Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 6 2007 10.5194/bg-4-1041-2007 http://www.biogeosciences.net/4/1041/2007/ http://www.biogeosciences.net/4/1041/2007/bg-4-1041-2007.html http://www.biogeosciences.net/4/1041/2007/bg-4-1041-2007.pdf 1041 1058 2007-11-29 Optical backscattering properties of the "clearest" natural waters M. S. Twardowski mtwardo@wetlabs2.com H. Claustre S. A. Freeman D. Stramski Y. Huot Department of Research, WET Labs, Inc., Narragansett, RI 02882, USA Laboratoire d'Océanographie de Villefranche, UMR-CNRS 7093, Villefranche sur Mer, France Marine Physical Laboratory, Scripps Institution of Oceanography, Univ. of California at San Diego, La Jolla, CA 92093-0238, USA 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. 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