River Plumes and Estuaries. SeaWiFS has occasionally viewed the Po
River plume, as seen in the image below. However, pervasive haze and pollution
in the Po River valley frequently obscures observations of the northern
Adriatic.
SeaWiFS image of Italy
and the Adriatic Sea. The Balkans to the west and the snow-covered Alps to the
north are also visible. The Po River valley is the hazy brown area just south
of the Alps. The plume of sediments carried by the Po River is seen on the
western side of the far northern Adriatic Sea.
Next, we look at the mightiest river on Earth, the Amazon, with one of the
newest instruments in orbit; MERIS, the Medium
Resolution Imaging Spectrometer carried on ENVISAT. MERIS captured this view of the
mouth of the Amazon River on March 25, 2002. The large mass of sediments
delivered by the Amazon to the Atlantic Ocean is carried by currents up the
coast into the Caribbean Sea.
MERIS image of the mouth
of the Amazon River and sediments on the coast of Brazil. Click on this image
to see a larger version. Image credit: European Space Agency (ESA).
The Ganges River in India, along with the Yellow and Yangtze River in
China and the Mekong River in Vietnam and Cambodia, carries some of the
heaviest mass of sediments of any river in the world. The sediments
of the Ganges are derived from erosion of the Himalaya mountain range.
The MODIS image at the top of this article shows the delta region of the
Ganges and the massive amount of sediments entering the Bay of Bengal.
Rivers, then, are obviously one of the primary sources of sediments
to the marine environment. However, many rivers do not carry a large
amount of sediments to the ocean except under special conditions;
specifically, floods. On September 23, 1999, SeaWiFS acquired a
remarkable view of the North Carolina coast one week after the passage
of Hurricane Floyd. The rain-swollen rivers of North Carolina delivered
an immense amount of sediment into the Atlantic Ocean, which was
then carried into the open ocean by the Gulf Stream.
SeaWiFS image of the U.S. East Coast acquired
one week after the passage of Hurricane Floyd (see image below).
The sediments generated by the flood waters of rivers in North Carolina
are seen entering the Gulf Stream off of Cape Hatteras. Also note
the increased turbidity in the sounds and river estuaries and
persistent sediment suspension southward along the coast.
Hurricane Floyd was interesting for another reason, which is the concluding
topic of this article. The sediments carried by rivers can be classified as
riverine, predominantly inorganic sediments, because they are usually composed
of mineral grains that have been eroded from the land surface. Another form of
sediments which is found primarily in the ocean are biogenic sediments,
which means that they are composed of minerals formed by organisms. The main
two mineral forms of this type that are found in the ocean are calcium
carbonate (CaCO3) and silica (SiO2). Calcium carbonate
is formed by numerous organisms: coccolithophorids, foraminifera, pteropods,
corals, and coralline algae, to name a few. Silica is predominantly formed by
diatoms.
CaCO3 is important to the marine carbon cycle because this
mineral can dissolve in seawater, if the seawater chemistry is
undersaturated with respect to the mineral. Seawater contains ions of
calcium (Ca2+), which doesn't vary much in concentration in
seawater, and both bicarbonate (HCO3-) and carbonate
(CO32-), which do vary considerably, particularly with
depth. Seawater is usually oversaturated with respect to CaCO3 at
the surface of the ocean, and undersaturated deep in the ocean. The depth at
which CaCO3 will start to dissolve, the saturation horizon ,
varies. It can be nearly 3000 meters deep in the Atlantic Ocean, and as little
as 150 meters deep in the northern Pacific Ocean.
Another factor is that CaCO3 can exist in a variety of mineral
forms. Calcite, formed by coccolithophorids and foraminifera, is the least
soluble in seawater. Aragonite, formed by pteropods, is more soluble than
calcite. And corals and coralline algae form calcite that contains a
considerable amount of magnesium (Mg), which makes this "high-Mg" calcite the
most soluble form of biogenic calcium carbonate.
Coral reefs, and the shallow carbonate banks that are commonly
located near them, are the most productive areas for biogenic CaCO3 in the ocean. (Well-known examples of carbonate banks are the Bahamas
islands and the island of Bermuda.) A small area of coral reef and
carbonate bank can produce the same amount of CaCO3 as hundreds of
square kilometers on the surface of the open ocean.
But there's a problem, which has been a research question for
marine geologists and geochemists for many years and which is still
very uncertain. Both the mechanisms by which this biogenic CaCO3
is transferred from the shallow banks and reefs to the deep sea,
and the amount of this material that is transported, are uncertain.
Remote sensing provides one way to clarify these questions. Because
the sediments produced on carbonate banks are highly reflective,
they are relatively easy to detect from space. Once these sediments
reach the open ocean, however, they will begin sinking. So to be
visible from space, a sufficient amount of sediments must be transported
a sufficient distance from the islands.
There are several mechanisms that can move these sediments, and the
most notable is hurricane-force winds. Before Hurricane Floyd moved
inland over North Carolina and the states in the U.S. Northeast, it
traveled directly over the Bahamas islands. On September 16, 1999,
SeaWiFS captured this remarkable view of the U.S. East Coast, with
Hurricane Floyd's center near Washington, D.C. The path of Floyd
carried it over the U.S. continental shelf, which is clearly visible
due to the suspension of sediments on the sea floor.
SeaWiFS image of Hurricane Floyd, the U.S.
East Coast, and the Bahamas Banks, acquired on September 16, 1999.
Sediment suspension along the U.S. continental shelf is prominent in
this image.
Close examination of the Bahamas islands in this image revealed the presence
of suspended carbonate sediments moving off the banks. This image shows
several different plumes of sediment. The largest amount of sediments appears
to be transported northeastward off the northern margin of Little Bahama Bank
(north of Great Abaco Island). Other smaller plumes are visible on the
northwestern corner of Little Bahama Bank and on the northeastern and
northwestern sides of Great Bahama Bank to the south. Three of these plumes
are labeled I, II, and III.
Magnified September 16 SeaWiFS image of the Bahamas
Banks, showing several areas where carbonate sediments suspended by
the winds of Hurricane Floyd are being transported off the banks.
However, it took the next hurricane of the 1999 season, Hurricane Gert, to
provide the clearest image of CaCO3 sediments transported off a
carbonate bank: in this case, the island of Bermuda. On September 25, just
hours after Hurricane Gert had passed near the island, SeaWiFS captured this
image of the island. The true-color image is on the left, and the image
portraying normalized water-leaving radiance at 555 nanometers data, nLw(555),
is on the right. The 555 nm band is particularly good at detecting the
reflection of light from suspended sediments.
 
SeaWiFS images of Bermuda acquired on September
25, one day after the passage of Hurricane Gert. The plume of sediments
generated by Gert is visible in both images. (Left) True-color image.
(Right) Normalized water-leaving radiance at 555 nm image. Data processing
has placed a mask over the bright land of the island of Bermuda and the
shallow waters of the carbonate bank surrounding the island.
These two events provided a fairly clear indication of what suspended
carbonate sediments transported from carbonate banks look like from space.
The next step is to attempt to quantify the mass of sediments that is
transported in such events. These sediments are particularly amenable
to quantification because they are reflective, they are mainly composed of
one mineral type, and because they are not associated with large amounts
of organic matter. They therefore provide good "test cases" for
algorithms that use remote-sensing data to quantify sediment concentrations.
If these algorithms prove to be reliable, then the principles used
in the algorithms may be applied to quantifying sediment concentrations
for more complex conditions.
Links
References
James G. Acker, Christopher W. Brown, Albert C. Hine, Edward Armstrong,
and Norman Kuring, 2002: Satellite remote sensing observations and
aerial photography of storm-induced neritic carbonate transport from
shallow carbonate banks. International Journal of Remote Sensing,
23(14), 2853-2868.
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