of POC Using Satellite Data calibrated with Transmissometer and
POC Data from JGOFS/WOCE
Our group has collected transmissometer data since the early
1980's all around the world during JGOFS (North Atlantic
Bloom Experiment, Equatorial Pacific, Arabian Sea, Ross
Sea, Antarctic Polar front Zone-Pacific sector), WOCE (Pacific,
Indian and Southern Oceans), SAVE (South Atlantic), and
other programs. Many of these data have been published and
a synthesis of the WOCE and JGOFS data is performed as part
of this NSF-SMP grant. These data include transmissometer
measurements from 51 cruises and corrected data collected
(1991-1995) and BATS
Fig. 1. SeaTech instrument
attenuation data from the areas noted above include 6678
profiles, beginning in 1983. Measurements were made using
a SeaTech transmissometer
interfaced with CTD rosette.
A total of 16 different instruments were used on WOCE cruises
(serial ##: 15, 63D, 100D, 102D, 114D, 115, 135, 148, 151D,
152D, 156, 173D, 203D, 264AD, 265D, 266D).
measures the beam attenuation coefficient in the red spectral
band (l = 660 nm). Attenuation
of the light beam across the transmissometer's 25 cm path
length (r) was obtained using the same procedure
for all cruises making the data comparable and uniform.
In brief, the percent transmission (Tr) of
light was measured and was converted to beam attenuation
(c) using the equation c = -Ln(Tr)/r.
Beam attenuation (c)
is the sum of attenuation due to particles (cp),
water (cw), and colored
dissolved organic matter (ccdom):
c = cp
+ cw + ccdom.
According to several studies, ccdom
is small enough to be ignored in measurements at 660 nm
in open waters. Attenuation due to water cw
is essentially constant for this instrument at a factory-calibrated
value of 0.364 1/m.
Transmissometer Data reduction
majority of original raw transmissometer data were acquired
during both down and up casts of the CTD/rosette. The down
trace is generally the preferred trace because the sensors
are less obstructed during descent, However, water bottles
are routinely tripped during the ascent, so it is essential
to record transmissometer data at the time and depth of
the bottle trip. Having both down and up traces provides
an opportunity to compare the two profiles to check for
instrumental errors in the data and to use the up trace
if there are problems with the down trace. Temperature hysteresis
can cause slight differences between down and up traces,
especially where temperature gradients are large (Gardner
et al., 1985; Bishop, 1986). Compared
to the typical signal magnitude in surface waters, the hysteresis
data were sampled at high frequency (30Hz) and binned
to a standard 2 db pressure interval. The data reduction
procedure was applied uniformly to all data. This procedure
was quite complicated and consisted of several steps briefly
Raw-data files were processed using customised software
algorithms, which processed down and up casts stored in
these files. This processing included:
Pressure checking and filtering - due to ship heave
during the cast the CTD-probe sometimes experienced
a brief reverse excursion, so pressure values were checked
for non-monotonic values and breaks were filtered;
An initial large-spike removal was performed using two
filter windows depending on depth. Window size was 0.274-1.245
1/m (15300-12000 counts of the raw data) for upper 100
m depth and 0.274-0.572 1/m (15300-14200 counts) below
spike checking and removal was performed using a gradient
binning - data were averaged over 2 db intervals centred
at the even numbers (i.e. 0, 2, 4, 6, ... db pressure),
but data between 2 and 6 db were often excluded because
of air bubble contamination in surface waters;
Instrument calibrations - data were recalculated from
the volts to the physical units using the pre-cruise,
during-cruise and post-cruise calibration values. When
several during-cruise calibrations were made, those
values were applied according to the most closely associated
Smoothing by means of a running 5-point average; (this
smoothing was not done in the transmissometer data stored
in the JOGFS data base)
Profile minimum determination.
After the first step all data were
loaded into a preliminary data base for visual checking
and examination which included:
Checking for remaining spikes likely representative
of individual large particles (see Fig.
Checking for the "nose"-feature, which sometimes
occurs with SeaTech instruments. The "nose"
is a smooth, roughly normally-distributed (with depth)
peak in beam attenuation that occurs between 200-800
m water depth (see Fig.
3). The manufacturer believes it is due to condensation
on electronic components inside the instrument (not
on the windows) and is most likely to occur when the
instrument has been heated in the sun on the ship
deck prior to deployment. Up casts seldom had the
Checking for excessive noise;
Checking for the necessity of a uniform profile shift,
which can occur due to "dirty windows,"
sensor trend or instrumental offsets;
After the second step data were manually edited:
Final removal of any remaining spikes which passed through
the software filtration;
Profile editing - when profiles with artefacts such
as the "nose" had no associated up profile,
we eliminated the "nose" portion of the data
and used the surface data and deep-water data.
Assessment of general cruise trends and minimal values
Fig. 4. Instrumental trend
The minimum value, its depth and the station bottom
depth were plotted for each profile on every cruise.
This allowed detection of any cruise-long decay in the
instrument settings. This cruise trend assessment was
based on the minimum signal values recorded only at
open-ocean, deep-water stations (see
Cruise trend correction and profile normalization has
been made by shifting the entire profile so that the
profile's minimum value in deepwater (from the zone
deeper than 750 m and more than 750 m above the seafloor)
was set equal to the cruise's minimum. For the shallow-water
stations the cruise-mean minimum deep-water value was
E. Final database loading was performed
after all the above steps have been completed. The final
database for each cruise was used for further global quality
checks and map and section construction.
Global Quality check
twenty-plus years of measurements, multiple transmissometer
units have been used to collect data. Sometimes instruments
have been switched within the same cruise due to malfunction
or use of multiple CTDs. Therefore it is necessary to determine
the variability of the data caused by using of multiple
instruments. Our basic assumption has been that deep-waters
are highly stable and constant in terms of hydro-optical
characteristics. We used the minimum beam c
values measured in deep water as an absolute minimum value
of every cruise. The manufacturer (Bartz, personal communication)
has used this approach and we have used it in modified ways
during processing of the JGOFS transmissometer data. It
has been proven to be widely applicable as long as you have
some stations in deep water.
5. Crossing points
a means of comparing data between cruises (which could be
seasons or years apart in time), we overlay beam c
profiles obtained during different cruises where stations
were located in close proximity (within 1° longitude-latitude
circle) to each other. We called such points "crossings".
A total of 24 crossings with two or more stations measured
during different cruises were chosen: 10 in the Atlantic
(22 stations), 10 in the Pacific (20 stations) and 4 in
the Indian (8 stations) Oceans (see Fig.
Comparison of the mid-water part of the beam c
profiles (~1000 m - ~4000 m) shows very good agreement of
data from different cruises for nearly all crossings (see
Fig. 6). Reproducibility
at crossings is better when the same instrument is used
rather than different instruments, suggesting there may
be some instrumental differences in the data at depth. Transmissometers
may be perfectly calibrated in air over a range of temperatures,
but it appears that there may be different responses to
pressure that cannot be easily tested for without a simultaneous
full-depth deployment of instruments. We have made a few
simultaneous deployments of two SeaTech transmissometers
from the same production batch and found that they produced
identical profiles, but that does not guarantee that all
SeaTech transmissometers used would track each other throughout
the entire water column. The most important data come from
the upper 100 m, where the signal is strongest, and although
it is not possible to distinguish between temporal and instrumental
differences from these data, we do not believe there is
much difference in euphotic zone data obtained with different
instruments. If differences exist, they appear to occur
primarily in deeper water as differing instrumental responses
6. Crossing profiles
Beam Attenuation coefficient
Sections of transects and some maps we have processed are
placed on this CD and also can be viewed on the project
web-site. One can click on any cruise-line on the map
or table listed at the bottom of each page and obtain an
instant view of the section of beam attenuation along that
transect. Placing the mouse pointer on or off the image
switches the sections from 0-500 m to 0-6000 m water layers.
Images and data can be downloaded as PDF files. Some sections
are also displayed as POC concentration through a conversion
Regressions for different regions/programs
This large data set allows us to assess the relationship
between POC and beam cp
in different regions and during different seasons.
Since beam cp is a function
of particle size, shape and index of refraction, one might
expect the beam cp to
POC relation to vary regionally and temporally during the
cycle of a bloom and spatially as regimes with different
community structures are encountered.
The beam c profiles exhibit very little structure
below 200-300 m except on some continental margins. In areas
where resuspension of bottom sediments creates bottom nepheloid
layers the beam attenuation signal increases. Regression
of beam cp vs. POC does
not apply on those regions because most of the material
in the water is fine-grain clays, not POC.
published by our group using the data included in this
W.D., P.E. Biscaye, J.R.V. Zaneveld and M.J. Richardson,
1985. Calibration and comparison of the LDGO nephelometer
and the OSU transmissometer on the Nova Scotian Rise.
Mar. Geol. 66:323-344.
W.D., M.J. Richardson, I.D. Walsh, and B.L. Berglund,
1990. In-situ optical sensing of particles for determination
of oceanic processes: what satellites can't see, but
transmissometers can. Oceanography 3:11-17.
W.D., I.D. Walsh, and M.J. Richardson, 1993. Biophysical
forcing of particle production and distribution during
a spring bloom in the North Atlantic. Deep-Sea Res.
M.J., G.L. Weatherly, and W.D. Gardner, 1993. Benthic
storms in the Argentine Basin. Deep-Sea Res. 40:957-987.
E.P., W.D. Gardner, M. J. Richardson, and M. Kominz,
1994. Abyssal currents and advection of resuspended
sediment along the northeastern Bermuda Rise. Mar.
W.D., S.P. Chung, M.J. Richardson, and I.D. Walsh,
1995. The oceanic mixed-layer pump. Deep-Sea Res.
II 42: 757-775.
I.D., S.P. Chung, M.J. Richardson and W.D. Gardner,
1995. The diel cycle in the Integrated Particle Load
in the Equatorial Pacific: A Comparison with Primary
Production. Deep-Sea Res. II 42: 465-477.
S.P., W.D. Gardner, M.J. Richardson, I.D.Walsh, and
M.R. Landry, 1996. Beam attenuation and microorganisms:
Spatial and temporal variations in small particles
along 140° W during 1992 JGOFS-EqPac transects.
Deep-Sea Res. II 43: 1205-1226.
W.D., 1997. Visibility in the ocean and the effects
of mixing, Quarterdeck 5: 4-9.
M.J. and W.D. Gardner, 1997. Tools of the trade, Quarterdeck
D., J. Aiken, W. Balch, R. Barber, J. Dunne, W. D.
Gardner, C. Garside, C. Goyet, E. Johnson, D. Kirchman,
M. McPhaden, J. Newton, E. Peltzer, L. Welling, J.
White and J. Yoder, 1997. A meeting place of great
ocean currents: shipboard observations of a convergent
front at 2° N in the Pacific. Deep-Sea Res. II
I.D., W. D. Gardner, M. J. Richardson, S-P. Chung,
C.A. Plattner and V. Asper, 1997. Particle dynamics
as controlled by the flow field of the Eastern Equatorial
Pacific. Deep-Sea Res. II 44: 2025-2047.
S.P. W.D. Gardner, M.R. Landry, M.J. Richardson and
I.D. Walsh, 1998. Beam attenuation by microorganisms
and detrital particles in the equatorial Pacific.
J. Geophysical Research 103: 12,669-12,681.
J.S., W.D. Gardner, M. J. Richardson and I.D. Walsh,
1998. Effects of monsoons on the seasonal and spatial
distributions of POC and chlorophyll in the Arabian
Sea. Deep-Sea Res. II 45: 2103-2132.
W.D., Gundersen, J.S., M. J. Richardson and I.D. Walsh,
1999. The role of diel variations in mixed-layer depth
on the distribution, variation, and export of carbon
and chlorophyll in the Arabian Sea. Deep-Sea Res.
II 46: 1833-1858.
J., L.A. Codispoti, S.L. Smith, K. Wishner, C. Flagg,
W.D. Gardner, S. Gaurin, S.W.A. Naqvi, V. Manghnani,
L. Prosperie and. J. Gundersen, 1999. The oxygen minimum
zone in the Arabian Sea during 1995. Deep Sea Res.
II 46: 1903-1931.
W.D., M.J. Richardson, and W.O. Smith, 2000. Seasonal
Patterns of Water Column Particulate Organic Carbon
and Fluxes in the Ross Sea, Antarctica. Deep-Sea Res.
II, 47: 3423-3449.
W.D., J.C. Blakey, I.D. Walsh, M.J. Richardson, S.
Pegau, J.R.V. Zaneveld, C. Roesler, M.C. Gregg, J.A.
MacKinnon, H.M. Sosik and A.J. Williams, III, 2001
(May). Optics, particles, stratification and storms
on the New England continental shelf. Journal of Geophysical
Research, 106: 9473-9497.
E., W.S. Pegau, W.D. Gardner, J.R.V. Zaneveld, A.H.B.
Barnard, M.S. Twardowski, G.C. Chang, and T.D. Dickey,
2001. The Spectral Particulate Attenuation and Particle
Size Distribution in the Bottom Boundary Layer of
a Continental Shelf, Journal of Geophysical Research,
J., S. Gaurin, L.A. Codispoti, T. Takahashi, F.J.
Millero, W.D. Gardner, and M.J Richardson, 2001. Seasonal
evolution of the hydrographic properties during the
Antarctic Circumpolar Current at 170° W during
1997-1998, 2001. Deep-Sea Res. II, 48: 3943-3972.
W.D., M.J. Richardson, C.A. Carlson, and D.A. Hansell,
A.V. Mishonov, 2003. Determining True Particulate
Organic Carbon: Bottles, Pumps and Methodologies.
Deep_Sea Research II, 50: 655 - 674. [PDF]
A.V., W.D. Gardner, and M. J. Richardson, 2003. Remote
sensing and surface POC concentration in the South
Atlantic. Deep-Sea Research II, 50: 2997-3015. [PDF]
Y., W.D. Gardner, and M. J. Richardson. Nepheloid
layers on the central Louisiana shelf, Continental
Shelf Res. (in revision).
W.D. and M.J. Richardson, 1999. Temporal and spatial
variability of particulate matter in the Ross Sea,
Spring-Fall 1996-1997. Antarctic Journal of the U.S.
W.D. A.V. Mishonov, and M.J. Richardson, 2004. Global
POC Distribution Based on WOCE, JGOFS Transmissometer
Profiles of Beam Attenuation. Deep-Sea Research II
J.K.B. The correction and suspended particulate matter
calibration of Sea Tech transmissometer data. Deep-Sea.
Res. 1986; 33: 121-134.