This data set consists of global ocean calcite (CLCT) data derived from the
Moderate Resolution Imaging Spectroradiometer (MODIS), in orbit on the Aqua
(formerly EOS PM) platform. MODIS ocean products are processed and
distributed by the Ocean Biology Processing Group (OBPG). Processing details
can be found at http://oceancolor.gsfc.nasa.gov/DOCS/MODISA_processing.html
Coccolithophores are ... small marine phytoplankton which form external CaCO3
scales (diameter ~ a few m and thickness 250 to 750 nm) called coccoliths. The
coccoliths can form multiple layers and eventually detach. Coccolithophores are
the largest source of calcium carbonate on earth [Westbroek et al., 1985].
Dissolution of calcite depends on the depth of the calcite compensation depth.
Globally, calcium carbonate is responsible for about 75% of the deposition of
carbon on the sea floor having a marine origin [Groom and Holligan, 1987;
Honjo, 1986; Honjo, 1990], and exceeds organic burial by a factor of seven.
Thus, coccolith production is an important part of the biogenic carbon cycle.
A global-scale study of their distribution is an important application for
MODIS imagery. MODIS is ideal for such a study because of two unique
characteristics: first its high radiometric sensitivity should, in principle,
allow the detection of smaller quantities of coccoliths than CZCS; and second,
the existence of the land bands on MODIS, e.g., band 3 and 4, with their high
saturation radiance will enable study of very high concentrations of coccoliths
that would saturate a typical ocean color instrument. The Aqua MODIS Level-3
calcite products are generated routinely for daily, 8-day, monthly, seasonal
and annual time periods. Spatial resolutions of 4 and 9 km are available.
The algorithm for extracting the detached coccolith concentration from surface
waters is based on the semianalytic model of ocean color of Gordon et al.
. Briefly, the model relates the normalized water-leaving radiance, i.e.,
the radiance that would exit the ocean if the sun were at the zenith and the
atmosphere were removed [Gordon and Clark, 1981], to the absorption and
scattering properties of the constituents of the water using radiative transfer
theory. The absorption and scattering properties are then related to the
constituent concentrations through statistical analysis of direct measurements.
Therefore, they represent oceanic- or regional-averaged relationships.
For more detailed information, see the Algorithm Theoretical Basis Document
(ATBD) for MODIS Detached Coccolith Concentration
The Aqua MODIS Level 3 Calcite (CLCT) data files are available from the FTP
site at ftp://oceans.gsfc.nasa.gov/MODISA/L3SMI/