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| Titel |
Optical backscattering properties of the "clearest" natural waters |
| VerfasserIn |
M. S. Twardowski, H. Claustre, S. A. Freeman, D. Stramski, Y. Huot |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 4, no. 6 ; Nr. 4, no. 6 (2007-11-29), S.1041-1058 |
| Datensatznummer |
250001997
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| Publikation (Nr.) |
 copernicus.org/bg-4-1041-2007.pdf |
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| Zusammenfassung |
During the BIOSOPE field campaign October–December 2004, measurements of
inherent optical properties from the surface to 500 m depth were made with a
ship profiler at stations covering over 8000 km through the
Southeast Pacific Ocean. Data from a ~3000 km section containing the
very clearest waters in the central gyre are reported here. The total volume
scattering function at 117°, βt(117°), was measured with
a WET Labs ECO-BB3 sensor at 462, 532, and 650 nm with estimated
uncertainties of 2×10-5, 5×10-6, and 2×10-6 m−1 sr−1, respectively.
These values were approximately 6%, 3%, and
3% of the volume scattering by pure seawater at their respective wavelengths.
From a methodological perspective, there were several results:
– distributions were resolvable even though some of the values from the
central gyre were an order of magnitude lower than the lowest previous measurements in the literature;
– Direct in-situ measurements of instrument dark offsets were necessary to accurately
resolve backscattering at these low levels;
– accurate pure seawater backscattering values are critical in determining particulate
backscattering coefficients in the open ocean (not only in these very clear waters); the pure water
scattering values determined by Buiteveld et al. (1994) with a [1+0.3S/37] adjustment for salinity
based on Morel (1974) appear to be the most accurate estimates, with aggregate accuracies as low as a few percent; and
– closure was demonstrated with subsurface reflectance measurements reported by Morel
et al. (2007) within instrument precisions, a useful factor in validating the backscattering measurements.
This methodology enabled several observations with respect to the
hydrography and the use of backscattering as a biogeochemical proxy:
–The clearest waters sampled were found at depths between 300 and 350 m, from
23.5° S, 118° W to 26° S, 114° W, where total backscattering at 650 nm was not distinguishable from pure seawater;
–Distributions of particulate backscattering bbp across the central gyre
exhibited a broad particle peak centered ~100 m;
–The particulate backscattering ratio typically ranged between 0.4% and 0.6% at 650 nm through
the majority of the central gyre from the surface to ~210 m, indicative of "soft" water-filled particles
with low bulk refractive index; and
– bbp showed a distinct secondary deeper layer centered ~230 m
that was absent in particulate attenuation cp data. The particulate backscattering ratio was
significantly higher in this layer than in the rest of the water column, reaching 1.2% in some locations.
This high relative backscattering, along with the pigment composition and ecological niche of this layer, appear
to be consistent with the coccolithophorid Florisphaera profunda.
Moreover, results were consistent with several expectations extrapolated
from theory and previous work in oceanic and coastal regions, supporting the
conclusion that particulate and total backscattering could be resolved in
these extremely clear natural waters. |
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