An Enlightened View of Calcite in the Ocean with MODIS
College students in an introductory class on mineralogy are very likely to
encounter a specimen of a clear, transparent crystal that goes by the name of
"Iceland spar" calcite. One of the most obvious properties of this mineral
specimen is the visual property of double refraction: viewing something
through the crystal provides two images, as shown below.
Crystal of Iceland spar calcite showing double
refraction of light. (Image courtesy of Jo Edkins.)
The visual clarity of calcite might make it a great material for lenses,
(if that minor double image problem could be overcome). Trilobites, the
ubiquitous organisms that swam in Earth's primordial seas, actually used
calcite in the lenses of their multiple eyes. A close-up of the eye of
Phacops appears below.
Microphotograph of the multiple eye of the
trilobite Phacops, showing the calcite lenses in the eye.
The chemical formula for calcite is CaCO3,
or calcium carbonate. CaCO3 is an important
component of Earth's carbon system, primarily because oceanic phytoplankton
and zooplankton form shells (also called tests) and skeletons out of this
material. One of the most familiar forms of CaCO3 in the marine realm is the hard skeletons of
coral, which form coral reefs and the tropical ring-shaped coral islands
called atolls. Beautiful Penrhyn Atoll in the Cook Islands (below) was
photographed from the Space Shuttle. (This photograph is from the Oceanography
from the Space Shuttle Web site.
Penrhyn Atoll, Cook Islands
Two major types of phytoplankton, coccolithophorids and foraminifera,
create shells made out of calcite. Surprisingly, two types of zooplankton,
pteropods and heteropods, form shells made out of aragonite, which is also
CaCO3 but which has a different mineral
structure. One of the primary questions regarding these organisms is how much
CaCO3 they make, globally, every year.
This question is an important part of the oceanic and global carbon cycle,
and it is difficult to reliably estimate. Remote sensing with instruments
such as the Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide
Field-of-view Sensor (SeaWiFS) has provided remarkable views of vast blue-white
blooms of coccolithophores, but actually quantifying how much CaCO3 is in those
blooms is considerably more difficult.
A quick definition of terms: a coccolithophorid is a particular form
of phytoplankton that forms disks of CaCO3
called coccoliths. The organisms form spherical shells ("coccospheres")
out of the coccoliths. Coccolithophorids are microscopic.
A few years before SeaWiFS was launched, Dr. Christopher Brown (now with the
National Oceanic and Atmospheric Administration) and Dr. James Yoder of the
University of Rhode Island (now with the National Science Foundation) used CZCS
data to map the occurrence of coccolithophorid blooms in the world ocean.
Click on the map to learn more about monitoring of coccolithophorid blooms, and
to see a large microphotograph of the most common coccolithophorid in the
ocean, Emiliania huxleyi.
Global map of coccolithophorid bloom occurrence based on CZCS data.
One area in which these blooms frequently occur is in the southern Atlantic
Ocean, near the coast of Argentina and the Falkland Islands. A previous
Science Focus! article, More Than Meets The Eye,
demonstrated how SeaWiFS data can be used to diagnose the presence of
coccolithophorid blooms in SeaWiFS data for an image acquired by SeaWiFS over
this oceanic region. In fact, Dr. Brown developed the algorithm that the
SeaWiFS Project uses to detect coccolithophorid blooms and "flag" them as
areas where the reflective properties of these organisms will lead to
erroneous calculation of chlorophyll concentration.
Blooms of coccolithophorids, particularly the widespread Emiliania
huxleyi, are easily recognized and detected due to their optical
characteristics, which give the water a milky turquoise color. The Bering Sea: Seasons and Cycles of Change has
some SeaWiFS images of large coccolithophore blooms in the Bering Sea. Since
these blooms can be observed so readily, the next logical research step is to
attempt to determine how much calcite they are producing: i.e., to quantify
the concentration of the coccolithophores and their coccoliths in the ocean.
And that's where the Moderate Resolution Imaging Spectroradiometer (MODIS)
comes into play. Two members of the MODIS Ocean science team, Dr. William
"Barney" Balch of the Bigelow Laboratory for Ocean Sciences in Maine, and Dr.
Howard Gordon of the University of Miami, have developed an algorithm that uses
MODIS data to quantify the amount of coccolithophorid calcite in these
blooms. The algorithm relies on Dr. Gordon's semi-analytical model of
water-leaving radiances and Dr. Balch's ongoing research into the optical
properties of coccolithophorids and coccolithophorid blooms.
Ever since the MODIS-Terra instrument began acquiring data, and especially
following the initiation of observations by MODIS-Aqua, this algorithm has
been applied to the global quantification of calcite in the oceans. A small
image of the MODIS calcite concentration product for March 2003 is
shown below. Some coccolithophore blooms occurring with the early
North Atlantic spring bloom can be seen near the
coasts of England and France.
Global map of MODIS coccolithophore calcite product for March 2003.
Go to the Earth Observatory Calcite Data page and construct your own
animations of the calcite concentration product to see the seasonal patterns
of calcite production in the world's oceans.
The researchers who developed the calcite concentration algorithm devised
two ways to check its accuracy. One way was to sample the recurring E.
huxleyi blooms in the Gulf of Maine and compare that data to the results
calculated by the algorithm. However, it took a few years for nature to
cooperate and provide coccolithophore blooms in the Gulf of Maine
following the launch of MODIS-Terra in 1998. So the researchers initially
went to their "Plan B" to test the algorithm - they dispersed 13 tons of
coccolith chalk in the ocean to make an offshore micro-patch of
suspended calcite with the same reflectance as naturally occurring blooms
(which contain hundreds of thousands of tons of coccolith calcite.
The name of this experiment was "Chalk-Ex".
Researchers put tons of chalk, composed of fossil
coccoliths, into the ocean for "Chalk-Ex". (Click on the image to see what the first "Chalk-Ex" looked like from the viewpoint of SeaWiFS. In this
image, only 2 bluish-white pixels are the actual chalk patch; the rest of the
white area is cloud.)
Since then, the Gulf of Maine has come through with some nice blooms
that could be used to test the accuracy of the algorithm. The MODIS image of
the Gulf of Maine shown below was acquired on June 11, 2002. Cape Cod is in
the lower left corner of the image.
MODIS image of a coccolithophorid bloom in the
Gulf of Maine, June 11, 2002. Nova Scotia is the land mass at the upper
right of the image, and Cape Cod is clearly visible at lower left.
Now, it's still possible for the algorithm to provide results for conditions
that are optically similar to coccolithophore blooms, which is why further
analysis of the data is still required. The algorithm shows significant
concentrations of coccolithophore calcite in the Southern Ocean, where diatoms
made out of silica would be expected to be the dominant organisms. Other
conditions, such as suspended sediments near the coast, and even the strange eruptions of hydrogen sulfide gas off the
coast of South Africa and Namibia, might also be similar enough to be
mis-identified as coccolithophore blooms.
Even though the algorithm still needs to be "tweaked", it represents another
great step forward in understanding the oceanic carbon cycle. The reason that
calcite and CaCO3 in general are important is
due to the fact that both their formation and dissolution involve carbon and
carbon dioxide (CO2).
Calcite, Carbon, and Carbon Dioxide in the Global Ocean
It would take a few book chapters to go into the details of the chemistry of
carbon and carbon dioxide system in seawater, so we'll only discuss the
relevant parts of the system. Dr. Joceline Boucher of the Corning School of
Ocean Studies at the Maine Maritime Academy has an excellent on-line resource
about marine geochemistry. The three relevant "lecture notes" are shown below:
When marine organisms form CaCO3, the
biological process of "calcification", CO2 is
produced. However, this process is just one part of the ocean carbon cycle.
In the open ocean, the organic matter that composes organisms is
continuously decomposed by bacteria after the organisms perish. This process,
known as respiration, converts the organic carbon into inorganic carbon, which
is somewhat confusingly called "mineralization". The two major dissolved
forms of inorganic carbon in the oceans are bicarbonate ion, HCO3-, and carbonate ion, CO32-, as shown in this slide:
This slide also shows another process that occurs in the deep ocean, the
dissolution of CaCO3. The combined effect of
these processes is to increase the quantities TCO2 and seawater's total
alkalinity. The net effect of these processes is to decrease the
concentration of CO32- in the deep
ocean as the water masses age, which means that the water becomes progressively
more corrosive to CaCO3, i.e., the dissolution
of CaCO3 is increasingly favored. Because
the oceans generally circulate from the north Atlantic Ocean to the north
Pacific Ocean, as the water moves it becomes progressively more corrosive to
CaCO3. The distribution of carbonate
sediments composed of coccoliths and foraminiferal CaCO3 is directly influenced by this process; the
sediments are found in much deeper waters in the Atlantic Ocean compared
to the Pacific Ocean.
All of that is nicely summarized on this slide. (If
you use the single right arrow button at the bottom to advance two slides, you
can see plots of the distribution of inorganic carbon in the Atlantic and
Pacific Oceans. North is to the right. The white lines show where calcite
and aragonite would start to dissolve. In the Atlantic Ocean, they would
essentially start to dissolve at the same depth, but in the Pacific Ocean,
there is enough difference in their solubility that aragonite will dissolve at
a slightly shallower depth than calcite.)
The reason that these processes are important is that the increasing levels
of atmospheric CO2 will eventually be
absorbed by the oceans, and they will slowly change the chemistry of seawater
with respect to CaCO3. The dissolution of
CaCO3 will actually neutralize (in several
thousand years) the CO2 in the atmosphere.
But in the short term, increasing concentrations of CO2 actually inhibit the ability of organisms to form
CO2. Thus, the full effects of the
interaction of the biology and chemistry in the oceans and atmosphere are still
very hard to determine. Getting a better estimate of how much calcite is
being produced by myriads of coccolithophorids in the world's oceans is one
vital element in increasing our understanding of how the system works, and
allowing improved predictions of what might happen in the future.
Acknowledgements
We would like to thank Dr. Joceline Boucher for her excellent Web pages
on ocean carbonate chemistry; Dr. Christopher Brown for his image of
coccolithophore bloom occurrence based on CZCS data, and Dr. Barney Balch
for images of the Chalk-Ex experiment.
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